The basic unit of measurement for electrical voltage is volt. Depending on the value, the voltage can be measured in volts(V), kilovolts(1 kV = 1000 V), millivolts(1 mV = 0.001 V), microvolts(1 μV = 0.001mV = 0.000001V). In practice, most often, you have to deal with volts and millivolts.
There are two main types of voltages - permanent and variable... Batteries and accumulators serve as a constant voltage source. The source of alternating voltage can be, for example, the voltage in the electrical network of an apartment or house.
To measure voltage use voltmeter... Voltmeters are turnout(analog) and digital.
Today, dial voltmeters are inferior to digital voltmeters, since the latter are more convenient to use. If, when measuring with a pointer voltmeter, the voltage readings have to be calculated on a scale, then in a digital one the measurement result is immediately displayed on the indicator. And in terms of dimensions, the pointer device loses to the digital one.
But this does not mean that dial gauges are not used at all. There are some processes that cannot be seen with a digital device, therefore, switches are more used in industrial enterprises, laboratories, repair shops, etc.
On electrical schematic diagrams, a voltmeter is indicated by a circle with a capital Latin letter " V" inside. Next to the symbol of the voltmeter, its letter designation is indicated " PU"And the serial number in the diagram. For instance. If there are two voltmeters in the circuit, then next to the first one they write “ PU 1", And about the second" PU 2».
When measuring DC voltage, the polarity of the voltmeter connection is indicated on the diagram, but if AC voltage is measured, then the polarity of the connection is not indicated.
The voltage is measured between two points circuits: in electronic circuits between positive and minus poles, in electrical circuits between phase and zero... The voltmeter is connected parallel to the voltage source or parallel to the chain section- a resistor, lamp or other load on which you want to measure the voltage:
Consider connecting a voltmeter: in the upper diagram, the voltage is measured across the lamp HL1 and at the same time on the power supply GB1... In the lower circuit, the voltage is measured across the lamp HL1 and resistor R1.
Before measuring the voltage, determine it view and approximate magnitude... The fact is that the measuring part of voltmeters is designed for only one type of voltage, and from this the measurement results are different. A voltmeter for measuring a constant voltage does not see an alternating voltage, and a voltmeter for an alternating voltage, on the contrary, can measure a constant voltage, but its readings will not be accurate.
It is also necessary to know the approximate value of the measured voltage, since voltmeters operate in a strictly defined voltage range, and if you make a mistake with the choice of the range or value, the device can be damaged. For instance. The measuring range of the voltmeter is 0 ... 100 Volts, which means that the voltage can be measured only within these limits, since when the voltage is measured above 100 Volts, the device will fail.
In addition to devices that measure only one parameter (voltage, current, resistance, capacitance, frequency), there are multifunctional devices that measure all these parameters in one device. Such a device is called tester(these are mainly dial gauges) or digital multimeter.
We will not dwell on the tester, this is a topic for another article, but let's go straight to the digital multimeter. For the most part, multimeters can measure two types of voltage in the range of 0 ... 1000 Volts. For the convenience of measurement, both voltages are divided into two sectors, and in the sectors into subranges: the DC voltage has five subranges, and the variable voltage has two.
Each sub-range has its own maximum measurement limit, which is indicated by a digital value: 200m, 2V, 20V, 200V, 600V... For instance. At the "200V" limit, the voltage is measured in the range of 0 ... 200 Volts.
Now the measurement process itself.
1. Measurement of DC voltage.
First, we determine with kind measured voltage (DC or AC) and move the switch to the desired sector. For example, let's take a finger-type battery with a constant voltage of 1.5 volts. We select the constant voltage sector, and in it the measurement limit is "2V", the measurement range of which is 0 ... 2 Volts.
The test leads must be inserted into the slots as shown in the lower figure:
Red the probe is usually called positive, and it is inserted into the socket opposite which the icons of the measured parameters are shown: "VΩmA";
black the probe is called minus or common and it is inserted into the slot, opposite which there is a "COM" icon. All measurements are made against this probe.
We touch the positive pole of the battery with the positive probe, and the negative pole with the negative one. The measurement result of 1.59 Volts is immediately visible on the indicator of the multimeter. As you can see, everything is very simple.
Now for another nuance. If the probes on the battery are reversed, then a minus sign will appear in front of the unit, signaling that the polarity of the multimeter is reversed. The minus sign is very convenient in the process of setting up electronic circuits, when you need to determine the plus or minus bus on the board.
Well, now let's consider the option when the voltage value is unknown. Let's leave the finger battery as a voltage source.
Let's say we don't know the voltage of the battery, and in order not to burn the device, we start the measurement from the very maximum limit of "600V", which corresponds to the measurement range of 0 ... 600 Volts. With the probes of the multimeter, we touch the poles of the battery and on the indicator we see the measurement result equal to “ 001 ". These figures indicate that there is no voltage or its value is too low, or the measurement range is too large.
We go down below. We move the switch to the "200V" position, which corresponds to the range of 0 ... 200 Volts, and touch the poles of the battery with the probes. The indicator shows readings equal to " 01,5 ". In principle, these readings are already enough to say that the voltage of the finger-type battery is 1.5 Volts.
However, the zero in front suggests dropping one more limit lower and measuring the voltage more accurately. We decrease to the limit "20V", which corresponds to the range of 0 ... 20 Volts, and again we make a measurement. The indicator showed the readings “ 1,58 ". Now we can say with accuracy that the voltage of the finger battery is 1.58 Volts.
In this way, not knowing the magnitude of the voltage, they find it, gradually decreasing from a high measurement limit to a low one.
There are also situations when, when measuring in the left corner of the indicator, the unit " 1 ". The unit signals that the measured voltage or current is higher than the selected measurement limit. For instance. If a voltage equal to 3 Volts is measured at the "2V" limit, then a unit will appear on the indicator, since the measurement range of this limit is only 0 ... 2 Volts.
There is one more limit “200m” with a measuring range of 0… 200 mV. This limit is intended to measure the very small voltages (millivolts) that sometimes have to be encountered when setting up an amateur radio design.
2. Measurement of alternating voltage.
The process of measuring AC voltage is no different from measuring DC. The only difference is that the polarity of the probes is not required for the alternating voltage.
The AC voltage sector is divided into two sub-ranges 200V and 600V.
At the "200V" limit, you can measure, for example, the output voltage of the secondary windings of step-down transformers, or any other voltage in the range of 0 ... 200 Volts. At the "600V" limit, you can measure voltages of 220 V, 380 V, 440 V or any other voltage within the range of 0 ... 600 Volts.
As an example, let's measure the voltage of a home network of 220 volts.
We move the switch to the "600V" position and plug the multimeter probes into the socket. The indicator immediately showed a measurement result of 229 Volts. As you can see, everything is very simple.
And one moment.
Before measuring high voltages, ALWAYS make sure that the insulation of the probes and wires of the voltmeter or multimeter is in good condition, and also check the selected measurement limit additionally. And only after all these operations take measurements... This will save yourself and the device from unexpected surprises.
And if something is not clear, then watch the video, which shows the measurement of voltage and current using a multimeter.
- Tutorial
Introduction
Hello everyone! After the completion of the cycle on the sensors, there were questions of a different plan for measuring the parameters of consumption of household and not very electrical appliances. Who consumes how much, how what to connect to measure, what are the subtleties and so on. It's time to reveal all the cards in this area.In this series of articles, we will look at the topic of measuring electricity parameters. There are actually a very large number of these parameters, which I will try to gradually tell you about in small series.
