Prelude
While somehow studying the vast expanses of the Internet for Chinese usefulness, I came across a digital voltmeter module:The Chinese "rolled out" these performance characteristics: 3-digit red color display; Voltage: 3.2~30V; Working temperature: -10~65"C. Application: Voltage testing.
It didn’t quite fit me in the power supply (the readings are not from zero - but this is a retribution for the power from the circuit being measured), but it’s inexpensive.
I decided to take it and sort it out on the spot.
Schematic diagram of the voltmeter module
In fact, the module was not so bad. I soldered the indicator, drew a diagram (the numbering of parts is shown conditionally):Unfortunately, the chip remained unidentified - there is no marking. Maybe it's some kind of microcontroller. The value of the capacitor C3 is unknown, I did not begin to measure it. C2 - presumably 0.1mk, also did not solder.
File in place...
And now about the improvements that are needed to bring this "display" to mind.
1. In order for it to start measuring a voltage of 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 undesirable - the DA1 stabilizer is 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 the revision according to item 1, the measured voltage range increases to 99.9V (previously it was limited by the maximum input voltage of the DA1 stabilizer - 30V). The division factor of the input divider is about 33, which gives us a maximum of 3 volts at the input of DD1 at 99.9V at the input of the divider. I applied a maximum of 56V - I don’t have any more, nothing burned out :-), but the error also increased.
4. To move or completely turn off the point, you need to solder the R13 10kΩ chip resistor, which is located next to the transistor, and instead solder a regular 10kΩ 0.125W resistor between the pad farthest from the tuning chip resistor and the corresponding control segment output DD1 - 8, 9 or 10.
Normally, the point is illuminated at the middle digit and the base of the transistor VT1, respectively, is connected to the pin through a 10kΩ CHIP. 9DD1.
The current consumed by the voltmeter was about 15mA and varied depending on the number of illuminated segments.
After the described alteration, all this current will be consumed from an external power source, without loading the measured circuit.
Total
And in conclusion, a few more photos of the voltmeter.
factory condition
With soldered indicator, front view
With soldered indicator, rear view
I received a couple of electronic built-in voltmeters from AliExpress model V20D-2P-1.1 (measurement of direct voltage), the issue price is 91 cents a piece. In principle, you can now find it cheaper (if you search well), but it’s not a fact that this will not be at the expense 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)
- it is possible to adjust
- update rate: about 500ms/time
- promised measurement accuracy: 1% (+/-1 digit)
And everything would be fine, I put it in place and used it, but I came across information about the possibility of their refinement - adding a current measurement function.
Digital Chinese Voltmeter I prepared everything you need: a two-pole toggle switch, output resistors - one MLT-1 for 130 kOhm and the second wire for 0.08 Ohm (made from a nichrome spiral with a diameter of 0.7 mm). And for the whole evening, according to the scheme found and the manual for its implementation, he connected this economy with wires to a voltmeter. To no avail. Either there was not enough ingenuity in understanding the unsaid and underdrawn in the found material, or there were differences in the schemes. The voltmeter didn't work at all.
We connect the digital voltmeter module I had to solder the indicator and study the circuit. Here it was already required not a small soldering iron, but a tiddly one, so that it tinkered fairly. But over the next five minutes, when the whole scheme became available for review, I understood everything. In principle, I knew that I should start with this, but I really wanted to solve the issue “easily”.
V-meter refinement scheme
Refinement scheme: ammeter to voltmeter So this scheme was born for connecting additional electronic components with those already existing in the voltmeter circuit. The standard circuit resistor marked in blue must be removed. I will 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 drew only the part that is necessary for refinement. I consider it obvious that the power supply of the voltmeter should be done separately, after all, the starting point in the readings should start from zero. Later it turned out that power from a battery or accumulator would not work, because the current consumption of a voltmeter at a voltage of 5 volts is 30 mA.
Board - Chinese voltmeter module After assembling the voltmeter, he took up the essence of the action. I won’t be wise, I’ll just show and tell you what to combine with what to make it work.
Step-by-step instruction
so, act one- an SMD resistor with a resistance of 130 kOhm is soldered from the circuit, standing at the input of the positive power wire, between the diode and the trimming resistor of 20 kOhm.
We connect the resistor to the voltmeter-ammeter Second. On the released contact, a wire of the desired length is soldered from the side of the trimmer (it is convenient for the sample to be 150 mm and preferably red)
Solder SMD resistor Third. On the track connecting the 12 kΩ resistor and the capacitor, a second wire (for example, blue) is soldered from the "ground" side.
Testing a new circuit
Now, according to the diagram and this photo, we “hang” an addition on the voltmeter: a toggle switch, a fuse and two resistors. The main thing here is to properly solder the newly installed red and blue wires, however, not only them.
We convert the block voltmeter into an A-meter And here there are more wires, although everything is simple:
» - an electric motor is connected with a pair of connecting wires« separate power supply for the voltmeter"- battery with two more wires
« power supply output"- a couple more wires
After power was supplied to the voltmeter, “0.01” immediately appeared, after power was supplied 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. Switching was performed when the power supply to the load was turned off. In the future, instead of a toggle switch, I will put a button without fixation, it is safer for the circuit and more convenient for operation. I was pleased that everything worked on the first try. However, the readings of the ammeter were different from the readings on the multimeter by more than 7 times.
Chinese voltmeter - ammeter after alteration Here it turned out that the wire resistor instead of the recommended resistance of 0.08 ohms has 0.8 ohms. I made a mistake in the measurement during its manufacture in counting zeros. I got out of the situation like this: a crocodile with a 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 ampervoltmeter coincided and became the moment of truth. The resistance of the involved section of the nichrome wire was 0.21 Ohm (measured with a prefix to the multimeter at the limit of "2 Ohm"). So it didn’t even turn out badly that instead of 0.08, the resistor turned out to be 0.8 ohms. No matter how you count, according to the formulas, you still have to adjust. For clarity, the result of his efforts was recorded on a video.
Video
I consider the acquisition of these voltmeters successful, but it’s a pity that their current price in that store has grown a lot, almost $ 3 apiece. Written by Babay from Barnaula.
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. 
