I received from AliExpress a couple of electronic built-in voltmeters model V20D-2P-1.1 (DC voltage measurement), the price is 91 cents each. In principle, you can now find it cheaper (if you look hard enough), but it’s not a fact that this will not be to the detriment of the build quality of the device. Here are its characteristics:

• operating range 2.5 V - 30 V
• glow color red
• overall size 23 * 15 * 10 mm
• does not require additional power (two-wire version)
• there is a possibility of adjustment
• refresh rate: about 500ms/time
• Promised measurement accuracy: 1% (+/-1 digit)

And everything would be fine, put it in place and use it, but I came across information about the possibility of improving them - adding a current measurement function.

Digital Chinese voltmeter

I prepared everything I needed: a two-pole toggle switch, output resistors - one MLT-1 for 130 kOhm and a second wire resistor for 0.08 Ohm (made from a nichrome spiral with a diameter of 0.7 mm). And the whole evening, according to the found circuit and instructions for its implementation, I connected this equipment with wires to a voltmeter. To no avail. Either there was not enough insight in understanding what was left unsaid and incompletely drawn in the material found, or there were differences in the schemes. The voltmeter didn't work at all.

Connecting the digital voltmeter module

I had to unsolder the indicator and study the circuit. What was needed here was not a small soldering iron, but a tiny one, so it took quite a bit of fiddling. But over the next five minutes, when the entire scheme became available for review, I understood everything. In principle, I knew that this was where I needed to start, but I really wanted to solve the issue “easy.”

V-meter modification scheme

Refinement scheme: ammeter to voltmeter

This is how this scheme for connecting additional electronic components with those already existing in the voltmeter circuit was born. The standard resistor of the circuit marked in blue must be removed. I’ll say right away that I found differences from other circuits given on the Internet, for example, the connection of a tuning resistor. I didn’t redraw the entire voltmeter circuit (I’m not going to repeat it), I only drew the part that was necessary for modification. I think it’s obvious that the voltmeter’s power supply needs to be separate; after all, the starting point in the readings should start from zero. Later it turned out that power from a battery or accumulator will not work, because the current consumption of the voltmeter at a voltage of 5 volts is 30 mA.

Board - Chinese voltmeter module

After assembling the voltmeter, I got down to the essence of the action. I won’t split hairs, I’ll just show and tell you what to connect with what to make it work.

Step by step instructions

So, action one– an SMD resistor with a resistance of 130 kOhm is removed from the circuit, standing at the input of the positive power wire, between the diode and the trimming resistor 20 kOhm.

We connect the resistor to the voltmeter-ammeter

Second. On the freed contact, on the side of the trimmer, a wire of the desired length is soldered (for testing, conveniently 150 mm and preferably red)

Unsolder the SMD resistor

Third. A second wire (for example, blue) is soldered to the track connecting the 12 kOhm resistor and the capacitor from the “ground” side.

Testing a new circuit

Now, according to the diagram and this photo, we “hang” an addition to the voltmeter: a toggle switch, a fuse and two resistors. The main thing here is to correctly solder the newly installed red and blue wires, but not only them.

We convert the voltmeter block into an A-meter

But here there are more wires, although everything is simple:

» — a pair of connecting wires connects the e/motor
« separate power supply for voltmeter"- battery with two more wires
« power supply output"- a couple more wires

After applying power to the voltmeter, “0.01” was immediately displayed; after applying power to the electric motor, the meter in voltmeter mode showed a voltage at the output of the power supply equal to 7 volts, then switched to ammeter mode. The switching was performed when the power supply to the load was turned off. In the future, instead of a toggle switch, I will install a button without locking, it will be safer for the circuit and more convenient to use. I was pleased that everything worked on the first try. However, the ammeter readings differed from the multimeter readings by more than 7 times.

Chinese voltmeter - ammeter after modification

Here it turned out that the wirewound resistor, instead of the recommended resistance of 0.08 Ohm, has 0.8 Ohm. I made a mistake in the measurements during its manufacture in the counting of zeros. I got out of the situation like this: the crocodile with the negative wire from the load (both black) moved along a straightened nichrome spiral towards the input from the power supply, the moment when the readings of the multimeter and the now modified ampere-voltmeter coincided and became the moment of truth. The resistance of the involved section of the nichrome wire was 0.21 Ohm (measured with a multimeter attachment at the “2 Ohm” limit). So it didn’t even turn out bad that instead of 0.08 the resistor turned out to be 0.8 Ohm. Here, no matter how you count, according to the formulas, you still have to adjust. For clarity, I recorded the result of my efforts on video.