So far there are three episodes in the plans:
- Measurement of electricity.
- Power quality.
- Devices for measuring the parameters of electricity.
Based on the results of the entire cycle, we will make a kind of smart electric meter with Internet access. Quite ardent lovers of controlling the electrical appliances of their smart home can provide all possible assistance in the implementation of the communication part on the basis of, for example, MajorDomo. Let's make OpenSource smart home better, so to speak.
In this series, we'll cover the following questions in two parts:
- Connection of current and voltage sensors in DC devices, as well as single-phase and three-phase AC circuits;
- Measurement of effective values of current and voltage;
- Power factor measurement;
- Full, active and reactive power;
- Electricity consumption;
1. Connecting sensors
In the last cycle about voltage and current sensors, I talked about the types of sensors, but did not talk about how to use them and where to put them. It's time to fix it
Connecting DC Sensors
It is clear that the entire cycle will be devoted to AC systems, but we will quickly go over DC circuits as well, as this can be useful to us when developing DC power supplies. Take a classic PWM buck converter for example:
Fig 1. Buck converter with PWM
Our task is to provide a stabilized output voltage. In addition, on the basis of information from the current sensor, it is possible to control the operating mode of the choke L1, preventing its saturation, and also to implement current protection of the converter. And to be honest, there are no special options for installing sensors.
A voltage sensor in the form of a resistive divider R1-R2, which is the only one capable of operating on direct current, is installed at the output of the converter. As a rule, a specialized converter microcircuit has a feedback input, and makes every effort to ensure that a certain voltage level, prescribed in the documentation for the microcircuit, appears at this input (3). For example 1.25V. If our output voltage matches this level - everything is fine - we directly apply the output voltage to this input. If not, then set the divisor. If we need to provide an output voltage of 5V, then the divider must provide a division factor of 4, that is, for example R1 = 30k, R2 = 10k.
The current sensor is usually installed between the power supply and the converter and on the microcircuit. By the potential difference between points 1 and 2, and with a known resistance, the resistors Rs can determine the current value of the current of our choke. Installing a current sensor between the sources and the load is not a good idea, since the filter capacitor will be cut off by the resistor from the impulse current consumers. Installing a resistor in the break of the common wire also bodes well - there will be two ground levels with which it is still a pleasure to tinker.
Voltage drop problems can be avoided by using non-contact current sensors such as hall sensors:
Fig 2. Non-contact current sensor
However, there is a trickier way to measure current. Indeed, the voltage drops across the transistor in exactly the same way and the same current flows through it as the inductance. Therefore, by the voltage drop across it, you can also determine the current value of the current. To be honest, if you look at the internal structure of converter microcircuits, for example, from Texas Instruments, then this method occurs just as often as the previous ones. The accuracy of this method is certainly not the highest, but this is quite enough for the current cutoff to work.
Fig 3. Transistor as a current sensor
We do the same in other circuits of similar converters, be it boost or inverting.
However, it is necessary to separately mention the transformer forward and flyback converters.
Fig 4. Connection of current sensors in flyback converters
They can also use either an external resistance or a transistor in its role.
This completes the connection of sensors to DC / DC converters. If you have any suggestions for other options, I will gladly supplement the article with them.
1.2 Connecting sensors to single-phase AC circuits
In AC circuits, we have a much larger selection of possible sensors. Let's consider several options.The simplest is to use a resistive voltage divider and a current shunt.
Fig 5 Connection of resistor sensors
However, she has a couple of significant disadvantages:
First, either we will provide a significant signal amplitude from the current shunt, having allocated a large amount of power on it, or we will be content with a small signal amplitude and subsequently amplify it. And secondly, the resistor creates a potential difference between the neutral of the network and the neutral of the device. If the device is isolated, it does not matter, if the device has a ground terminal, then we risk being left without a signal from the current sensor, since we will short-circuit it. Perhaps it is worth trying sensors that work on other principles.
For example, we will use current and voltage transformers, or a hall effect current sensor and a voltage transformer. There are much more opportunities for working with equipment, since the neutral wire has no losses, and most importantly, in both cases there is a galvanic isolation of the measuring equipment, which can often come in handy. However, it must be borne in mind that transformer current and voltage sensors have a limited frequency response and if we want to measure the harmonic composition of distortions, then this is not a fact for us.
Fig. 6 Connection of transformer and proximity sensors of current and voltage
1.3 Connecting sensors to polyphase circuits of alternating current networks
In multi-phase networks, our ability to connect current sensors is slightly less. This is due to the fact that it will not work at all to use the current shunt, since the potential difference between the phase shunts will fluctuate within hundreds of volts and I do not know of a single general-purpose controller whose analog inputs can withstand such mockery.One way to use current shunts is of course - for each channel it is necessary to make a galvanically isolated analog input. But it is much easier and more reliable to use other sensors.
In my quality analyzer I use resistive voltage dividers and external hall effect current sensors.
Figure 7 Current sensors in a three-phase network
As you can see from the picture, we are using a four-wire connection. Of course, instead of hall effect current sensors, you can take current transformers or Rogowski loops.
Instead of resistive dividers, voltage transformers can be used for both four-wire and three-wire systems.
In the latter case, the primary windings of the voltage transformers are connected with a delta, and the secondary with a star, the common point of which is the common point of the measuring circuit
Figure 8: Using voltage transformers in a three-phase network
2 RMS value of current and voltage
It's time to solve the problem of measuring our signals. First of all, the effective value of current and voltage is of practical importance for us.
Let me remind you of the materiel from the sensor cycle. Using the ADC of our microcontroller, at regular intervals, we will record the instantaneous voltage value. Thus, for the measurement period, we will have an array of data on the level of the instantaneous voltage value (for current, everything is the same).
Fig 9. Series of instantaneous voltage values
Our task is to calculate the effective value. First, let's use the integral formula:
(1)In a digital system, you have to limit yourself to a certain quantum of time, so we go to the sum:
(2)Where is the sampling period of our signal, and is the number of samples for the measurement period. Somewhere here, in the video, I begin to rub the game about equality of areas. I should have slept that day. =)
In the MSP430FE4252 microcontrollers, which are used in single-phase Mercury electricity meters, 4096 readings are made for a measurement period of 1, 2 or 4 seconds. We will rely on T = 1c and N = 4096 in the future. Moreover, 4096 points per second will allow us to use fast Fourier transform algorithms to determine the harmonic spectrum up to the 40th harmonic, as required by GOST. But more on that in the next series.
Let's sketch out the algorithm for our program. We need to ensure a stable start of the ADC every 1/8192 second, since we have two channels and we will measure this data alternately. To do this, set the timer and the interrupt signal will automatically restart the ADC. All ADCs can do this.
We will write the future program on arduino, since many have it at hand. We have a purely academic interest so far.
Having a 16MHz system quartz frequency and an 8-bit timer (so that life does not seem like honey), we need to ensure the frequency of operation of at least any timer interrupt with a frequency of 8192Hz.
We are sad about the fact that 16MHz is not divided as a whole as we need, and the final frequency of the timer is 8198Hz. Close our eyes to 0.04% error and still read 4096 samples per channel.
We are sad about the fact that the overflow interrupt in arduino is busy timing (responsible for millis and delay, so this will stop working normally), so we use the comparison interrupt.
