Galvanostat

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A galvanostat is a control and measuring device capable to keep constant the current through an electrolytic cell in coulometric titrations, disregarding changes in the load itself. A synonym is "amperostat".

Its main feature is its nearly "infinite" (i.e. extremely high respect to common loads) internal resistance.

The designation "galvanostat" is mainly used in electrochemistry: this device differs from common constant current sources by its ability to supply and measure a wide range of currents (from picoamperes to amperes) of both polarities.

[edit] Technical realization

The simpler galvanostat consists of a high-voltage source producing a constant voltage U with a resistor Rx connected in series: in order to force an almost constant current through a load, this resistor shall be much higher than the load resistor Rload. As a matter of fact, the current I through the load is given by

I = \frac{U}{R_x + R_{load}}

and if Rx>>Rload, the current I is approximately determined by Rx as follows

I \cong \frac{U}{R_x}

This simple realization requires rather high voltages (~100 V) to keep the load current constant with sufficient approximation for all practical purposes. Therefore, more complex versions of galvanostats, using electronic amplifiers with feedback and lower voltages, have been developed and produced. These instruments are capable to feed constant currents in the ranges from few picoamperes (pA) to several amperes (A); typical construction for use in the lower range of feed currents uses operational amplifiers.

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