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-   -   Bypass capacitors on diodes? (http://www.diyaudio.com/forums/power-supplies/133916-bypass-capacitors-diodes.html)

wwenze 28th November 2008 12:10 AM

Bypass capacitors on diodes?
 
I saw this on another forum (not a good one for audio unfortunately):

http://i528.photobucket.com/albums/d...iodebypass.jpg

But with my limited knowledge I can't tell if it's beneficial or destructive.

Any comments?

raudio1969 29th November 2008 02:16 AM

I have seen this done when the diodes are used as a bridge rectfier. I was told it would reduce noise.

Eva 1st December 2008 02:46 PM

The impedance (and capacitance) of a diode changes very abruptly when it stops conducting (after reverse recovery is completed). This together with a series inductive component usually results in moderate voltage spikes or ringing. A parallel impedance (capacitor) helps to make this change not so abrupt and the spikes smaller.

The ideal solution is not a simple capacitor in parallel with each diode, though. An additional RC may be required to damp ringing. Also, a single C (plus optional RC) in parallel with each transformer secondary should be enough instead of four.

sreten 1st December 2008 02:51 PM

Quote:

Originally posted by Eva

The ideal solution is not a simple capacitor in parallel with each diode, though.

Hi,

True. But by deliberately using "poor" capacitors, with low Q at the
frequencies of interest it was found the resistor(s) can be omitted.

:)/sreten.

Eric Juaneda 4th December 2008 08:15 PM

Quote:

Originally posted by Eva
...The ideal solution is not a simple capacitor in parallel with each diode, though. An additional RC may be required to damp ringing...
Link to an interesting article about this.
Calculating optimum snubbers by Jim Hagerman

Eric

Eric Juaneda 4th December 2008 08:22 PM

Hi sreten
Quote:

Originally posted by sreten
True. But by deliberately using "poor" capacitors, with low Q at the
frequencies of interest it was found the resistor(s) can be omitted...

Avoid using poor capacitors like ZU5 shown in the picture. Prefer MKP, it sounds better. Calculating optimum snubber and using good capacitors is important event with smooth, fast or Schottky diodes.

Eric

Eva 4th December 2008 09:48 PM

The problem is that the optimum damping resistance is usually way higher than the internal resistive component of any lossy capacitor.

zigzagflux 5th December 2008 12:23 AM

I have found that to be true; just a capacitor, no matter what kind, never improved what I saw on the scope. Better off adding the correct capacitor and the right resistor for the application.

I think an RC snubber across the transformer is much more beneficial than caps across diodes, especially when using schottky's.


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