And we suddenly realize that the signal is bipolar, and that the msp430fe4252 copes with it perfectly. We are content with a unipolar ADC, so we assemble a simple bipolar-to-unipolar converter on an operational amplifier:
Fig 10 Bipolar to unipolar converter
Moreover, our task is to ensure the oscillation of our sinusoid relative to half of the reference voltage - then we either subtract half of the range or activate the option in the ADC settings and get signed values.
The Arduino has a 10-bit ADC, so we subtract half from the unsigned result in the range of 0-1023 and get -512-511.
We check the model assembled in LTSpiceIV and make sure that everything works as it should. In the video, we are additionally convinced experimentally.
Fig 11 simulation result. Green is the source signal, blue is the output
Arduino sketch for one channel
void setup () (autoadcsetup (); DDRD | = (1<
The program was written in the Arduino IDE for the ATmega1280 microcontroller. On my debug board, the first 8 channels are routed for the internal needs of the board, so the ADC8 channel is used. It is possible to use this sketch for a board with an ATmega168, however, you must select the correct channel.
Inside the interrupts, we juggle a couple of service pins to clearly see the working digitizing frequency.
A few words about where the 102 coefficient came from. At the first start, a signal of various amplitudes was supplied from the generator, the actual voltage value was read from the oscilloscope, and the calculated value in absolute ADC units was taken from the console.
| Umax, V | Urms, B | Counted |
| 3 | 2,08 | 212 |
| 2,5 | 1,73 | 176 |
| 2 | 1,38 | 141 |
| 1,5 | 1,03 | 106 |
| 1 | 0,684 | 71 |
| 0,5 | 0,358 | 36 |
| 0,25 | 0,179 | 19 |
Dividing the values of the third column by the values of the second, we get an average of 102. This will be our "calibration" factor. However, you can see that the accuracy drops sharply with decreasing voltage. This is due to the low sensitivity of our ADC. In fact, 10 digits for accurate calculations are catastrophically small and if the voltage in the outlet is measured in this way it will work out, then putting a 10-bit ADC to measure the current consumed by the load will be a crime against metrology.
We will interrupt at this point. In the next part, we will consider the other three questions of this series and we will smoothly move on to creating the device itself.
The presented firmware, as well as other firmware for this series (since I shoot videos faster than preparing articles), can be found in the repository on GitHub.
Measurement of direct currents is most often performed by magnetoelectric galvanometers, microammeters, milliammeters and ammeters, the main part of which is a magnetoelectric measuring mechanism (meter). The device of one of the common designs of a dial gauge is shown in Fig. 1. The meter contains a horseshoe-shaped magnet 1. In the air gap between its pole pieces 2 and a stationary cylindrical core 5 made of soft magnetic material, a uniform magnetic field is created, the induction lines of which are perpendicular to the surface of the core. In this gap, a frame 4 is placed, wound with a thin copper insulated wire (0.02 ... 0.2 mm in diameter) on a lightweight paper or aluminum rectangular frame. The frame can be rotated together with the axis 6 and the arrow 10, the end of which moves above the scale. Flat spiral springs 5 are used to create a moment that counteracts the rotation of the frame, as well as to supply current to the frame. One spring is fixed between the axle and the body. The second spring is attached at one end to the axis, and the other to the corrector lever 7, the fork of which covers the eccentric shaft of the screw 8. By turning this screw, the arrow is set to the zero division of the scale. Counterweights 9 serve to balance the moving part of the meter in order to stabilize the position of the arrow when the position of the instrument changes.
Rice. 1. The device of the magnetoelectric measuring mechanism.
The measured current, passing through the turns of the frame, interacts with the magnetic field of the permanent magnet. The torque generated in this case, the direction of which is determined by the well-known left-hand rule, causes the frame to rotate by such an angle at which it is balanced by the opposing moment arising from the twisting of the springs 5. Due to the uniformity of the constant magnetic field in the air gap, the torque, and, consequently, the angle the deviations of the arrow turn out to be proportional to the current flowing through the frame. Therefore, magnetoelectric devices have uniform scales. Other quantities that affect the value of the torque, the magnetic induction in the air gap, the number of turns and the area of the frame, remain constant and, together with the spring force, determine the sensitivity of the meter.
When the frame is rotated, currents are induced in its aluminum frame, the interaction of which with the field of a permanent magnet creates a braking torque that quickly calms the moving part of the meter (the settling time does not exceed 3 s).
The meters are characterized by three electrical parameters: a) total deflection current Ii, which causes the pointer to deflect to the end of the scale; b) the voltage of the total deflection Ui, that is, the voltage on the meter frame, which creates a current Ii in its circuit; c) internal resistance Ri, which is the resistance of the frame. These parameters are interconnected by Ohm's law:
In radio measuring devices, various types of magnetoelectric meters are used, the total deflection current of which usually lies in the range of 10 ... 1000 μA. Meters in which the total deflection current does not exceed 50-100 μA are considered highly sensitive.
Some meters are equipped with a magnetic shunt in the form of a steel plate that can be moved closer to or away from the end surfaces of the pole pieces and the magnet. In this case, the total deflection current I will accordingly decrease or increase within small limits, due to a change in the magnetic flux acting on the frame due to the branching of a part of the total magnetic flux through the shunt.
The total deviation voltage Ui for most meters lies in the range of 30-300 mV. The frame resistance Ri depends on the frame perimeter, the number of turns and the wire diameter. The more sensitive the meter, the more turns from the thinner wire its frame has and the greater its resistance. An increase in the sensitivity of the meters is also achieved by using more powerful magnets, frameless frames, springs with a small opposing moment and a suspension of the moving part on stretch marks (two thin threads).
In sensitive meters with frameless frames, the arrow, deflecting under the action of the current passing through the frame, makes a series of oscillations before stopping in the equilibrium position. To reduce the settling time of the arrow, the frame is shunted with a resistor with a resistance of the order of thousands or hundreds of ohms. The role of the latter is sometimes performed by the electrical circuit of the device, connected in parallel to the frame.
Gauges with movable frames allow you to obtain an angle of full deflection of the arrow up to 90-100 °. Small-sized meters are sometimes made with a fixed frame and a movable magnet mounted on the same axis with the arrow. In this case, it is possible to increase the angle of complete deflection of the arrow up to 240 °.
Particularly sensitive meters used to measure very low currents (less than 0.01 μA) and voltages (less than 1 μV) are called galvanometers. They are often used as null indicators (indicators of the absence of current or voltage in a circuit) when measuring by comparison methods. According to the method of counting, dial and mirror galvanometers are distinguished; in the latter, the reading of the risk on the scale is created with the help of a light beam and a mirror, fixed on the moving part of the device.
Magnetoelectric meters are suitable for direct current measurements only. Changing the direction of the current in the frame lead to the change in the direction of the torque and the deflection of the arrow in the opposite direction. When the meter is connected to an alternating current circuit with a frequency of up to 5-7 Hz, the arrow will continuously oscillate around zero of the scale with this frequency. At a higher frequency of the current, the mobile system, due to its inertia, does not have time to follow the changes in the current and the arrow remains in the zero position. If a pulsating current flows through the meter, then the deflection of the arrow is determined by the constant component of this current. In order to exclude the jitter of the arrow, the meter is shunted with a large capacitor.