Video

I consider the purchase of these voltmeters a success, but it’s just a pity that their current price in that store has increased significantly, almost 3 dollars apiece. Author Babay iz Barnaula.

Do-it-yourselfers, designing, developing and implementing a variety of charger or power supply circuits, are constantly faced with an important factor - visual monitoring of the output voltage and current consumption. Here Aliexpress very often lends a helping hand, promptly supplying Chinese digital measuring instruments. In particular: a digital ampere-voltmeter is a very simple device, affordable and displays quite accurate information data.

But for beginners, commissioning (connecting an ampere-voltmeter to the circuit) can be a problematic task, since the measuring device comes without documentation and not everyone can quickly connect the color-coded wires.

An image of one of the most popular voltammeters among homemade people is posted below,

This is a 100 volt/10 amp ampere-voltmeter and comes with a built-in shunt. Many radio amateurs quite often purchase such measuring instruments for their homemade products. Digital device can be powered from separate sources,

and from one operated and measured voltage source. But there is a small nuance hidden here; the condition must be met - the voltage of the power source used was within 4.5-30 V.

For DIYers who still don’t quite understand: connect the thick black wire to the minus of the power supply, the thick red wire to the plus of the power supply (the voltmeter scale readings will light up),

We connect a thick blue wire to the load, the second end from the load goes to the plus of the power supply (the ammeter scale readings will light up).

A miniature Chinese voltmeter can simplify the process of measuring voltage and the amount of current consumed on a power supply or homemade charger. Its cost rarely exceeds 200 rubles, and if you order it from China through affiliate programs, you can also get a significant discount.

To charger

Those who like to design their own chargers will appreciate the ability to monitor the volts and amperes of the network, without the help of bulky portable devices. This will also appeal to those who work on expensive equipment, the operation of which can be adversely affected by regular drops in network voltage.

Using a Chinese ampere-voltmeter, which is no larger than a box of matches, you can easily monitor the condition electrical network. One of the tangible problems that new electricians have may be the language barrier and wire markings that differ from the standard ones. Not everyone will immediately understand which wire needs to be connected where, and the instructions are usually only in Chinese.

100 V/10 A devices are very popular among independent designers. It is also desirable that the device have a shunt to finalize the connection process. A tangible advantage of this device is that it can be connected to a power source charger or to a separate battery.

*The power supply voltage of the ammeter and voltmeter must be in the range from 4.5 to 30 V.

The connection diagram is as follows:

• The black wire is negative. It also needs to be connected to minus.
• The red wire, which should be thicker than the black one, is a plus; it must accordingly be connected to the power source.
• The blue wire connects the load to the network.

If everything has been connected correctly, two scales should light up on the display.

To power supply

Power supplies play an important role in leveling the network readings to the desired state. If not operated correctly, they can cause serious damage to expensive equipment by causing overheating. In order to avoid problems during their operation, and especially in cases where the power supply is made manually, it is advisable to use an inexpensive ammeter and voltmeter.

From China you can order the most different models, but for standard devices operating from home network those that measure current from zero to 20 A and voltage up to 220 V are suitable. Almost all of them are small-sized and can be installed in small power supply cases.

Most devices can be adjusted using built-in resistors. In addition, they have high accuracy, almost 99%. The display displays six positions, three each for voltage and current. They can be powered either from a separate or built-in source.

To connect the voltmeter you need to understand the wires, there are five of them:

• Three thin ones. Black minus, red plus, yellow to measure the difference.
• Two fat ones. Red plus, black minus.

The first three cords are most often combined for convenience. The connection can be made through a special socket connector, or using soldering.

*A connection by soldering is more reliable; with minor vibrations, the socket mount of the device may become loose.

Step-by-step connection:

1. It is necessary to decide from which power source the device will operate, separate or built-in.
2. The black wires are connected and soldered to the minus of the power supply. Thus, a general minus is created.
3. In the same way, you need to connect the thin red and yellow contacts. They are connected to the power contact.
4. The remaining red pin will connect to the electrical load.

