Capacitor bypass of diodes in bridge rectifiers

[Speakerguy]

I have see on several audio projects that people will bypass the diodes in a bridge rectifier (discrete diodes or monolithic part) with small .1-.01uF parts. Is this for RF shunting or what purpose do they serve? Thanks!

 

[dknguyen]

THey might also be used for EMI since a lot of diodes have snap recovery and the capacitor might help soften this. Kind of like a capacitive snubber.

What part of the circuit is the bridge rectifier part of?

 

[ericgibbs]

hi,

They reduce HF noise caused by the reverse recovery current being switched off when the current carriers in the diodes are depleted.

The fast switching times of this current edge produces HF noise, the capacitor acts as short to the HF noise and prevents it from causing interference with the powered system.

 

[Speakerguy]

What's an appropriate size? Just any .1 or .01uF cap, or should I size it based on the speed of the diode? This is for the main power supply bridge rectifier. Thanks guys

 

[ericgibbs]

What's an appropriate size? Just any .1 or .01uF cap, or should I size it based on the speed of the diode? This is for the main power supply bridge rectifier. Thanks guys

Hi,
It would depend upon the psu's usage, eg: if it was powering an audio system I would use 100nF.
For a bog standard application I would either use 10nF or none at all.!

Whats the size of the reservoir cap.?

 

[Hero999]

I've seen them before, not on schematics but on the PCB of a ready-made piece of kit.

I assumed they were for RF bypass, so just use 10nF to 100nF and it doesn't matter what type, cheap and nasty ceramics are fine.

 

[Speakerguy]

Whats the size of the reservoir cap.?

Totally variable, active crossover circuits to power amplifier supplies.

 

[ericgibbs]

Totally variable, active crossover circuits to power amplifier supplies.

hi,
The reason for asking, is that in some DIY psu projects, the builder uses a huge value capacitor just after the bridge,
thinking it will kill any noise.

The problem is the very high recharge current that flows for a very short time thru the transformer and rectifiers,
this can cause interference in the powered application.

 

[Nigel Goodwin]

I have see on several audio projects that people will bypass the diodes in a bridge rectifier (discrete diodes or monolithic part) with small .1-.01uF parts. Is this for RF shunting or what purpose do they serve? Thanks!

I've always been told that they are to prevent mains spikes killing the rectifier diodes - I must admit I've never fitted them in anything I've ever made, but I always use well over rated diodes.