Snubbers on Power Transformer Secondaries - diyAudio
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Old 11th April 2012, 04:54 PM   #1
Magz is offline Magz  United States
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Default Snubbers on Power Transformer Secondaries

Hello all,

I've been reading up on snubbers for power transformer secondaries, including the work of John Swenson and Jim Hagerman. One bit isn't clear to me though - when using a center tapped HT transformer, should the snubber RC network be run from one end of the secondary to the other, or should there be two RC networks, each running from one end of the secondary to the center tap? After plowing through several web and DIYAudio searches I have found examples of both; is there a standard "best practice"?
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Old 11th April 2012, 05:23 PM   #2
Serge66 is offline Serge66  Switzerland
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Hi Magz,

Personally, I would put the snubber on the primary. Most, if not all, power transformers used in tube equipment are elevating the primary voltage. So the spike is also put through the voltage ratio.
My 2 cents. Any other thoughts, anyone?
CHeers,

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Old 11th April 2012, 05:34 PM   #3
Magz is offline Magz  United States
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Serge, thanks for the reply.

I usually do put an RC network on the primary, to remove noise coming from the mains.

What I'm talking about in this post, though, is a snubber on the secondary meant to absorb any ringing that occurs between the transformer secondary and the rectifier. There has been quite a bit posted on this subject but I can't seem to find a clear answer regarding how best to do it for CT secondaries; it's quite clear with a bridge rectifier that it can only be placed completely across the secondary winding. Most use a 330ohm resistor with a .022uF cap for this.
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Old 11th April 2012, 06:31 PM   #4
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A single network across the entire secondary will suffice. Even if your rectifier is FWCT as opposed to a 4-device bridge. As long as you have the correct values for R and C, (and a scope, of course) it will work.

Personally, all articles attempting to predict RC values have not helped me. The best way is by experiment with a properly loaded circuit. You start by identifying the smallest C that makes a change, and pick the largest R that effectively snubs. Trial and error will quickly show you which value needs to go up or down from there. Measuring ringing frequencies can help to pick ballpark values, but at the end of the day there is an ideal set for optimal performance. Too much C (or too low R) just consumes excess power, and too small C (or too high R) does not snub the transients.
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Old 11th April 2012, 06:48 PM   #5
Magz is offline Magz  United States
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Thanks, Zig

I'll try it with a single network then. I do have a scope and I have done a similar thing for output transformers in the past, getting the cap value in the ballpark and then using a pot to dial in the perfect resistance.
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Old 11th April 2012, 06:56 PM   #6
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Quote:
Originally Posted by Magz View Post
Serge, thanks for the reply.

I usually do put an RC network on the primary, to remove noise coming from the mains.
"Snubbers" are used to remove ringing in switching devices under inductive loads, to make a lossy tank and discharge leakage inductances in transformers or motors, they donīt remove any line noise, you must use a line filter to do this.
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Old 11th April 2012, 07:35 PM   #7
DF96 is offline DF96  England
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Yes, a snubber on the primary won't stop mains noise but it may stop arcing on the on/off switch.
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Old 12th April 2012, 12:34 AM   #8
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The snubber is there to re-route leakage type current flow from the diode switching. The best route is to place the snubber as close to the section of winding that is undergoing the switching - in the normal CT secondary case, this is from each end of the secondary to the CT, and as close as possible to the transformer, rather than out at the diodes (but in practise you don't typically get any option). The loop area of the secondary to CT helps radiate any noise, so best to twist any such secondary leads, and not pass them near to sensisitve inputs (as you would do for heater distribution).
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Old 12th April 2012, 02:09 AM   #9
Magz is offline Magz  United States
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OK, so as I mentioned in the first post, there is not agreement as to the "best practice". Maybe I'll try it both ways.
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Old 12th April 2012, 09:45 AM   #10
DF96 is offline DF96  England
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The two secondaries (or halves of a CT secondary) are magnetically coupled together, so a snubber on one alone will reduce ringing on the other.

If you want "agreement" on technical issues then you either need to find a forum where everyone thinks alike (but this does not guarantee truth) or be able to filter out those who don't understand the problem and then take a consensus from those remaining.
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