Automotive, laboratory power supplies can have currents that reach up to 20 amperes or more. It is clear that a couple of amperes can be easily measured with a regular cheap multimeter, but what about 10, 15, 20 or more amperes? After all, even at not very heavy loads, the shunt resistors built into ammeters can overheat over a long measurement time, sometimes even hours, and in the worst case, melt.
Professional instruments for measuring large currents are quite expensive, so it makes sense to assemble the ammeter circuit yourself, especially since there is nothing complicated about it.
The circuit, as you can see, is very simple. Its operation has already been tested by many manufacturers, and most industrial ammeters work in the same way. For example, this scheme also uses this principle.
The peculiarity is that in this case a shunt (R1) with a resistance of a very low value is used - 0.01 Ohm 1% 20W - this makes it possible to dissipate very little heat.
The adjustments will consist of zeroing the output of the ammeter when there is no current, and calibrating it by comparing it with another, exemplary current measuring instrument. The ammeter is powered by a stable symmetrical voltage. For example, from 2 9 volt batteries. To measure current, connect the sensor to the line and a multimeter in the 2V range - see the readings. 2 volts will correspond to a current of 20 amperes.
Using a multimeter and a load, such as a small light bulb or resistance, we will measure the load current. Let's connect the ammeter and get current readings using a multimeter. We recommend running a few tests with different loads to compare the readings to a reference ammeter and make sure everything is working correctly. You can download the printed armor file.
Digital VOLTMETER and AMMETER for laboratory power supply (unipolar and bipolar) on a specialized ICL7107 chip
It so happened that there was a need to manufacture an ammeter and a voltmeter for laboratory power supplies. To solve the problem, I decided to scour the Internet and find an easily repeatable scheme with an optimal price-quality ratio. There were thoughts of making an ammeter and a voltmeter from scratch based on an LCD and a microcontroller (MK). But I think to myself, if it’s a microcontroller, then not everyone will be able to repeat the design - after all, you need a programmer, and I don’t even really want to buy or make a programmer for programming once or twice. And people probably won’t want it either. In addition, all microcontrollers (that I have dealt with) measure the positive polarity input signal relative to the common wire. If you need to measure negative values, you will have to deal with additional operational amplifiers. Somehow all of this was stressful! My eye fell on the widespread and affordable ICL7107 chip. Its cost turned out to be half the cost of MK. The cost of a 2x8 character LCD turned out to be three times more than the cost of the required number of seven-segment LED indicators. And I like the glow of LED indicators more than LCD. You can also use a similar, even cheaper, domestically produced m/skh KR572PV2. I found the diagrams on the Internet and went ahead to check the functionality! There was an error in the diagram, but it was corrected. It turned out that when calibrating the readings, the m/sx ADC works quite accurately and the accuracy of the readings will completely satisfy even the most picky user. The main thing is to take a good quality multi-turn tuning resistor. The counting is very fast - without brakes. There is a significant drawback - bipolar power supply ±5V, but this issue can be easily solved using a separate mains power supply on a low-power transformer with positive and negative stabilizers (I will give the diagram later). To obtain -5V, you can use a specialized ICL7660 microcircuit (visible in the photo at the top of the page) - cool stuff! But it has an adequate price only in an SMD package, and in a regular DIP it seemed to me a little expensive, and it’s much more difficult to buy than conventional linear stabilizers - it’s easier to make a negative stabilizer. It turned out that the ICL7107 perfectly measures both positive and negative voltages relative to the common wire, and even the minus sign is displayed in the first digit. In fact, in the first digit only the minus sign and the number “1” are used to indicate the polarity and value of hundreds of volts. If for a laboratory power supply a voltage indication of 100V is not needed and there is no need to indicate the voltage polarity, since everything should be written on the front panel of the power supply, then the first indicator can not be installed at all. For an ammeter the situation is the same, but only a “1” in the first digit will indicate that a current of ten Amperes has been reached. If the power supply has a current of 2...5A, then you can not install the first indicator and save money. In short, these are just my personal thoughts. The schemes are very simple and start working right away. You only need to set the correct readings on the control voltmeter using a trimming resistor. To calibrate the ammeter, you will have to connect a load to the power supply and use the control ammeter to set the correct readings on the indicators and that’s it! To power ammeters in a bipolar power supply circuit, it turned out that it is best to use a separate small network transformer and stabilizers with a common wire isolated from the common wire of the power supply itself. In this case, the inputs of the ammeters can be connected to the measuring shunts “at random” - m/sx will measure both “positive” and “negative” voltage drops on the measuring shunts installed in any part of the power supply circuit. This is especially important when both stabilizers in a bipolar power supply are already connected via a common wire without measuring shunts. Why do I want to make a separate low-power power supply for meters? Well, also because if you power the meters from the transformer of the power supply itself, then when you receive a voltage of 5 V out of 35 V, you will need to install an additional radiator, which will also generate a lot of heat, so it’s better to use small sealed transformers on a small board. And in the case of a power supply with a voltage of more than 35 V, say 50 V, you will have to take additional measures to ensure that for five Voltage stabilizers at the input the voltage is no more than 35 V. You can use high-voltage switching stabilizers with low heat generation, but this increases the cost. In short, if not one thing, then another ;-)
Voltmeter circuit:
Ammeter circuit:
Photo view of the printed circuit board of a voltmeter and ammeter (board size 122x41 mm) with seven-segment LED indicators of type E10561 with digits 14.2 mm high. The power supply for the voltmeter and ammeter is separate! This is necessary to ensure the ability to measure currents in a bipolar power supply. The ammeter shunt is installed separately - a 0.1 Ohm/5 W cement resistor.
