P
Phil Allison
- Jan 1, 1970
- 0
** 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
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