The Volt Meter

The basic circuit for measuring electric potential using the TLO81 OPAMP is shown in figure 17. The TLO81 OPAMP chip is shown at the top on the left hand side of the circuit diagram. The chip is about 1 cm long and 8 mm wide and 2mm thick. At the centre of one side is circular disk or hole and if this side is placed on the left with the pins pointing down, as shown in the diagram, the eight connecting pins are numbered one to 8 starting at the bottom left and counting counter clockwise to the upper left. The pins can be connected into a circuit employing a standard ‘bread board’ or a correctly prepared printed circuit board (PCB).

Figure 17. A Volt Meter Based on the TLO81 Operational Amplifier

The OPAMP in figure 16 is wired as a non-inverting amplifier that means that a positive input voltage will provide a positive output voltage. Pin 3 is the input and pin 6 the output. A negative DC supply is applied to pin 4 and a positive DC supply applied to pin 7. The power supply can range from ± 5 volts to ±16 volts, the chosen supply being ±6 volts. Pin 2, in this instance is the feed back pin and the amplification obtained from the system is given by,

Thus, if R₁ is 27k and R₂ 270k then,

Thus the amplification factor is 11.

Pins 1 and 5 are zero adjustment pins and are connected to a 10k trimmer as shown on the right of the figure. The input is shorted, and the output adjusted to zero using the trimmer. Once it has been adjusted it will need no further attention unless something catastrophic happens to the apparatus.  

The input impedance of the TLO81 is extremely high and for the purposes of this circuit can be considered infinite. It follows that resistance R₁ must also be very high to ensure little or virtually no current is take from the source; a value of 10 M would be appropriate. If used for measuring electrolytic cell voltages no part of the voltmeter circuit may be connected to earth and the meter circuit must remain completely electrically isolated except for the two input connections. Thus, taking the meter input from the working electrode to the reference electrode, the output will be proportional to the difference between the potential of the working electrode and the potential of the reference electrode. However, this is an ideal case as other potentials develop in the circuit that must be taken into account as previously discussed.