Topic: how to put a current and voltage meter on a power source.
It is quite convenient when an indicator is installed on the power supply, showing a constant voltage and current. When powering the load, you can always see the voltage drop, the amount of current consumed. But not all power supplies are equipped with ammeters and voltmeters. Purchased, more expensive power supplies have them, but cheap models do not. Yes, and they are not always installed in home-made power supplies. Today it is possible to purchase for little money a digital module measuring indicator of direct current and voltage (Chinese voltmeter ammeter). This module costs about 3 bucks. You can buy it by sending from China, at the nearest radio market, electronic components store.
This Chinese digital voltmeter, ammeter module itself measures direct current (up to 10, 20 amperes, depending on the model) and voltage (up to 100, 200 volts). It has a small, compact size. It can be easily mounted in any suitable housing (you need to cut the appropriate hole and just insert it there). On the back of the board, there are two trimming resistors that can be used to correct the readings of the measured values of current and voltage. The accuracy of this digital Chinese voltmeter and ammeter module is quite high - 99%. The screen has a three-character display of red (for voltage) and blue (for current) colors. This unit is powered by DC voltage from 4 to 28 volts. Consumes little current.
The installation itself, the electrical connection to the power supply circuit is quite simple. There are the following wires on the current and voltage measuring module: three thin wires (black minus and red plus power supply of the module, yellow for measuring DC voltage relative to any black), two thick wires (black minus and red plus for measuring DC current).
This Chinese ammeter, voltmeter module can be powered both from the source itself, on which we measure electrical quantities, and from an independent power supply. So, after mounting in the meter case, we solder together two black wires (thin and thick), this will be a common minus, which we solder to the minus of the power supply. We solder together thin wires of red and yellow color, connect them to the output (plus) of the power source. To the thick red wire, relative to the soldered black wires, we connect the electrical load itself (these will be the output wires of the power supply).
It is important to note that the polarity of the current wires is important for correct DC current measurement. That is, it is the thick red wire that should be the output of the power supply. Otherwise, this digital ammeter will show zeros on its display. On a conventional power supply (without a voltage regulation function), only the voltage drop can be monitored on the indicator. But on an adjustable power source it will be clearly visible what voltage you now have when it is set.
Video on this topic:
P.S. In general, connecting this digital Chinese voltmeter, ammeter module should not be difficult. The next time you use it, you will appreciate its work, you will like it. The most popular is a three-character measuring unit, although a four-character measuring unit will be a little more expensive, in which the measurement accuracy is no longer 99%, but 99.9%. These digital modules that measure direct current and voltage are of a separate type, that is, one such unit is either an ammeter or a voltmeter. They have a bigger screen.
Topic: how to put a current and voltage meter on a power source.
It is quite convenient when an indicator is installed on the power supply, showing a constant voltage and current. When powering the load, you can always see the voltage drop, the amount of current consumed. But not all power supplies are equipped with ammeters and voltmeters. Purchased, more expensive power supplies have them, but cheap models do not. Yes, and they are not always installed in home-made power supplies. Today it is possible to purchase for little money a digital module measuring indicator of direct current and voltage (Chinese voltmeter ammeter). This module costs about 3 bucks. You can buy it by sending from China, at the nearest radio market, electronic components store.
This Chinese digital voltmeter, ammeter module itself measures direct current (up to 10, 20 amperes, depending on the model) and voltage (up to 100, 200 volts). It has a small, compact size. It can be easily mounted in any suitable housing (you need to cut the appropriate hole and just insert it there). On the back of the board, there are two trimming resistors that can be used to correct the readings of the measured values of current and voltage. The accuracy of this digital Chinese voltmeter and ammeter module is quite high - 99%. The screen has a three-character display of red (for voltage) and blue (for current) colors. This unit is powered by DC voltage from 4 to 28 volts. Consumes little current.
The installation itself, the electrical connection to the power supply circuit is quite simple. There are the following wires on the current and voltage measuring module: three thin wires (black minus and red plus power supply of the module, yellow for measuring DC voltage relative to any black), two thick wires (black minus and red plus for measuring DC current).
This Chinese ammeter, voltmeter module can be powered both from the source itself, on which we measure electrical quantities, and from an independent power supply. So, after mounting in the meter case, we solder together two black wires (thin and thick), this will be a common minus, which we solder to the minus of the power supply. We solder together thin wires of red and yellow color, connect them to the output (plus) of the power source. To the thick red wire, relative to the soldered black wires, we connect the electrical load itself (these will be the output wires of the power supply).
It is important to note that the polarity of the current wires is important for correct DC current measurement. That is, it is the thick red wire that should be the output of the power supply. Otherwise, this digital ammeter will show zeros on its display. On a conventional power supply (without a voltage regulation function), only the voltage drop can be monitored on the indicator. But on an adjustable power source it will be clearly visible what voltage you now have when it is set.
Video on this topic:
P.S. In general, connecting this digital Chinese voltmeter, ammeter module should not be difficult. The next time you use it, you will appreciate its work, you will like it. The most popular is a three-character measuring unit, although a four-character measuring unit will be a little more expensive, in which the measurement accuracy is no longer 99%, but 99.9%. These digital modules that measure direct current and voltage are of a separate type, that is, one such unit is either an ammeter or a voltmeter. They have a bigger screen.
