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Old 28th November 2008, 12:10 AM   #1
wwenze is offline wwenze  Singapore
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Default Bypass capacitors on diodes?

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

Click the image to open in full size.

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

Any comments?
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Old 29th November 2008, 02:16 AM   #2
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I have seen this done when the diodes are used as a bridge rectfier. I was told it would reduce noise.
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Old 1st December 2008, 02:46 PM   #3
Eva is offline Eva  Spain
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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.
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Old 1st December 2008, 02:51 PM   #4
sreten is offline sreten  United Kingdom
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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.
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Old 4th December 2008, 08:15 PM   #5
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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
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Old 4th December 2008, 08:22 PM   #6
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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
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Old 4th December 2008, 09:48 PM   #7
Eva is offline Eva  Spain
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The problem is that the optimum damping resistance is usually way higher than the internal resistive component of any lossy capacitor.
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Old 5th December 2008, 12:23 AM   #8
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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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