Scheme of the simplest mains power supply for joint and separate power supply of voltmeters and each of the ammeters (maybe a nonsense idea, but it works):
And a photo view of printed circuit boards using compact sealed transformers 1.2...2 W (board size 85x68 mm):
Voltage polarity converter circuit (as an option for obtaining -5 V from +5 V):
Video of voltmeter operation
Video of workammeter
I won’t make kits or boards, but if anyone is interested in this design, you can download the printed circuit board drawings.
Thank you all for your attention! Good luck, peace and goodness to your home! 73!
Figure 1 shows a circuit of a digital ammeter and voltmeter, which can be used as an addition to circuits of power supplies, converters, chargers, etc. The digital part of the circuit is implemented on a PIC16F873A microcontroller. The program provides voltage measurement 0... 50 V, measured current - 0... 5 A.
LED indicators with a common cathode are used to display information. One of the operational amplifiers of the LM358 chip is used as a voltage follower and serves to protect the controller in emergency situations. Still, the price of the controller is not so low. The current is measured indirectly, using a current-voltage converter made by the operational amplifier DA1.2 of the LM358 microcircuit and the transistor VT1 - KT515V. You can also read about such a converter. The current sensor in this circuit is resistor R3. The advantage of this current measurement circuit is that there is no need for precise adjustment of the milliohm resistor. You can simply adjust the ammeter readings with trimmer R1 and within a fairly wide range. The load current signal for further digitization is removed from the load resistor of the converter R2. The voltage on the filter capacitor located after the rectifier of your power supply unit (stabilizer input, point 3 on the diagram) should not be more than 32 volts, this is due to the maximum supply voltage of the op-amp. The maximum input voltage of the KR142EN12A microcircuit stabilizer is thirty-seven volts.
Adjusting the voltammeter is as follows. After all the procedures - assembly, programming, checking for compliance, the product you have assembled is supplied with supply voltage. Resistor R8 sets the voltage at the output of the KR142EN12A stabilizer to 5.12 V. After this, the programmed microcontroller is inserted into the socket. Measure the voltage at point 2 with a multimeter that you trust, and use resistor R7 to achieve the same readings. After this, a load with a control ammeter is connected to the output (point 2). In this case, equal readings of both devices are achieved using resistor R1.
You can make a current sensor resistor yourself, using, for example, steel wire. To calculate the parameters of this resistor, you can use the program “Did you download the program?” Have you opened it? So, we need a resistor with a nominal value of 0.05 Ohm. To make it, we will choose steel wire with a diameter of 0.7 mm - this is what I have, and it does not rust. Using the program, we calculate the required length of the segment having such resistance. Let's look at the screenshot of this program's window. 
And so we need a piece of stainless steel wire with a diameter of 0.7 mm and a length of only 11 centimeters. There is no need to twist this segment into a spiral and concentrate all the heat at one point. Look like that's it. What is not clear, please go to the forum. Good luck. K.V.Yu. I almost forgot about the files.
The diagram of an on-board automobile voltmeter with indication is shown in the figure below:
The device is a six-level linear indicator, in the range from 10 to 15 volts. DA1, on K142EN5B at pin 8, produces a voltage of 6 volts to power the digital chip DD1 type K561LN2. The inverters of the K561LN2 microcircuit serve as threshold elements, representing nonlinear voltage amplifiers, and resistors R1 - R7 set the bias at the inputs of these elements. the input voltage of the inverter exceeds the threshold level, a low level voltage will appear at its output, and the LED at the output of the corresponding inverter will light up.
The printed circuit board of the on-board LED voltmeter with a diagram of the arrangement of parts on it, measuring 80x45 mm, is shown in the figures below:
When setting up an on-board LED voltmeter, instead of a battery, connect a laboratory stabilized 10-volt source, installing a temporary trimming resistor instead of resistor R1. By changing resistance R1, we achieve the moment when LED HL1 turns on. The remaining levels are set automatically. When checking the remaining levels in detail, the resistances R2 – R6 are specified, respectively.
The miniature graphical ammeter displays a bar display with a current range of 0 to 1A using planar red smd leds. This histogram has 20 segments of the same color, where each step is approximately 0.05 amps of current. Switching control is performed by a PIC16F686 microcontroller with a 10-bit ADC. This measuring device (as a separate module) can be used in various circuits and devices. Each LED output is provided with a jumper to set the output trigger, which can be configured for control, alarm, start relay, DC motor protection.
