In this article we learn how to build a digital voltmeter and a digital ammeter combined circuit module for measuring DC volts and current through different ranges, digitally.


Electrical parameters like voltage and current are inherently associated with electronics and with electronic engineers.

Any electronic circuit would be just incomplete without appropriate supply of voltage and current levels.

Our mains AC supply an alternating voltage at the potentials of 220 V, for implementing these voltages in electronic circuits we incorporate DC power adapters which effectively step down the mains AC voltages.

However, most power supplies don't include power monitoring systems in them, meaning the units don't incorporate voltage or current meters for displaying the relevant magnitudes.

Mostly the commercial power supplies use simple ways to display the voltages like a calibrated dial or ordinary moving coil type meters. These may be OK as long as the involved electronic operations are not critical, but for complex and sensitive electronic operations and troubleshooting, a hi-end monitoring system becomes imperative.

A digital volt meter and an ammeter become very handy for monitoring voltages and current perfectly without compromising safety parameters.

An interesting and accurate digital voltmeter and ammeter circuit has been explained in the present article which can be easily built at home, however the unit will require a well designed PCB for the sake of accuracy and perfection.

Simulation and Working

The circuit employs IC 3161 and 3162 for the required processing of the input voltage and current levels.

The processed info can be directly read over three 7-segment common anode display modules.

The circuit requires a 5 volt well regulated power supply section for operating the circuit and should be included without fail as the IC strictly requires a 5 volt supply for operating correctly.

The displays are powered by individual transistors which make sure that the displays are lit brightly.

The transistors are BC640, however you may try other transistors like 8550 or 187 etc.

The proposed digital voltmeter, ammeter circuit module can be effectively used with a power supply for indicating the voltage and current consumption by the connected load through the attached modules.

Referring to the circuit diagram below, the 3 digit digital display module is build through the ICs CA 3162 which is an analogue to digital converter IC, and the complementary CA 3161 IC which is BCD to 7 segment decoder IC, both these ICs are manufactured by RCA.

Circuit diagram for digital ammeter voltmeter circuit

How the Displays Work

The 7-segment displays used are common anode type and are connected across the shown T1 to T3 transistor drivers for indicating the relevant readings.

The circuit includes the facility for the decimal point selection as per the load specs and range.

For example in the voltage readouts, when the decimal point illuminates at LD3 signifies a 100mV range.

For the current measurement the selection facility enables you to choose from a couple ranges, that is through a 0 to 9.99, and the other from 0 to 0.999 amps (using the link b). Which implies that the current sensing resistor is either a 0.1 ohm, or a 1 ohm resistor, as shown in the diagram below:

In order to ensure that R6 has no effect on the output voltage this resistor needs to be positioned prior to the voltage divider network which becomes responsible for controlling the output voltage.

S1 which is a DPDT switch is used for selecting either the voltage or the current reading as per the users preference.

With this switch set for measuring voltage P4 along with R1 provides an attenuation of around 100 for the fed input voltage.

Additionally the point D is enabled at a lower voltage level for allowing the illumination of the decimal point on the LS module, and the figure "V" become brightly illuminated.

With the selection switch held towards the Amp range, the voltage drop acquired across the sensing resistor is applied straight over to the points of the Hi-Low inputs of IC1 which is the DAC module.

The significantly low value of the sensing resistors ensures a negligible effect on the voltage divider outcome.

Adjustment Ranges for the Displays

You will find 4 adjustment ranges supplied in the proposed digital voltmeter ammeter circuit module.

P1: for nulling the current range.

P2: For enabling full scale calibration of the current range.

P3: for nulling the voltage range.

P4: For enabling full scale calibration of the voltage range.

It is recommended that the presets are adjusted in the above order only wherein P1, and P3 appropriately used for correctly nullifying the respective parameters of the module.

P1 helps to compensate the regulator operating quiescent current consumption value, which results in a minor negative deviation across their voltage range, which is in turn effectively compensated by P3.

The voltage/current display module works using the unregulated supply from the supply source without any issues (not to exceed 35V max), note the point E and F in the second figure above. In that case the bridge rectifier B1 can be eliminated.

The system might be designed like a twofold to acquire concurrent V and I readings. It ought to be recognized, however, that the current sensing resistor is short-circuited by means of the ground links each time the two devices are provided from the identical source. There are basically two methods to defeat this disorder.

The first is to hook up the V module from a different source, while the l module from the "host" supply. The second is a lot more graceful and necessitates hard wiring areas E to the left side of the current sensing resistor.

Be aware, although, that the highest possible V reading in that case turns into 20.0 V (R6 declines l V max.), because the voltage at pin ll usually will not surpass l.2 V.

Bigger voltages tend to be showed by choosing the lower current quality, ` i.e., R6 gets to be 0R1. Instance: R6 falls 0.5V at a current usage of 5 A, to ensure 1.2 - 0.5 = 0.7V continues to be for the voltage reading, whose optimum display is in that case 100 x 0.7: 70 V Just as before, these kinds of complications simply develop whenever a couple of of these units are employed all in one supply.

PCB design for making the above discussed modules

PCB design for digital ammeter voltmeter circuit

Need Help? Please send your queries through Comments for quick replies!