The Amp Meter

The basic circuit for measuring electric current using the same TLO81 OPAMP is shown in figure 18.  The circuit is very similar to that of the volt meter except, in this case, the circuit is set up to measure current (i) instead of voltage. To do this, resistance R₅ is placed in series with the counter electrode - working electrode circuit and the current is measured by determining the voltage across the inserted resistance R₅.

         Now,       and    

Thus, as     then           ₍₂₅₎

Now it is important that any extra potentials introduced into the circuit must be minimized for reasons previously discussed. Consequently, R₅ must be made as small as possible. Assume the current range that needs to be measured is 0 to 100 ma and assume that the maximum extraneous voltage that will be allowed  is 1 mv.

Then, the value of R₅ will be given by       

Now assume that the output required to represent 100 ma is 1 volt,

then V_out (max) = 1v and substituting in equation (25),

Solving for (R₂) assuming a value for (R₃) of 500 ohms gives a value for (R₂) of 500,000 ohms. Thus, a practical value for (R₂) would be 500k. There is a choice of values for (R₂) and (R₃). For example if (R₃) is made 100 ohms then (R₂) would be 100,100 ohms and practical value for (R₂) would be 100k.

These are simple examples of the use of the operational amplifier for use in electrochemistry. However, the circuits can become far more complicated when compensating circuits are introduced (e.g. temperature compensation etc.) but the basic circuits given above (or appropriately modified for specific applications) can provide accurate voltage and current data if so required.

Figure 18. An Ammeter Based on the TLO81 Operational Amplifier