We connect a Chinese digital voltammeter. Shunt for ammeter

Prelude

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

File in place...

2. After modification according to item 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

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 removing the indicator or ringing the circuits with a tester and off you go!

I decided to write this article when I was making a power supply for my home laboratory. From own experience it was noticed that on adjustable block there must be food voltmeter, to estimate the installed voltage. And ampermetR, for an approximate estimate of the current consumed by the load. It was decided to install these useful elements into the new power source: a voltmeter and an ammeter. After searching in the drawers, I found two suitable measuring heads (the main criterion is minimum dimensions). With a maximum current of 50µA 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: maximum current the needle deviation is 30mA in my case. The maximum voltage that our voltmeter should measure is 30V.

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

Means shunt(resistor) with a resistance of 1 kOhm must be connected in series with the ammeter. In this case we will get a voltmeter. Those. If a current of 30 mA flows through such a series circuit, then the voltage drop across this resistor is 30 V. 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. We should also not forget that the resistor must be of suitable capacity, which we determine using the formula P = I*I*R, we get P=30mA*30mA*1kOhm=0.9W, we set it with a margin of at least 1W.

We also need to take into account internal resistance device. Then the additional resistor is calculated as follows: Rd=Up/Ii-Ri.
Rd - resistance of the additional resistor;
Up - max. the value of the selected voltage measurement limit;
Ii is the total deviation current of the selected ammeter;
Ri is the internal resistance (of the device frame) of the selected ammeter, it is indicated.

Let's do 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 measurement 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 and one part will flow through the ammeter, and the other through the resistance. It is necessary to select such a resistance so that a current of 1.5A is divided into two, 50 μA through an ammeter, and the rest of the current through a 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 using the fitting method. 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 deflects to the extreme position.

To do this you will need an adjustable voltage source and load, I used a car light bulb. Next, we adjust the shunt in this way, increasing the length of the wire if you need to reduce the maximum current, or shorten the wire if you need to increase it maximum value ammeter scales.

I got this four-layer shunt. I sealed the edges with silicone glue.

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

Ammeter from voltmeter It is done by analogy with the first option, only the shunt is installed not in series but in parallel. It also happens that voltmeters have internal resistors, which can be removed to create an ammeter.

It should be remembered that the ammeter must have minimal resistance, and the voltmeter must have very high resistance.

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.

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 charger power source 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, and accordingly must 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 a 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 from a built-in source.

• 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 female 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.

Note! Accurate voltage values ​​can only be obtained by regulated source nutrition. 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 direct current 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 of 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.

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.

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.

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.

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

This is how this scheme for connecting additional electronic components with those already existing in the voltmeter circuit. 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.

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 trimmer resistor 20 kOhm.

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)

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.

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

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 for operation. I was pleased that everything worked on the first try. However, the ammeter readings differed from the multimeter readings by more than 7 times.

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: a crocodile with 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.

Do-it-yourselfers, designing, developing and implementing the most different schemes chargers or power supplies, 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,