>-----Original Message-----
>From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] 
>Sent: 20 November 2005 08:45
>To: 'Microcontroller discussion list - Public.'
>Subject: [EE] capacitors on a diode bridge
>
>
>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?
>
>Wouter van Ooijen

One potentialy important function which I've not seen mentioned is that the capacitors provide a stable RF impedance from the point of view of the powered circuit looking back in the mains.  The impedance of the diodes obviously varies considerably at a rate of either 50 or 60Hz, and if a circuit conducts any RF back into the mains, this variation can cause problems.

A classic case which I've experienced is powering one of the simple FM "bug" transmitters via a "wall wart" style supply.  Inevitably the received audio will be drowned in 50/60Hz buzz, that seemingly no amount of capacitive or inductive filtering on the supply lines will eliminate.  Bypass the bridge recitifer with small value ceramic caps and the hum all but vanishes.

Regards

Mike

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