At incorrect connection the instrument display will show zero values. In order for the measurements to be as close as possible to the actual ones, it is necessary to correctly observe the polarity of the supply contacts. Only connecting a thick red wire to the load will give an acceptable result.

Pay attention! Accurate voltage values ​​can only be obtained with a regulated power supply. In other cases, the display will only show the voltage drop.

A popular voltmeter model that is often used by radio amateurs. Has the following characteristics:

• Operating voltage DC from 4.5 to 30 V.
• Power consumption less than 20 mA.
• The display is two-color red and blue. Resolution 0.28 inches.
• Performs measurements in the range 0 – 100 V, 0 – 10 A.
• The lower limit is 0.1 V and 0.01 A.
• Error 1%.
• Temperature operating conditions from -15 to 75 degrees Celsius.

Connection

Using a voltmeter, you can measure the current voltage in the power supply network. To do this, you need the following:

• Connect the thick black wire to the negative of the power supply.
• Red connects to the load, and then to the power.

This connection diagram does not provide for the use of a thin black contact.

If a third-party power source is used, the connection will be as follows:

• Thick cords are connected in the same way as in the previous example.
• Thin red connects to the plus of a third-party source.
• Black with a minus.
• Yellow with source plus.

This voltmeter and ammeter is also convenient because it is sold in an already calibrated state. But even if inaccuracies in its operation were noticed, they can be corrected using two tuning resistors on the rear panel of the device.

Which digital voltmeters are the most reliable?

The electrical equipment market is crowded with manufacturers who provide a wide variety of choices. However, not every device brings positive emotions from use. With a large number of products, it is not always possible to find a reliable and inexpensive copy.

Tested and reliable voltmeters include:

• TK 1382. Inexpensive Chinese, the average price of which rarely rises above 300 rubles. Equipped with tuning resistors. Performs measurements in the ranges 0-100 Volts, 0-10 Amps.
• YB27VA. Almost a twin of the previous voltmeter, it differs in the marking of the wires and at a reduced price.
• BY42A. It is more expensive than previous models, but also has an increased upper measurement limit of 200 V.

These are the most popular representatives of this type voltmeters, which can be freely purchased for conversion on the radio market or ordered via the Internet.

Calibration of Chinese voltmeter ammeter

Over time, any equipment wears out. Since the operation of measuring instruments is affected not only by their own faults, but also by faults in the connected devices, sometimes it is necessary to make adjustments.

Most models have special resistors on their housing. By rotating them, you can change the zero values.

All measuring instruments have a measurement error, which is indicated in the documentation.

Conclusion

Including inexpensive voltmeters in the circuit avoids problems with inappropriate network voltage. For a small fee, you can find out whether the equipment works in suitable conditions. To connect them, you need to know the markings of all wires and the location of the plus and minus of the energy source.

For my next project (converting an ATX 580W power supply into a laboratory one), I bought the above-mentioned indicator. It was not immediately and at the right time that it became clear that its power input was galvanically connected to the minus input of the shunt. This introduces a noticeable error when the indicator is powered from the same source from which the current is measured (the error is up to an ampere with my 50A shunt!). It was possible, of course, to install another duty station and power the indicator from it, but it seemed too bold to me and I decided to hack the indicator itself.

By searching on the Internet I found its twin brother YB27VA and its typical circuit. I’ll say right away that the circuit of my device is slightly different. The essence of the modification is to decouple the differential input of the ad8605 operational amplifier (labeled as B3A) from the common power wire. To remake, you will need basic reverse engineering skills (to make sure that the circuit is the same), soldering of small parts and knowledge of Ohm's law :)

Scheme before modification:

Scheme after:

The cut paths are marked in red. I decided to abandon resistor R6, since it seems that it is only needed so that the ammeter shows “0” when the shunt is disconnected. Also, transferring the ad8605 power supply (2 legs) is not necessary (judging by the tests in the simulator).

