DIYers, designing, developing and implementing a variety of schemes chargers or power supplies, are constantly faced with an important factor - visual control of the output voltage and current consumption. Here, Aliexpress often lends a helping hand, promptly supplying Chinese digital measuring instruments. In particular: a digital ampervoltmeter is a very simple device, affordable and displays quite accurate information data.

But for beginners, commissioning (connecting an ammeter voltmeter to the circuit) can be a problematic task, because the measuring device comes without documentation and not everyone can connect the wires quickly marked with color.

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

this is a 100 volt/10 amp ammeter and comes with a built-in shunt. Many radio amateurs often purchase such measuring instruments for their homemade products. The digital device can be powered both from separate sources,

and from one operated and measured voltage source. But here a small nuance is hidden, it is necessary to observe the condition - the voltage of the power source used was within 4.5-30 V.

For DIYers who are not yet entirely clear: we connect a thick black wire to the minus of the power supply, a thick red wire to the plus of the power supply (the readings of the voltmeter scale will light up),

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

I decided to write this article when I was making a power supply for my home laboratory. From my own experience, it has been noticed that on an adjustable power supply there should be voltmeter, to evaluate the installed voltage. And ammetR, for an approximate estimate of the current consumed by the load. It was decided to install these useful elements in a new power source: a voltmeter and an ammeter. Looking in the boxes, I found two suitable measuring heads (the main criterion is the minimum dimensions). With a maximum current of 50uA and 30mA.

Let's do it first voltmeter from ammeter

So, let's move on to the calculations.

The easiest way is to make a voltmeter from an ammeter, I use a second ammeter. For calculations, we need: the maximum deflection current of the arrow is 30mA in my case, the maximum voltage that our voltmeter should measure is 30V.

Using Ohm's law find the resistance: R=U/I, R=1kOhm.

Means shunt(resistor) with a resistance of 1 kΩ must be connected in series with an ammeter. In this case, we get a voltmeter. Those. if a current of 30mA flows through such a series circuit, then the voltage drop across this resistor is 30V. In my case, I don’t even need to change the scale of the device, it’s enough to stick the letter “V” to make it clear that this is a voltmeter.

It should be remembered that a current of 0-30mA will always flow through such a voltmeter, depending on the measured voltage from 0-30V. And since it is used in the power supply, this is not critical. Also, one should not forget that the resistor must be of a suitable capacity, which we determine by the formula P \u003d I * I * R, we get P \u003d 30mA * 30mA * 1kOhm \u003d 0.9W, set with a margin of at least 1W.

We must also take into account the internal resistance of the device. Then the additional resistor is considered as follows: Rd \u003d Up / Ii-Ri.
Rd - resistance of the additional resistor;
Up - max. the value of the selected voltage measurement limit;
Ii - total deflection current of the selected ammeter;
Ri is the internal resistance (framework of the device) of the selected ammeter, it is indicated.

Doing ammeter from ammeter which has a small scale.

The first ammeter has a scale of 50 μA, which is very small, I need 1.5A. To expand the measuring range of the ammeter, you need to install a shunt, but not in series, but in parallel with the measuring head. It turns out that the current will branch out and one part will flow through the ammeter, and the other through the resistance. It is necessary to choose such a resistance that the current of 1.5A is divided by two, 50 μA through the ammeter, and the rest of the current through the resistor.

For calculations, you will need to know the resistance of the ammeter, but since I don’t know it, I will make the shunt by fitting. To do this, you need to take a copper wire with a diameter of 0.8-1 mm and a length of 1 meter and measure the current at which the arrow deviates to its extreme position.

This will require an adjustable voltage source and load, I used a car light bulb. Further, in this way we adjust the shunt by increasing the length of the wire if it is necessary to reduce the maximum current or shorten the wire if it is necessary to increase the maximum value of the ammeter scale.

I got this shunt in four layers. I glued the edges with silicone glue.

It should be remembered that if the shunt accidentally breaks off, then a large current will flow through the microammeter and it will fail.

ammeter from voltmeter is done by analogy with the first option, only the shunt is installed not in series but in parallel. It also happens that internal resistors are installed in voltmeters, removing which you can get an ammeter.

It should be remembered that the ammeter should have a minimum resistance, and the voltmeter should have a very high resistance.

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

The indicator is tinted with an automotive tint film (20%) to reduce brightness and improve the visibility of the indicator in the light.
I highly recommend toning it. Scraps of tint film will be happy to give you free of charge at any car service engaged in tinting.

There are also other modifications of this module on the Internet, but the essence of the alterations does not change from this - if you come across a different module, simply correct the circuit according to the board by dropping the indicator or ringing the circuit with a tester and go!