On November 20, 2005 12:45 am, Wouter van Ooijen wrote:
> In circuits I sometimes see capacitors over each of the fout diodes
> in a diode bridge. What is the prurpose and effect of these
> capacitors? I recently tried to help a client get rid of some
> interference problem and these capacitors turned out to do a good
> job. But I would like to know exactly (if possible) why?

You didn't mention how-big the caps were, but guessing 100nF, then the 
"resistance" of the capacitors could be seen as:

X = 1 / ( 2 * pi * f * C)

X = 1/(2*3.141*50hz*100nF)=31k (sort-off open circuit to AC freq)

possible electric motor noise
X = 1/(2*3.141*5000hz*100nF)=318ohms (low impedance to hi-freq)

possible fluorescent ballasts
X = 1/(2*3.141*50000hz*100nF)=32ohms (very low resistance)

Electrolytic-type capacitors are fine at low frequencies but have an 
inductive component at high frequencies, plus being a chemical 
reaction, don't filter that quickly.

Hope that helps.
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