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Old 4th February 2004, 01:00 AM   #1
K-amps is offline K-amps  United States
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Question Snubbing Cap values on bridge rectifiers.

Whats the largest value for snubbing caps used on bridge recifiers.

I got a good surplus stock of 0.474 uF 250v caps and was wondering if this would be detrimental instead of beneficial if used as snubbing caps.

Also what is conventional wisdom say on the same.




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Old 4th February 2004, 02:26 AM   #2
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If your math is good, try this ap note:

http://www.hagtech.com/pdf/snubber.pdf

see section titled "snubber Capacitor".
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Old 4th February 2004, 03:09 AM   #3
K-amps is offline K-amps  United States
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I read it... lol, didnt get it.
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Old 4th February 2004, 04:41 AM   #4
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Quote:
I read it... lol, didnt get it.
It basically says that using a resistor in series with a capacitor connected in parallell with each diode is better then just using a single capacitor as the resitor helps by damping out the ringing after less then one cycle.

I have tested this in an amplifier power supply and can confirm that the theory seems to be correct, in my experience the resistor is not that critical but it is important to have it there, for a power supply of 220VAC 2A I used 100 ohm and 10nF and for a power supply of 350VAC and 100mA I used 10nF and 470 ohm, all components selected for min ringing.

Regards Hans
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Old 4th February 2004, 11:07 AM   #5
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If you are going to use snubber caps on the rectifier for the mains 50/60 Hz VAC I would suggest you to use simple and plain 100 nF lossy ceramic capacitors with high dielectrica in parallel with each rectifier diode.

The PDF doc at
http://www.hagtech.com/pdf/snubber.pdf does not address the real issue with diodes, talking about only the intrinsic capacitance in the diode is actually a lack of "knowledge"
But the math is ok and applicable.

To be accurate you must measure the di/dt for the diodes Dtrr if you really want to calculate on the proper snubber, but I don't think this is a matter for you and your 50/60Hz rectifier, it's something else when designing SMPS's.

BTW, don’t use too large cap, the capacitors own resonance frequency decrease and thereby the snubbering capability for high frequency energies, eg. a small one can actually be more efficient.

Good luck!
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Old 4th February 2004, 12:08 PM   #6
K-amps is offline K-amps  United States
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Thanks Tubetvr and especially Ultima Thule,

Assuming secondaries of (dual) 60vac and 60Hz Mains (i.e. 120Hz at the rectifier) will 0.474 uF be too much or will it help,

Should I use a resistor with this value of 0.474 uF?

thanks again.
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Old 4th February 2004, 12:51 PM   #7
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Question Cap value

Quote:
Originally posted by K-amps
Should I use a resistor with this value of 0.474 uF?
Hi,

it's very unlikey that you have caps which are 0.474 uF. It may be marked 474 but this does not imply that it is 0.474 uF.

James
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Old 4th February 2004, 01:02 PM   #8
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474 is probably 470000 pF, which is 0.47uF. Pretty close to what you thought.
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Old 4th February 2004, 01:03 PM   #9
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IMO, 0.47 seems to big.

A Sony amp I have here uses 0.01 caps across the silicon rectifier diodes.
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Old 4th February 2004, 01:08 PM   #10
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I'll guess 10-100 nF is pretty normal without measuring anything. If you want to insert a resistor you must know more.
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