Chinese voltammeter connection diagram. We connect a Chinese digital voltammeter. Which digital voltmeters are the most reliable?

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 (error 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)

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 instruction

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 20 kOhm trimming resistor.

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, however, 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 a 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.

Currently, from all kinds of electronic devices that are taken out of service for one reason or another, various power supplies remain, both switching and those assembled on step-down transformers. Their use by beginning radio amateurs as a laboratory power supply is complicated by the fact that they have a certain stabilized voltage at the output. However, inexpensive miniature voltage and current regulator modules that have appeared on the market make it possible, together with the same miniature digital voltmeters and ammeters, to successfully convert them into laboratory power supplies, sometimes even without making a new, more capacious case.

What remained was the power supply, which provided a stabilized voltage of 5V at the output. Naturally, there was a desire to use it more intensively in my amateur radio needs. Moreover, the voltage adjustment of 5.5 volts to the maximum, which could be done using a trimming resistor, was already available. And the output current easily reached almost one ampere.

To achieve what you want, you need to install a measuring device on the front panel - a voltammeter, a voltage regulator (a variable resistor instead of a trimmer), a switch for the type of measurement (voltmeter - ammeter) and connecting terminals.

It turned out to be not difficult at all. A Chinese-made voltmeter modified using this method to be able to measure current too, for smoother and more accurate settings, a PK-1 push-button switch and two types of connecting terminals - standard for power supplies and an RCA “tulip” connector - which has proven to be very convenient in this quality.

Block connection diagram

The connection diagram for additionally introduced devices is not at all complicated, and its implementation takes even less time than drawing. It is better to make the power supply of the voltammeter separate, through an integrated 5-volt stabilizer, alternatively from suitable batteries or accumulators, then the output voltage indication will start from zero. The switch for the type of measured quantity is PK-1; the necessary additional electronic components of the circuit are installed on it. A fuse is required.

Everything fit, except that we had to slightly file the edge of the printed circuit board and the module with the rectifier and voltage stabilizer from the additional winding of the standard transformer, place it in an insulated “box” (it’s orange) and give it a place inside the radiator (it doesn’t heat up).

Adjustment of the voltmeter and ammeter readings went without complications. The readings of the voltmeter are adjusted by the SMD trimming resistor located on its board, and the ammeter by changing the resistance of the measuring resistor, indicated in the diagram as “R measuring resistor 0.2 Ohm”. Current readings are made in amperes. The readings relative to the standard meter are set quite accurately, but there is a nuance that is not yet fully understood: I set the voltmeter readings and they coincide with the standard ones perfectly, but after setting the ammeter readings, the voltmeter readings are somewhat off. And vice versa. Therefore, we had to choose whose readings would correspond, and whose readings would have to be corrected.

This is how the power supply turned out in the end: with a display of adjustable output voltage, with the ability to find out the current current consumption (you need to press the non-fixed button of the PC-1 switch) and two types of connecting terminals. A novice radio amateur should not assemble his first power supply from scratch; the best option is to modify a ready-made one to suit his needs. Author Babay iz Barnaula.

Discuss the article CONNECTION DIAGRAM FOR A DIGITAL VOLTAMMETER

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 (error 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)

For many purposes it is often necessary to use a voltammeter. Whether it's a laboratory power supply or a charger. In this article we will talk about a fairly cheap, but very common Chinese voltammeter marked dsn-vc288. This rather miniature 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 ranges from 5 to 36 Volts, and the measured voltage is actually the one that we will measure. The second two-pin connector is designed to measure current and is connected to the open circuit of the measured circuit. There are also two variable resistors on the board with the designations I_ADJ and V_ADJ. This is the current and voltage calibration respectively.

The first turn on of the dsn-vc288 voltammeter 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, but at a current of 1 A the readings float away and the further the further the further). First of all, suspicions fell on the shunt. Instead, I took several resistors of standard 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 limit of current measurement, but increase the accuracy at low currents).

But such a replacement of the shunt did not give the desired effect - the nonlinearity persisted. And then, on the Internet, I discovered another modification to this voltammeter, which consisted of installing an additional jumper (the photo shows where it goes and where it comes from). It needs to be done 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 this, 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 there would be no need to calibrate the device again if the setting floated.

After such modifications, I assembled a dsn-vc288 voltammeter. The device is now ready for use.