Meters designed to operate in a DC circuit, the direction of which is unchanged, have a one-sided scale, one of the ends of which is a zero division. To obtain the correct deflection of the arrow, it is necessary that the current flow through the frame in the direction from the terminal marked "+" to the terminal marked "-". Meters designed to operate in DC circuits, the direction of which can be changed, are equipped with a two-sided scale, the zero division of which is usually located in the middle; when current flows in the device from the "+" terminal to the "-" terminal, the arrow deviates to the right.
Magnetoelectric meters can withstand short-term overload, reaching 10 times the current Ii, and 3 times long-term overload. They are insensitive to external magnetic fields (due to the presence of a strong internal magnetic field), consume little power during measurements and can be performed in all accuracy classes.
For measurements on alternating current, magnetoelectric meters are used in conjunction with semiconductor, electronic, photoelectric or thermal converters; taken together, they form, respectively, rectifier, electronic, photoelectric or thermoelectric devices.
In measuring instruments, electromagnetic, electrodynamic and ferrodynamic meters are sometimes used, which are suitable for direct measurement of both direct currents and rms values of alternating currents with a frequency of up to 2.5 kHz. However, meters of these types are significantly inferior to magnetoelectric meters in terms of sensitivity, accuracy and power consumption in measurements. In addition, they have an uneven scale, compressed in the initial part, and are sensitive to the influence of external magnetic fields, for the weakening of which it is necessary to use magnetic shields and complicate the design of the devices.
Determination of electrical parameters of magnetoelectric meters
When a measuring mechanism of an unknown type is used as a meter of a magnetoelectric device, the parameters of the latter - the total deflection current Ii and the internal resistance Ri - have to be determined empirically.
Rice. 2. Circuits for measuring electrical parameters of magnetoelectric meters
The resistance of the frame Ri can be approximately measured with an ohmmeter that has the required measurement limit. Care should be taken when checking highly sensitive meters, as the high current of the ohmmeter can damage them. If a multi-range battery-powered ohmmeter is used, the measurement should start at the highest resistance limit at which the current in the ohmmeter's supply circuit is lowest. The transition to other limits is allowed only if this does not cause the meter arrow to overshoot.
Quite accurately, the parameters of the meter can be determined according to the diagram in Fig. 2, a. The circuit is powered from a constant voltage source B through a resistor R1, which serves to limit the current in the circuit. With the rheostat R2, the deflection of the meter needle AND the entire scale is achieved. In this case, the value of the current Ii is measured according to a reference (reference) microammeter (milliammeter) μA call reference). Then, in parallel with the meter, a reference resistance box Rо is connected, by changing the resistance of which, the current through the meter is reduced by exactly two times compared to the current in the common circuit. This will take place when the resistance Ro = Ri. Instead of a resistance box, you can use any variable resistor, followed by measuring its resistance Rо = R and using an ohmmeter or a DC bridge. It is also possible to connect in parallel with the meter an unregulated resistor with a known resistance R, preferably close to the expected resistance Ri; then the value of the latter is determined by the formula
Ri = (I / I1 - 1) * R,
where I and I1 are the currents measured, respectively, by the μA and I.
If the meter And has a uniform scale containing αп divisions, then you can apply the circuit shown in Fig. 2, b. The required parameters of the meter are calculated by the formulas:
Ii = U / (R1 + R2) * αп / α1; Ri = (α2 * R2) / (α1-α2) - R1,
where U is the supply voltage measured by the voltmeter V, α1 and α2 are the readings on the meter scale when the switch B is set to positions 1 and 2, respectively, and R1 and R2 are the known resistances of the resistors, which are taken of approximately the same denominations. The measurement error is the smaller, the closer the α1 reading to the end of the scale, which is achieved by the appropriate choice of resistance
Magnetoelectric milliammeters and ammeters
Magnetoelectric meters, when directly connected to electrical circuits, can be used only as DC microammeters with a measurement limit equal to the total deflection current Ii. To expand the measurement limit, the meter And is included in the current circuit in parallel to the shunt - a resistor of low resistance Rsh (Fig. 3); in this case, only a part of the measured current will flow through the meter, and the less, the less the resistance Rsh in comparison with the resistance of the meter Ri. In electronic measurements, the maximum required measurement limit for direct currents rarely exceeds 1000 mA (1 A).
At the selected limit value of the measured current Ip, the current of total deviation Ii must flow through the meter; this will be the case with shunt resistance
Rsh = Ri: (Ip / Ii - 1). (one)
For example, if it is necessary to expand the measurement range of a microammeter of the M260 type, having the parameters Ip = 0.2 mA and Ri = 900 Ohm, to a value of Ip = 20 mA, it is necessary to use a shunt with a resistance of Rsh = 900 / (100-1) = 9.09 Ohm.
Rice. 3. Calibration diagram of the magnetoelectric milliammeter (ammeter)
Shunts to milliammeters are made of manganin or constantan wire. Due to the high resistivity of the material, the dimensions of the shunts are small, which allows them to be connected directly between the clamps of the device inside or outside its casing. If the value of the current Ip (in amperes) is known, then the diameter of the shunt wire d (in millimeters) is chosen from the condition
d> = 0.92 I p 0.5, (2)
when performing which the current density in the shunt does not exceed 1.5 A / mm 2. For example, a milliammeter shunt with a measurement limit of Ip = 20 mA should be made of a wire with a diameter of 0.13 mm.
Having picked up a wire of a suitable diameter d (in millimeters), its length (in meters) required to make a shunt with resistance Rsh (in ohms) is approximately found by the formula
L = (1.5 ... 1.9) d 2 * Rsh (3)
and is precisely adjusted when the device is turned on according to the diagram in Fig. 3 in series with a reference milliammeter mA.
Shunts for high currents (to ammeters) are usually made from sheet manganin. To exclude the influence of contact resistances and resistances of connecting conductors, such shunts have four clamps (Fig. 4, a). External massive clamps are called current and are used to connect a shunt in the circuit of the measured current. Internal terminals are called potential and are designed to connect the meter. This design also excludes the possibility of damage to the meter by high current if the shunt is accidentally disconnected.
To reduce the temperature measurement error caused by the different temperature dependence of the resistances of the meter frame and the shunt, a Manganin resistor Rk is connected in series with the meter (Fig. 4, b); the error decreases as many times as the resistance of the meter circuit increases. Even better results are achieved when a thermistor Rk with a negative temperature coefficient of resistance is turned on. When calculating a device with temperature compensation, the resistance R and in the calculation formulas should be understood as the total resistance of the meter and the resistor Rk.
Rice. 4. Schemes for switching on a shunt for high currents (a) and a temperature compensation element (b)
Taking into account the effect of the shunt, the internal resistance of the milliammeter (ammeter)
Rma = RiRsh / (Ri + Rsh). (4)
To ensure a sufficiently high accuracy in a wide range of measured currents, the device must have several measurement limits; this is achieved by using a number of switchable shunts designed for different values of the limiting current Ip.
The transient factor of the scale N is the ratio of the upper limit values of two adjacent measurement limits. With N = 10, as, for example, in a four-limit milliammeter with the limits of 1, 10, 100 and 1000 mA, the instrument scale made for one of the limits (1 mA) can be easily applied to measure currents on the remaining limits by multiplying the count by the corresponding multiplier is 10, 100 or 1000. In this case, the measurement range will reach 90% of the indication range, which will lead to a noticeable increase in the measurement error of those current values that correspond to the readings on the initial sections of the scales.