The second modification solves the problem associated with the fact that the indicator does not “see” the first ~180 mA of current, that is, when 1A is applied to the shunt, the device shows 0.8A, if 0.2 is applied, then zero, etc. This is due to the input bias of the op amp and ADC. It can be calculated by knowing the resistance of the shunt and the amount by which the device “lies”. I got 270 µV at the input of the op-amp. This bias can be easily created artificially by adding one resistor to the circuit, as a result the device will start measuring from zero.

In my case, it was necessary to add a 1140 kOhm resistor from the 3V integrated stabilizer to the “+” input of the op-amp. This resistor, together with R7 and the shunt, forms a divider that sets the initial bias.

The composite resistor turned out to be exactly as much as needed due to the error of one of them :)

As a result, it now measures starting from 50mA, up to 50A with a minimum step of approximately 20mA (0 also shows). Linearity also does not disappoint, but sometimes it misses one, for example, it jumps from 0.12 to 0.14.

The achieved accuracy pleasantly surprised me; it turned out to be a real measuring device that can be used in a laboratory power supply as the main indicator. Which you can even trust :) (this applies, at least, to the current). It is not clear why the Chinese decided to save on a couple of cheap parts. Their cost is clearly an order of magnitude lower than other components, the same ad8605, for example. Use good equipment :)

More photos with measurement results:

P.S. I was about to publish an article, but I decided to check - how are things going with tension? It turned out that the situation was also not good - the device was lying at 0.1V, and this could not be fixed elegantly, because the lower resistor was a tuning resistor. But I still soldered a 20 MΩ resistor there and the result suited me)

Prelude

While somehow exploring the vast expanses of the Internet for Chinese utilities, I came across a digital voltmeter module:

The Chinese rolled out the following performance characteristics: 3-digit red color display; Voltage: 3.2~30V; Working temperature: -10~65"C. Application: Voltage testing.

It didn’t quite fit into my power supply (the readings are not from zero - but this is the price to pay for the power from the circuit being measured), but it’s inexpensive.
I decided to take it and figure it out on the spot.

Voltmeter module diagram

In fact, the module turned out to be not so bad. I unsoldered the indicator, drew a diagram (the numbering of parts is shown conventionally):

Unfortunately, the chip remained unidentified - there are no markings. Perhaps it's some kind of microcontroller. The value of capacitor C3 is unknown; I did not measure it. C2 - supposedly 0.1 microns, I didn’t solder it either.

File in place...

And now about the modifications that are necessary to bring this “show meter” to fruition.

1. In order for it to start measuring voltage less than 3 Volts, you need to unsolder the jumper resistor R1 and apply a voltage of 5-12V from an external source to its right (according to the diagram) contact pad (higher is possible, but not advisable - the DA1 stabilizer gets very hot). Apply the minus of the external source to the common wire of the circuit. Apply the measured voltage to the standard wire (which was originally soldered by the Chinese).

2. After modification according to claim 1, the range of the measured voltage increases to 99.9V (previously it was limited by the maximum input voltage of the DA1 stabilizer - 30V). The input divider ratio is about 33, which gives us a maximum of 3 volts at the DD1 input at 99.9V at the divider input. I supplied a maximum of 56V - I don’t have any more, nothing burned :-), but the error also increased.

4. To move or completely turn off the point, you need to unsolder the R13 10 kOhm CHIP resistor, which is located next to the transistor, and instead solder a regular 10 kOhm 0.125 W resistor between the contact pad farthest from the trimming CHIP resistor and the corresponding control segment pin DD1 - 8, 9 or 10.
Normally, the dot lights up at the middle digit and the base of transistor VT1 is connected to the pin via a 10kOhm CHIP. 9 DD1.

The current consumed by the voltmeter was about 15 mA and varied depending on the number of illuminated segments.
After the described modification, all this current will be consumed from an external power source, without loading the measured circuit.

Total

And finally, a few more photos of the voltmeter.

Factory condition

With desoldered indicator, front view

With desoldered indicator, rear view

The indicator is tinted with automotive tint film (20%) to reduce brightness and improve the visibility of the indicator in the light.
I highly recommend tinting it. You will be happy to be given scraps of tinting film for free at any auto repair shop that does tinting.

There are also other modifications of this module on the Internet, but the essence of the modifications does not change - if you come across the wrong module, simply adjust the circuit diagram on the board by unsoldering the indicator or ringing the circuits with a tester and off you go!

Connection