Ammeter voltmeter from China. Laboratory work.
Inexpensive digital voltmeters using three-digit digital LED indicators are available from online stores in China.
Voltmeters came across two sizes: 48 x 30 x 22mm and 36.6 x 14.8 x12mm.
The larger one is made in a black plastic case and simply fits into the window cut out in the front panel of the power supply. A small voltmeter is unpackaged and is attached to the "lugs" of the printed circuit board.
The devices are powered by direct current at a voltage of 4 to 30 V (through the built-in integral stabilizer) and measure direct voltage up to 30 or 99.9 V.
Detailed characteristics of voltmeters are posted on the websites of sellers. One of the sites provides a schematic diagram of one of these voltmeters.
The voltmeter is assembled on the STM8 microcontroller. In the above diagram, the input voltage divider consists of series-connected resistors R1 and R2 (390 kOhm and 10 kOhm). It is easy to calculate that when 1 V is applied to the input of the divider, a voltage of 0.025 V is applied to the measuring input of the processor. (Divider current I=U:R = 1: (390k+10k)=0.0025 mA; voltage drop across R2=I*R \u003d 0.0025 mA * 10k \u003d 0.025V).
If you put a measuring resistor of 0.025 Ohm in the output current circuit in the power supply, then when a current of 1A flows through it, a voltage of 0.025 V will drop on the measuring resistor. And if this voltage is applied to R2, then the voltmeter indicator will show the unit (1 Ampere). Thus, the voltmeter turned into an ammeter.
You can install a toggle switch and switch the meter to the voltmeter or ammeter mode according to the diagram below. Three circuits have to be switched:
Standard voltmeter input;
Additional meter input (measurement input of the processor);
The common wire of the voltmeter.
In order to use a two-pole toggle switch as a switch, I had to go for some trick - add "your" resistor R ext 330 kΩ in the input voltage divider circuit. The regular input of the voltmeter is not used and is not switched.
The diagram shows the inclusion in the "negative" circuit of the power source. So R measurements is switched on in pulse (computer) power supplies (when they are converted into various power supplies used for amateur radio purposes), where this measuring resistor is simultaneously used as a current sensor in the output current control circuit.
The “-Upit” output shown in the diagram is not connected anywhere, the device receives “minus” power through the measuring circuit switch. Since the “negative” power wire of the voltmeter is switched at the same time, at the moment of switching, the indicators of the device go out for a short time.
It should be noted that when the meter is built into a laboratory power supply, the device will start working only when the minimum voltage at its output is about 4 volts. For a battery charger, this is irrelevant. For a laboratory power supply, the meter will have to be powered from an independent power source that is not galvanically connected to the power supply.
Solutions can be different - rectifiers on a separate winding on the transformer, on a separate small transformer from the so-called "adapters" of power supply, a phone charger board or just a suitable battery.
In principle, the measuring resistor R measurements can also be included in the "positive" circuit of the power supply, taking into account the fact that in this case the voltmeter-ammeter will also have to be "powered" from an independent power source that is galvanically not connected to the power supply (otherwise, the entire potential of the output voltage and the processor will fail).
When the output terminals of the power source are short-circuited, when the battery is connected to the charger (if the rectifier is assembled in a bridge circuit), a large short-circuit current flows through the measuring resistor until the protective fuse burns out and a voltage pulse is generated on the resistor, which can damage the processor.
At first glance, two processor protection solutions are visible.
The first ("organizational" and the simplest) - instead of a toggle switch that switches the meter to the "voltmeter" - "ammeter" modes, install a non-fixed button and measure the current when the button is pressed. Since "polarity reversal" and a short circuit occur most often when the load is connected or disconnected and the operator's hands are busy with this process, the meter's switch button will be in the "voltmeter" state and the device will not suffer.
The second is schematic. Install a high-speed electronic device parallel to the measuring resistor (measuring input), which protects against exceeding the permissible voltage at the input of the measuring input of the processor, for example, a suppressor or a zener diode.
I came across voltmeters with a divider of 330 kOhm and 10 kOhm. Since I already used a standard 5-watt 0.1 Ohm resistor in a ceramic case as a measuring resistor in the circuit of a converted computer power supply, the voltage drop across it was too large to feed to the processor. I had to connect a multi-turn small-sized potentiometer in parallel with the measuring resistor (“it turned up under my arm” at 100 Ohms) and set the readings on the indicator using the “exemplary” tester.
This method can also be used in the case of using a homemade uncalibrated measuring resistor.
On sale there are voltmeters positioned by the manufacturer as "voltmeters for embedding on the car panel to measure the voltage of the on-board network" with an upper measurable limit of 24V. They have only two outputs (black "minus" and red "plus"). In these voltmeters, the input of the divider is connected to the "plus" of the power supply by a printed conductor, which is easy to cut. In such a voltmeter, the divider costs 91 kOhm and 10 kOhm. That is, a 5-watt resistor in a ceramic case with a nominal value of 0.1 ohm is well suited as a measuring resistor.