Rice. 5. Scales of multi-range magnetoelectric milliammeters
In order to increase the accuracy of measurements in some devices, the limiting values of the measured currents are selected from a number of numbers 1, 5, 20, 100, 500, etc., using a common scale with several rows of numerical marks for counting (Fig. 5, a). Sometimes the limiting values are chosen from a series of numbers 1, 3, 10, 30, 100, etc., which makes it possible to exclude counting along the first third of the scale; however, the scale should have two rows of marks, graduated in multiples of 3 and 10, respectively (Fig. 5, b).
The switching of shunts, necessary for the transition from one measurement limit to another, can be carried out by means of a switch when using common input terminals on all limits (Fig. 6) or using a system of split sockets, the halves of which are closed with each other by a metal plug of the measuring cord (Fig. 7 ). A feature of the circuits in Fig. 6, b, and 7, b is that the shunt of each measurement limit includes resistors of shunts of other, less sensitive limits.
Rice. 6. Schemes of multi-range milliammeters with switches of measurement limits.
When switching under the current of the measurement limit of the device, damage to the meter is possible if it is briefly connected without a shunt in the circuit of the measured current. To avoid this, the design of the switches (Fig. 6) must ensure the transition from one contact to another without breaking the circuit. Accordingly, the design of the split sockets (Fig. 7) should allow the plug of the measuring cord, when switched on, initially to close with the shunt, and then with the meter circuit.
Rice. 7. Circuits of multi-range milliammeters with plug-and-socket switching of measurement limits.
In order to protect the meter from dangerous overloads, a Kn button with an opening contact is sometimes placed in parallel to it (Fig. 7, b); the meter is included in the circuit only when the button is pressed. An effective way to protect sensitive meters is to bypass them (in the forward direction) with specially selected semiconductor diodes; in this case, however, a violation of the uniformity of the scale is possible.
Compared to devices with switchable shunts, multi-range devices with universal shunts are more reliable in operation. A universal shunt is a group of resistors connected in series, forming a closed circuit with the meter (Fig. 8). To connect to the circuit under investigation, a common negative terminal and a terminal connected to one of the shunt taps are used. In this case, two parallel branches are formed. For example, when switch B is set to position 2 (Fig. 8, a), one branch contains the resistors of the active section of the shunt having resistance Rsh.d = Rsh2 + Rsh3, in the second branch resistor Rsh1 is connected in series with the meter. The resistance Rsh.d should be such that at the maximum measured current Ip, the current of total deflection Ii flows through the meter. In general
Rsh.d = (Rsh + Ri) (Ii / Ip). (5)
where Rsh = Rsh1 + Rsh2 + Rsh3 + ... is the total resistance of the shunt.
The universal shunt as a whole performs the function of an acting shunt at limit 1, which corresponds to the smallest limit value of the measured current Iп1; its resistance can be calculated by the formula (1). If the limits of measurements are selected Ip2 = = N12 * Ip1; Ip3 = N23 * Ip2; Ip4 = N34 * Ip3, etc., then the resistances of individual sections of the shunt are determined by the expressions:
Rsh2 + Rsh3 + Rsh4 + ... = Rsh / N12;
Rsh3 + Rsh4 + ... = Rsh / (N12 * N23);
Rsh4 + ... = Rsh / (N12 * N23 * N34), etc. The resistance difference from two adjacent equalities allows you to determine the resistances of the individual components of the shunt Rsh1, Rsh2, Rsh3, etc.
Rice. 8. Circuits of multi-range milliammeters with universal shunts
From the above expressions, it can be seen that the transition factors N12, N23, N34, etc. are entirely determined by the ratio of the resistances of individual sections of the shunt and are completely independent of the meter data. Therefore, the same universal shunt, connected in parallel to different meters, will change their limits by the same number of times; in this case, the initial measurement limit is determined by the formula
Ip1 = Ii * (Ri / Rsh + 1). (6)
From the diagrams in Fig. 8 it can be seen that in devices with universal shunts, the measurement limits can be selected both using switches and using conventional sockets. Loss of contact in these circuits is safe for the meter. If the approximate value of the current to be measured is unknown, then before connecting the multi-limit device to the circuit under study, the highest upper measurement limit should be set,
Calibration of magnetoelectric milliammeters and ammeters
Calibration of a measuring device consists in determining its calibration characteristic, i.e., the relationship between the values of the measured quantity and the readings of the reading device, expressed in the form of a table, graph or formula. In practice, the graduation of the dial gauge is completed by drawing on its scale of divisions corresponding to certain numerical values of the measured value.
For magnetoelectric devices with uniform scales, the main task of calibration is to establish the correspondence of the final division of the scale to the limiting value of the measured value, which can be done using a circuit similar to that shown in Fig. 3. The device to be calibrated is connected to terminals 1 and 2. With a rheostat R in a circuit powered by a direct current source, set the limit value of the current Ip according to the reference device mA and mark the point of the scale to which the pointer of the meter I. deviates. If the device to be calibrated has one limit, then any point near the stop limiting the movement of the arrow can be taken as the end point of the scale. In multi-limit devices with multiple scales, such an arbitrary choice of the end of the scale can be made only at one limit, taken as the initial one.
If the arrow at the current Iп is not at the final division of the scale, the device must be adjusted. In single-range instruments or at the original limit of a multi-range instrument, this adjustment can be made with a magnetic shunt. In the absence of the latter, adjustment is carried out by adjusting the resistance of the shunts. If at current Ip the arrow does not reach the final division, then the shunt resistance Rsh should be increased; when the arrow goes off scale, the shunt resistance is reduced.
When calibrating multi-range devices operating according to the schemes shown in Fig. 6, b, 7, b and 8, the shunts must be fitted in a certain order, starting with the shunt resistance Rsh corresponding to the highest limiting current Ip3; then the resistances of the shunts Rsh2 and Rsh1 are sequentially adjusted. When switching the limits, it may be necessary to replace the reference device, the upper measurement limit of which in all cases must be equal to or slightly higher than the limit value of the graduated scale.
Knowing the positions of the initial and final divisions of a uniform scale, it is easy to determine the positions of all intermediate divisions. However, it should be borne in mind that some magnetoelectric devices, due to design flaws or features of the measuring circuit, may not have an exact proportionality between the angular movement of the arrow and the measured current. Therefore, it is advisable to check the graduation of the scale at several intermediate points by changing the current with a rheostat R. Resistor Ro serves to limit the current in the circuit.
Calibration should be performed with the fully assembled instrument under normal operating conditions. The obtained reference points are applied to the surface of the scale with a sharpened pencil (with the glass removed from the meter casing) or are fixed according to the marks of the existing scale of the instrument. If the old scale of the meter is unusable, then a new scale is made of thick, smooth paper, which is glued in place of the old scale with glue resistant to moisture. The position of the new scale must strictly correspond to the position occupied by the old scale when calibrating the device. Good results are achieved by drawing the scale with black ink on an enlarged scale and then making a photocopy of the required size.
The general principles of calibration discussed above are applicable to dial gauges for various purposes.
Features of measuring direct currents
To measure the current, the device (for example, a milliammeter) is connected in series to the circuit under investigation; this leads to an increase in the total resistance of the circuit and a decrease in the current flowing in it. The degree of this reduction is estimated (in percentage) by the coefficient of influence of the milliammeter
Bma = 100 * Rma / (Rma + Rts),
where Rts is the total resistance of the circuit between the connection points of the device (for example, terminals 1 and 2 in the diagram in Fig. 3).
