******Hi,

electro cap makers specify the maximum, safe "ripple current" for each category and size. Normally, two figures are given - one at AC supply frequencies and another at a much higher frequency - reflecting the differing ESR values at the two frequencies. In order to measure this current, a low value resistor needs to be installed in series with the particular electro and the resulting voltage measured with a true RMS volt meter or maybe a DSO that can do the same task.

Finding a suitable resistor and fitting it in series could prove very tedious (or even unsafe) in practice. However, there is a simple way around this that works for low and AC supply frequency measurements.

The classic formula " I = C dv/dt " shows the current flowing in an electro is proportional to the dv/dt of the voltage across the terminals any point in time - so if you derive a current proportional to dv/dt, then its RMS value is a measure of the ripple current.

A series RC network does the job, long as its time constant is short in relation to the high frequency components in the current wave. For AC supply frequencies a value of 100uS proves to be OK.

All you need then is a 100nF film cap and a 1kohm resistor, connected in series across the electro and simply measure the RMS voltage appearing across the resistor (Vrms). The voltage wave replicates the electro current wave almost exactly.

The necessary scale factor is simply the electro's value in uF multiplied by

10,000 ( as a 1mA current in the RC network requires a dv/dt of 10,000 V/S.)Electro ripple amps = C x Vrms x 10,000

BTW:

While ripple current in electros operating at AC supply frequencies is not regularly an issue - it can become one when the supply impedance is unusually low, as with direct mains operation or with large transformers and small value caps.

In these cases, the peak charging current can exceed the average load current by a factor of more than 10:1 , exaggerating the RMS current value considerably.

Larger than usual ripple voltage percentages also increase peak and RMS ripple currents, particularly if combined with the above.

.... Phil