Voltmeters of various manufacturers differ in circuit diagrams and processors used, but the principle of their use as an ammeter remains the same.
Below in the text are photographs of voltmeter boards that fell into the hands of the author. They indicate the location of the input divider resistors and the meter's input location.
When designing battery chargers and various power supplies, many radio amateurs use ready-made Chinese-made voltmeters-ammeters, which can be easily bought on the Internet, for example, on the Aliexpress website. Moreover, the cost of such ready-made devices is very attractive, and many suppliers, in addition to everything, carry out free delivery of goods to the buyer. Having found the most advantageous offer, we ordered a pair of WR-005 devices for testing, designed to measure voltage up to 100 Volts and current up to 10 Amperes. The order came, everything is in order with the blocks, there is no mechanical damage, but there was no passport or instructions describing how to connect the device. This was the reason for writing this article, because, most likely, we are not the only ones who are faced with issues of connecting the WR-005 to measurement circuits.
Similar measuring devices can be designed for other measurement parameters, but in any case, you will have two connectors on the board:
● The first connector has two thin wires, usually red and black. They serve to supply voltage to the measuring circuit. The supply voltage has a very wide range, you can apply from 4 to 30 volts, the red wire is positive, the black wire is negative. When power is applied to the circuit, the indicator will glow.
● The second connector is three-wire, the wires are thick, designed to connect the device to measuring circuits. But let's deal with the colors of the wires.
Apparently, indicators were produced earlier in which thick wires were black, red and yellow, so you can find this picture on the Internet:
In our case, this connector has blue, black and red wires, and the black wire is in the connector in the middle, so we decided to double-check them again.
As it turned out, globally nothing has changed:
● The black wire, as in the previous version, is a common wire (COM);
● Red wire - voltage measurement;
● Blue wire - current measurement.
For those who do not quite understand: the black thick wire is connected to the minus of the source, the red to the plus (the voltmeter starts to show), the blue thick wire is connected to the load, and from the second end of the load it goes to the plus of the source (the ammeter shows).
About the shunt. In devices up to 10 amperes, the shunt is built-in (soldered directly into the board), over 10 amperes, as a rule, an external shunt should be included, see the pictures below:
Our version of the device with a built-in shunt:
The external shunt looks like this:
Even after a correct connection, there is no guarantee that the readings of the voltmeter and ammeter will be correct, so it is worth checking them using, for example, an external multimeter. If necessary, you can correct the readings using trimming resistors located on the board of the WR-005 device.
The microcircuit on which the device is assembled does not have any identification marks, but the circuit diagram is as follows:
In conclusion, I would like to say that after connecting and testing the device, it showed itself on the positive side, the build quality is not bad, the reading errors correspond to those declared by the supplier, that is, the voltage error is 0.1 Volt, the current error is 0.01 Ampere, the current consumption of the measuring circuit does not exceed 20 mA. Any electronics is prone to fail over time, so how long this voltmeter-ammeter will serve us - time will tell. But, in principle, for such money, we believe that the WR-005 is a worthy purchase with quick installation and connection in devices that need to output digital indication of current and voltage parameters.
If anyone knows the brand of the microcircuit used in the device circuit, please write in the comments.
For many purposes, it is often necessary to use a voltammeter. Be it a laboratory power supply or a charger. This article will focus on a fairly cheap, but very common Chinese voltammeter marked dsn-vc288. This rather tiny device can measure voltage from 0 to 100 Volts and current in the range from 0 to 10 Amps. The resolution (step) for voltage is 0.1 Volt for current - 0.01 Ampere.
The device is connected simply: a three-pin connector is the power supply and the supply of the measured voltage. The power supply is in the range from 5 to 36 Volts, and the measured voltage is actually the one that we will measure. The second two-pin connector - designed to measure current is included in the break of the measured circuit. Also on the board are two variable resistors with the designations I_ADJ and V_ADJ. This is the calibration of current and voltage, respectively.
The first inclusion of the voltammeter dsn-vc288 revealed some problems. It measures voltage perfectly, but not so much current. The measurements are unstable, the numbers are constantly jumping, and the worst thing is non-linearity (we calibrate at a current of 100 mA, and at a current of 1 A, the readings drift away and the farther the more). The first suspicion fell on the shunt. Instead, I took several resistors of size 2512 and a resistance of 0.02 Ohm, and began to solder them one by one in parallel to select the desired resistance (by the way, this method can reduce the upper current measurement limit, but increase accuracy at low currents).
But such a replacement of the shunt did not give the desired effect - the non-linearity persisted. And then, on the Internet, I discovered another revision of this voltammeter, which consisted in installing an additional jumper (the photo shows where and where it comes from). You need to make it with a thicker wire.
I have a wire with a cross section of 0.75 mm, folded in half and covered with heat shrink. After that, the current readings of the voltammeter became stable and linear. Using a trimmer resistor, I calibrated the current, then measured its resulting resistance and replaced it with an assembly of two fixed resistors. This was done so that in the future it would not be necessary to calibrate the device again if the setting floats.
After such improvements, I assembled a dsn-vc288 voltammeter. Now the device is ready for use. 