Multiplying the numerator and denominator of the right side of the formula by the value of the current in the circuit I and taking into account that I * Rma is the voltage drop across the milliammeter Uma, and I (Rma + Rts) is equal to the emf. E acting in the investigated circuit, we obtain
Bma = 100 * Uma / E.
In a complex (branched) chain under e. etc. with. E you need to understand the open-circuit voltage between the break points of the circuit to which the device should be connected.
The limiting value of the voltage Uma is the voltage drop across the device Uп, which causes the deviation of its arrow to the final mark of the scale. Therefore, the maximum possible value of the coefficient of influence when using this device
Bp = 100Up / E. (7)
From the above formulas it follows that the smaller the e. etc. with. E, the more the instrument affects the measured current. For example, if Uп / E = 0.1, then Bp = 10%, that is, turning on the device can cause a decrease in the current in the circuit by 10%; at Uп / E = 0.01, the current decrease does not exceed 1%. Therefore, when measuring the filament current of radio tubes or the emitter current of transistors, a significantly greater change in the current in the circuit should be expected than when measuring anode, screen, or collector currents. It is also obvious that with the same measurement range, a device characterized by a lower voltage value Uп has a lesser effect on the measured current. In multi-range milliammeters with switchable shunts (Figs. 6 and 7), at all measurement ranges, the maximum voltage drop across the device is the same and equal to the voltage of the total deviation of the meter, that is, Uп = Ui = Ii / Ri, and the power consumed by the device is limited by the value
Pp = IiUi = Ip * Ii * Ri. In milliammeters with universal shunts (Fig. 8), the voltage drop across the device is equal to Ii * I and only at the initial limit 1. At other limits, it increases to the value Uп ≈ Ii * (Rп + Rsh) (with an increase in the power consumed by the device in (Ri + Rsh) / Ri times), since it is the sum of the voltage drops across the meter and the shunt section connected in series with it. Consequently, a device with a universal shunt, all other things being equal, has a stronger effect on the mode of the investigated circuits than a device with switchable shunts.
If we take the total resistance of the universal shunt Rsh >> Ri, then the lower limit of the milliammeter will be close to Ii, however, at other limits, the voltage drop across the device may turn out to be excessively large. If we take the resistance Rsh small, then the smallest limiting current Iп1 of the device will increase. Therefore, in each specific case, it is necessary to resolve the issue of the permissible value of the shunt resistance Rsh.
When a magnetoelectric device is connected to a pulsating or pulsed current circuit, in order to measure the constant component of this current, it is necessary to connect a large capacitor parallel to the device, which has a resistance for the variable component of the current that is much less than the internal resistance of the device Rma. In order to eliminate the effect of the device's capacitance relative to the housing of the installation under study, the place where the device is connected to high-frequency circuits is chosen so that one of its clamps is connected directly to the housing or through a high-capacity capacitor.
In some cases, permanent shunts are included in various circuits of the electronic device under study, which allows using the same magnetoelectric meter to alternately monitor the currents in these circuits without breaking them.
Task 1. Calculate the milliammeter circuit with a universal shunt (Fig. 8) for three measurement limits: 0.2; 2 and 20 mA with a transition factor N = 10. The meter of the device - a microammeter of the M94 type - has the data: Ii = 150 μA = 0.15 mA, Ri = 850 Ohm, Ui = Ii / Ri = 0.128 V. For each limit, find the drop voltage on the device at the limiting current, as well as the maximum possible effect of the device on the measured current, if e is acting in the circuit of the latter. etc. with. E = 20 V.
1. At the limit 1 (Ip1 = 0.2 mA), the shunt to the meter is a universal shunt as a whole. The impedance of the latter, determined by the formula (1), Rsh = 2550 Ohm.
The voltage drop across the device at the limiting current Uп1 = Ui = 0.128 V. The maximum possible coefficient of influence of the milliammeter Bp1 = (Uп1 / E) * 100 = 0.64%.
2. For limit 2 (Ip2 = 2 mA) the resistance of the shunting section of the universal shunt Rsh2 + Rsh3 = Rsh / N = 255 Ohm. Therefore, resistance Rsh1 = Rsh - (Rsh2 + Rsh3) = 2295 Ohm.
The limiting voltage drop across the device is Up2 = Ii / (Ri + Rsh1) = 0.727 V. The limiting coefficient of influence Bp2 = 100 * Up2 / E = 3.63%.
3. For limit 3 (Ip3 = 20 mA) Rsh3 = Rsh / N 2 = 25.5 Ohm; Rsh2 = 255-25.5 = 229.5 Ohm; Up3 = Ip * (Ri + Rsh1 + Rsh2) = 0.761 V; Bn3 = 100 * p3 / E = 3.80%.
Task 2. Calculate a milliammeter circuit with a universal shunt for three measurement limits: 5, 50 and 500 mA. The meter of the device - a microammeter of the M260M type - has the data: Ii = 500 μA, Ri = 150 Ohm. Determine the effect of the device on the measured current if measurements within 5 and 50 mA are made in circuits in which e. etc. with. not less than 200 V, and at the limit of 500 mA - in the heating circuit of a radio tube powered by a battery with an electromotive force. 6 B.
Answer: Rsh = 16.67 Ohm; Rsh1 = 15 Ohm; Rsh2 = 1.5 Ohm; Rsh3 = 0.17 Ohm; Uп1 = 75 mV; Bn1 = 0.037%; Up2 = 82.5 mV; Bn2 = 0.041%; Up3 = 83 mV; Bn3 = 1.4%.
Answer: 1) Rsh1 = 16.67 Ohm; Rsh2 = 1.52 0m; Rsh3 = 0.15 Ohm; 2) Rsh1 = 15.15 Ohm; Rsh2 = 1.37 Ohm; Rsh3 = 0.15 ohm.
DC transistor microammeters
If it is necessary to measure very small currents, much less than the total deflection current I and the available magnetoelectric meter, the latter is used in conjunction with a DC amplifier. The simplest and most economical are bipolar transistor amplifiers. Amplification of the current can be achieved by turning on transistors in a common emitter and a common collector circuit, but the former is preferable because it provides a lower input impedance to the amplifier.
Rice. 9. Circuits of single-transistor DC microammeters
The simplest circuit of a single-transistor microammeter powered from a source with an emf. E = 1.5 ... 4.5 V, shown in Fig. 9, a, with solid lines. The base current Ib is the measured current, at a certain nominal value of which Iн in the collector circuit current Ik flows, equal to the total deflection current I and the meter I. Static current transfer coefficient Vst = Ic / Ib = Ii / In, whence the nominal measured current In = Ii / Bst. For example, when using a transistor of the GT115A type with Bst = 60, and a meter of the M261 type with a current Ii = 500 μA, the rated current is In = 500/60 ≈ 8.3 μA. Since the relationship between the currents Ik and Ib is close to linear, the meter scale, calibrated in the values of the measured current, will be almost uniform (except for a small initial section of the scale up to 10% of its length). By connecting a specially selected shunt between the input terminals, you can increase the measured current limit to a value convenient for calculations (for example, up to 10 μA).
