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Old 9th September 2006, 08:14 AM   #1
Progg70 is offline Progg70  Sweden
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Cool Value for capacitors over the diodes to prevent from high frequency

I have an ordinary AC to DC diode bridge. I want to mount parallel capacitors over the diodes to prevent from high frequency. What value would I use? (see picture.)
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Old 9th September 2006, 08:26 AM   #2
singa is offline singa  Singapore
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0.1uF voltage rating 2.5 to 3 times or better if size permit of incoming AC voltage ( from secondary coil of transformer).Alternatively you can parallel another 0.01uF for higher frequency.
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Old 9th September 2006, 08:38 AM   #3
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Default 0,1 uF

Hi,
0,1 uF is generaly recommanded. I use Wima MKS4.
Low swiching noise diodes are also welcome. There is a nice article by John Camille about this topic in Sound Practices #7.
Alain
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Old 9th September 2006, 10:36 AM   #4
Progg70 is offline Progg70  Sweden
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Thanks!
Then itís just going ahead and does the physical job to put thing together.
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Old 12th September 2006, 04:04 PM   #5
AndrewT is offline AndrewT  Scotland
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Hi,
it might be better to put an RC snubber in parallel with each diode.
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Old 13th September 2006, 06:25 AM   #6
mrjam is offline mrjam  Italy
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I experimented some configurations (not simulations!)

A diode bridge should have a behaviur similar to a 1N4007 bridge, I found it best smoothed by 1ohm+22nF // 100nF across the secondary connection to the bridge. It's useless to put a snubber across each diode.
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Old 13th September 2006, 09:06 AM   #7
AndrewT is offline AndrewT  Scotland
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Hi,
Quote:
It's useless to put a snubber across each diode.
The diode manufacturers say otherwise (when they care to be informative).
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Old 13th September 2006, 02:01 PM   #8
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Quote:
Originally posted by mrjam
I experimented some configurations (not simulations!)

A diode bridge should have a behaviur similar to a 1N4007 bridge, I found it best smoothed by 1ohm+22nF // 100nF across the secondary connection to the bridge. It's useless to put a snubber across each diode.
depends upon the transformer -- and the voltage -- the frequency of the resonant circuit formed by the diode junction capacitance, and the leakage inductance, the ESR and ESL of the filter caps. the diode junction capacitance is a function of voltage (that's how we get diode tuning).

you can start off by measuring the transformer leakage inductance, but this is only a starting point...
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Old 14th September 2006, 07:17 AM   #9
mrjam is offline mrjam  Italy
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Quote:
depends upon the transformer -- and the voltage
.......
Sorry, I omitted to say: "in my test configuration"

Quote:
The diode manufacturers say otherwise (when they care to be informative).
I trust in my Tek digital storage 'scope!
With this net - in this test configuration - the oscillation is better smoothed.

Are you sure your manufacturers refer to standard diode bridges? ..or you're talking about SBVY or MUR series. In my tests - with my test configuration naturally - the former are best smoothed by a 22nF across each diode. RC doesn't give any improvement.



All I mean is that the RC snubber across each diode is not always the best solution. Let measure it ...but it's reaaaly difficult without a storage oscilloscope.
Next week I will have access to an exceptional tool, a 20000bucks Advantest R3131 3GHz Network/Spectrum Analyzer, so new measurements have to come. I'll let you know the results of the snubbering on the wide RF spectrum.
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Old 14th September 2006, 08:41 AM   #10
forr is offline forr  France
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Hi Jackinnj
---you can start off by measuring the transformer leakage inductance, but this is only a starting point...---

How to ?
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