High impedance DC voltmeter.
A high impedance DC voltmeter using a uA741 IC is shown here. The operational amplifier is used as a non-inverting DC amplifier in which the negative feedback is through a DC meter requiring 1mA for full scale deflection. Since R6 is 100 Ohms, the meter will show full scale reading when the DC input voltage to pin3 is equal to the voltage drop across R6, viz 0.1 volts. Choice of R1 and R2 for getting different voltage ranges are shown in the table. The diodes D1 and D2 protect the IC from accidental excessive input voltages and diodes D3 and D4 protect the meter from overloads.
Circuit diagram with parts list.
- Assemble the circuit on good quality PCB or common board.
- Mount the IC on a holder.
- The circuit can be powered from a +9V/-9V dual power supply.
- The meter M1 can be a 1mA FSD DC ammeter.
- The values of R1 & R2 for different voltage ranges can be obtained from the table given below.
Values of R1 ,R2 and the corresponding ranges.