In real circuits of transistor microammeters, measures are taken to stabilize the operating mode and correct its possible deviations. First of all, it is unacceptable (especially with an increased supply voltage) that the base circuit of the transistor opens, which may take place during the measurement. Therefore, the base is connected to the emitter through a resistor of small resistance, or, as shown by the dashed line in Fig. 9, a, with the negative pole of the source by means of a resistor Rb with a resistance of the order of hundreds of kilo-ohms. In the latter case, a bias voltage is applied to the base, which sets the operating mode of the amplifier. Then, in order to adjust the required rated current (suppose 10 μA for the above example), a trimmer resistor Rsh = (2 ... 5) Ri is connected in parallel with the meter (or in series with it).
It should be taken into account that in the absence of a measured current, the initial collector current Ic.n will flow through the meter, reaching 5-20 μA and due to the presence of an uncontrolled reverse collector current Ic.o and a current in the base resistor circuit Rb. The action of the current Ic.n can be compensated by setting the meter pointer to zero by the mechanical corrector of the device. However, it is more rational to perform an electrical zero setting before starting measurements, for example, using an auxiliary power supply element E0 and a rheostat R0 = (5 ... 10) Ri, creating a compensation current I0 in the meter circuit, equal in value, but opposite in direction to the current Iк. n. Instead of two power supplies, one can be used (Fig. 9, b) by connecting in parallel to it a voltage divider of two resistors R1 and R2 with resistances of the order of hundreds of ohms. In this case, a direct current bridge circuit is formed (see Bridge method for measuring electrical resistances), which is balanced by a change in the resistance of one of the arms (R0).
The need to complicate the original single-transistor amplifier circuit leads to the fact that the current gain
Ki = Ui / In (8)
turns out to be less than the current transfer coefficient Bst of the used transistor. Moreover, the reliable operation of a transistor microammeter can only be ensured if Ki<< Вст.
As you know, the parameters of the transistor significantly depend on the ambient temperature. A change in the latter leads to spontaneous oscillations (drift) of the reverse collector current Ic.o, which in germanium transistors increases almost 2 times for every 10 K increase in temperature. This causes a noticeable change in the current gain Ki and the input impedance of the amplifier, which can lead to a complete violation of the calibration characteristics of the device. One should also take into account the irreversible change in the parameters ("aging") of transistors observed over time, which creates the need for periodic checking and correction of the calibration characteristics of the transistor device.
If the change in current Ic.o can be compensated to some extent by setting zero before starting measurements, then special measures have to be taken to stabilize the gain Ki. So, the bias to the base (Fig. 9, b) is supplied by means of a voltage divider from resistors Rb1 and Rb2, and sometimes a thermistor with a negative temperature coefficient of resistance is used as the latter. The thermistor can be replaced with a diode D connected in parallel with the resistor Rb1. With increasing temperature, the reverse resistance of the diode decreases, which leads to such a redistribution of voltages between the electrodes of the transistor, which counteracts the increase in the collector current. Negative feedback between the collector and the base acts in the same direction, which appears due to the connection of the resistor Rb2 to the collector (and not to the supply minus). The most effective effect is exerted by negative feedback that occurs when a resistor Re is connected to the emitter circuit.
Increasing the stability of the amplifier through the use of a sufficiently deep negative feedback leads to a small ratio of Ki / Bst coefficients. Therefore, to obtain the gain Ki, equal to several tens, it is necessary to select a germanium transistor with a high current transfer coefficient for the microammeter: Vst = 120 ... 200.
In microammeters, it is possible to use silicon transistors, which, in comparison with germanium ones, have parameters that are more stable both in time and in relation to temperature effects. However, the Bst coefficient of silicon transistors is usually small. It can be increased by using a compound transistor circuit (Fig. 9, c); the latter has a current transfer coefficient Vst approximately equal to the product of the corresponding coefficients of its constituent transistors, i.e. Vst ≈ Vst1 * Vst2. However, the reverse collector current of the composite transistor is:
Ik.o ≈ Ik.o2 + Bst2 * Ik.o1
significantly exceeds the corresponding currents of its components and is subject to noticeable temperature fluctuations, which leads to the need to stabilize the amplifier mode.
The high stability of the operation of a transistor microammeter is easier to achieve when performing its amplifier according to a balanced circuit with two conventional or composite transistors specially selected for the identity of their parameters (first of all, according to the approximate equality of the coefficients Bst and currents Ik.o). A typical diagram of such a device with stabilization and correction elements is shown in Fig. 10. Since the initial collector currents of the transistors approximately to the same extent depend on the temperature and supply voltage, and they flow through the meter in opposite directions, compensating each other, the stability of the zero position of the meter arrow and the uniformity of its scale increase. The deep negative feedback provided by the resistors Re and Rb.k increases the stability of the current gain. The balanced circuit also increases the sensitivity of the microammeter, since the measured current creates potentials of different signs on the input electrodes of both transistors; As a result, the internal resistance of one transistor increases, and the other decreases, which increases the imbalance of the DC point, in the diagonal of which the I.
When setting up a balanced microammeter with a trimmer potentiometer Rk, the potential of the collectors is equalized, which is controlled by the absence of meter readings when the input terminals are short-circuited. Zero setting during operation is performed by potentiometer Rb by equalizing base currents with open input terminals. It should be borne in mind that these two adjustments are interdependent and when debugging the device, they must be repeated several times in succession.
Rice. 10. Balanced circuit of a transistor microammeter
The input resistance of the microammeter Rmka is mainly determined by the total resistance R = Rb1 + Rb2 + R6, acting between the bases of the transistors, and is approximately (0.8 ... 0.9) * R; its exact determination, as well as the nominal limiting current In, has to be carried out empirically. It is convenient to adjust the required value of the rated current using a shunt chain of resistors whose resistance must be taken into account when determining the input resistance Rmka.
The stability of the input impedance allows the expansion of the measurement limit in the direction of lowering the sensitivity using shunts. The shunt resistance required to obtain the limiting measured current Ip,
Rsh.p = Rmka * In / (Ip - In) = Rmka * Ii / (Ki * Ip - Ii) (9)
With the numerical data indicated in the diagram and the use of transistors with Bst ≈ 150, the balanced microammeter has a gain Ki ≈ 34 and, by means of a trimming resistor Rm, can be adjusted to the rated current In = 10 μA. If it is necessary to obtain a nominal current of about 1 μA, the amplifier is supplemented with a second stage, which is often performed according to the emitter follower circuit, which facilitates matching the output impedance of the amplifier with the low resistance of the I.
When checking power electrical circuits, it is often necessary to measure the current strength. To measure the magnitude of a direct current, as a rule, a resistor shunt is used, connected in series with the load, the voltage of which is proportional to the current. However, if it becomes necessary to measure large currents, then a shunt of impressive power will be required, so it is more advisable to use other measurement methods.
In this regard, I had the idea to assemble a current meter based on a Hall sensor. Its diagram is shown in the figure.
Features of the ammeter:
- Measurement of AC or DC current without electrical contact with the circuit
- TrueRMS current measurement regardless of waveform and maximum value over a period (approximately 0.5 seconds)
- Information output to the character LCD display
- Two measurement modes (up to 10A and up to 50A)
The scheme works as follows. The wire with the current is located inside the ferrite ring, thus creating a magnetic field, the magnitude of which is directly proportional to the strength of the current. A Hall sensor located in the air gap of the core converts the field induction into a voltage, and this voltage is fed to the operational amplifiers. Op amps are needed to bring the voltage levels from the sensor to the ADC input voltage range. The received data is processed by the microcontroller and displayed on the LCD display.
Preliminary calculation of the scheme
R20 * 10 * 7 ring made of N87 material was used as a core. Hall sensor - SS494B.
With the help of a needle file, a gap is machined into the ring of such a thickness that the sensor will fit there, that is, about 2 mm. At this stage, it is already possible to roughly estimate the sensitivity of the sensor to current and the maximum possible measured current.
Equivalent permeability of the core with a gap is approximately equal to the ratio of the length of the magnetic line to the value of the gap:
Then, substituting this value into the formula for calculating the induction in the core and multiplying it all by the sensitivity of the sensor, we find the dependence of the output voltage of the sensor on the current strength:
Here K B- the sensitivity of the sensor to the magnetic field induction, expressed in V / T (taken from the datasheet).
For example, in my case ls= 2 mm = 0.002 m,K B= 5 mV / Gauss = 50 V / T, whence we get:
The real sensitivity to current turned out to be equal 0.03V / A, that is, the calculation is very accurate.
According to the datasheet on SS494B, the maximum induction measured by the sensor is 420 Gauss, therefore the maximum measured current is:
Photo of the sensor in the gap:
Calculation of op-amp circuits
The ammeter has two channels: up to 10 A (23 MK output), and up to 50 A (24 MK output). Mode switching is handled by the ADC multiplexer.
An internal reference is selected as the ADC reference voltage, so the signal must be brought to the range of 0 - 2.56 V. When measuring currents of ± 10 A, the sensor voltage is 2.5 ± 0.3 V, therefore, it must be amplified and shifted so that the zero point was exactly in the middle of the ADC range. For this, op-amp IC2: A is used as a non-inverting amplifier. The voltage at its output is described by the equation:
Here, R2 means R2 and P2 connected in series, and R3 means R3 and P3, respectively, so that the expression does not look too cumbersome. To find the resistances of the resistors, write the equation twice (for currents -10A and + 10A):
We know the voltages:
Setting R4 equal to 20 kΩ, we obtain a system of two equations, where the variables are R2 and R3. The solution to the system can be easily found using mathematical packages, for example MathCAD (the calculation file is attached to the article).
The second circuit consisting of IC3: A and IC3: B is calculated in the same way. In it, the signal from the sensor first passes through the follower IC3: A, and then goes to the divider on the resistors R5, R6, P5. After the signal is attenuated, it is further biased by the op-amp IC3: B.
Description of the microcontroller
The ATmega8A microcontroller processes the signals from the op-amp and displays the results on the display. It is clocked by an internal 8 MHz oscillator. Fuses are standard, except for CKSEL. In PonyProg, they are exposed like this:
The ADC is configured to operate at 125 kHz (division ratio is 64). At the end of the ADC conversion, the interrupt handler is called. It stores the maximum current value, and also sums the squares of the currents of successive samples. As soon as the number of samples reaches 5000, the microcontroller calculates the RMS current value and displays the data on the display. Then the variables are reset and everything happens from the beginning. The diagram shows the WH0802A display, but any other display with the HD44780 controller can be used.
Microcontroller firmware, a project for CodeVision AVR and a simulation file in Proteus are attached to the article.
Setting up a schema
Setting up the device comes down to adjusting the trimming resistors. First you need to adjust the display contrast by turning P1.
Then, by switching the S1 button to the mode up to 10A, we set P2 and P3. We unscrew one of the resistors to the right as far as possible and, rotating the second resistor, we achieve zero readings of the device. We are trying to measure the current, the value of which is known exactly, while the ammeter readings should turn out to be lower than it actually is. We twist both resistors slightly to the left, so that the zero point is preserved, and again we measure the current. This time the reading should be slightly larger. We continue this until we achieve an accurate display of the current value.
Now let's switch to the mode up to 50A and configure it. With resistor P4 we set zero on the display. We measure any current and look at the readings. If the ammeter overestimates them, then turn P5 to the left; if it underestimates, then turn to the right. Again we set zero, check the readings at a given current, and so on.
Device photo
DC current measurement:
Due to insufficiently accurate calibration, the values are slightly overestimated.
Measuring alternating current with a frequency of 50 Hz, an iron is used as a load:
In theory, the rms sinusoid current is 0.707 of the maximum, but judging by the readings, this coefficient is 0.742. After checking the voltage waveform in the network, it turned out that it only resembles a sinusoid. Considering this, such readings of the device look quite reliable.
The device still has a drawback. Noises are constantly present at the sensor output. Passing through the op-amp, they get to the microcontroller, as a result of which it is impossible to achieve ideal zero (instead of zero, approximately 30-40 mA RMS is displayed). This can be corrected by increasing the capacitance C7, but then the frequency characteristics will deteriorate: at high frequencies, the readings will be underestimated.
Used sources
List of radioelements
| Designation | A type | Denomination | Quantity | Note | Score | My notebook |
|---|---|---|---|---|---|---|
| IC1 | MK AVR 8-bit | ATmega8A | 1 | DIP-28 | Into notepad | |
| IC2, IC3 | Operational amplifier | MCP6002 | 2 | SOIC-8 | Into notepad | |
| IC4 | Linear regulator | L78L05 | 1 | Into notepad | ||
| IC5 | Hall Sensor | SS494B | 1 | Into notepad | ||
| C1-C7 | Capacitor | 100 nF | 9 | K10-17b | Into notepad | |
| R1, R3, R6, R9 | Resistor | 10 kΩ | 4 | SMD 1206 | Into notepad | |
| R2 | Resistor | 12 kΩ | 1 | SMD 1206 | Into notepad | |
| R4 | Resistor | 20 kΩ | 1 | SMD 1206 | Into notepad | |
| R5 | Resistor | 6.8 k Ohm | 1 | SMD 1206 | Into notepad | |
| R7, R8 | Resistor | 100 kΩ | 2 | SMD 1206 | Into notepad | |
| P1 | Trimmer resistor | 10 kΩ | 1 | 3362P | Into notepad | |
| P2 | Trimmer resistor | 4.7 k Ohm | 1 | 3362P |
I would like to present to your attention an upgraded version for a laboratory power supply. Added the ability to disconnect the load when a certain predetermined current is exceeded. The firmware of the improved voltammeter is possible.
Digital current and voltage meter circuit
Several details have also been added to the diagram. From the controls - one button and a variable resistor with a nominal value of 10 kilo-ohms to 47 kilo-ohms. Its resistance is not critical for the circuit, and, as can be seen, it can vary over a fairly wide range. The appearance on the screen has also changed slightly. Added display of power and ampere * hours.
The trip current variable is stored in the EEPROM. Therefore, after shutdown, you will not need to configure everything again. In order to enter the current setting menu, press the button. By turning the knob of the variable resistor, it is necessary to set the current at which the relay will turn off. It is connected through a key on the transistor to the output PB5 microcontroller Atmega8.
At the moment of shutdown, the display will show that the maximum set current has been exceeded. After pressing the button, we will go back to the menu for setting the maximum current. You need to press the button again to switch to the measurement mode. To the exit PB5 the microcontroller will send log 1 and the relay will turn on. This current tracking also has its drawbacks. Protection will not be able to work instantly. It may take several tens of milliseconds to trigger. For most experimental devices, this disadvantage is not critical. This delay is not visible to humans. Everything happens at once. No new PCB was developed. Anyone who wants to repeat the device can slightly edit the printed circuit board from the previous version. The changes will not be significant.
