snubber cap question

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Hi,

I've been thinking of adding some snubber caps to my PS, across the rectifier, but I'm unsure what type of caps to use.

I have seen a few PS designs that use .1uF caps, but they don't specify the type. I need something 100V or higher I think, but then possibly even more than that (rail voltage +- 63V)....

The only easily obtainable caps with voltage 100V and .1uF are MKT and I'm not sure these are really suitable. (I'm pretty sure electros are a no go!)

I found that WIMA make caps specifically for snubber duty, these would I guess be ideal, but I can't source them.... (adilam lists them but has no stock).....

Any Suggestions?

Tony.
 
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Thanks, Now I just have to find some High voltage ceramics, all the ones I could find (in high capacitance values) were 50V :(

Ummm, cause I figure it can't hurt :).... I have some spurious noise in my amp, that I've been trying to get rid of for years, but haven't succeeded, thought it was worth a shot. (note that this noise is really quite low level (S/N is about 91db at least), but it bugs me!

edit: Hmmmm I had wondered about xover caps before, but thought the size might be a bit of a problem, if I can get the ceramics, I think I'll go with those.

Tony.
 
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Thanks Prune, That article is pretty heavy going will take a while to digest, but at least I now know it's not just a simple matter of chucking a few caps on :)

I also found a few threads where people advised against using snubber caps because they made things worse, could be they should have read the article you posted :)

Tony.
 
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Just using a capacitor lowers the ringing frequency but does not dampen it. Note also that the two capacitors and a resistor network discussed is across the transformer secondaries, not each rectifier.

The trouble with the article is that it is hard to get data on the transformers to plug into the calculations, and the alternative measurements are hard to perform; some trial and error and an oscilloscope is probably the easiest way to do it.
 
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Sch3mat1c said:
You only need one cap...which would allow the RF hash through anyway!

It's more than just "RF hash".
Quoting the article:

Ringing occurs in AC-DC supplies when the diode is just turning off....the previous example we had a natural ringing frequency of 560kHz. This is much higher than the audio band. However, it is modulated by the power line frequency. A half-wave bridge will "buzz" at the line frequency and a full-wave bridge at twice the line frequency. The ringing frequency acts as a carrier to couple with other circuits. To make matters worse the capacitance of the diode is nonlinear so the ringing frequency will contain plenty of harmonics....the circuit is physically large...resulting in a significant loop area. Coupling mechanisms to other circuits are both magnetic and capacitive.

Or an R+C (Zobel network) of appropriate values across the PT secondary.

The article has a Zobel network paralleled with another capacitor, which shows better performance in the simulation when correct values are used (fig. 10 RC snubber vs fig. 12 RC&C snubber).
 
I see no article besides what you quoted. Besides, just because it's on the internet doesn't mean it's correct. In fact, by that logic I could write some HTML, throw it on my site and you'd call it true... hmm I need to do that to some newsgroups :D :D :D

Erm internet rant beside... putting a C across it, depending on circumstances, will either dampen it (shunt the hash) or reduce the frequency. So of course it works at least as well, if not better. (If it only drops the frequency, then it will work as well as without the bypass\\\\\\tuning cap, as long as suitable values are used in the snubber.)

Tim
 
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I pretty much glossed over all the calculations thinking I'd need to look at it again when I had enough time to try and comprehend it (math isn't a strong point!).

I'll have another read and maybe make some guestimates of values I would need.

Tony.
 
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Looking at it again, I noticed that in his example C_eq is dominated by the second capacitor, C_x, so if your C_x also is far larger than the transformer interwinding capacitance + diode capacitance, then you don't need to measure the latter two. Of course, this begs the question of how to choose C_x, and I didn't figure out in the article why he picked 0.01uF.
 
It was not written, i think, but my feeling was that that value got the ringing down to almost audio band where it would be less harmful and then he damped it vith the rc network.
I found the article to be interesting and i fiddled with spice somewhat to try it out and it seems to be correct, i must admit though spice seemed to think the problem was even bigger than the article suggested. I got ringings that were as high as the ac signal itself. Probably a modelling issue of the rectifiers or the transformer.
Does anyone have some approximate values for the parameters of the transformer? I took the ones in the article but i have not found any other sources.
 
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I was actually thinking about trying to use speaker workshop, in impeadance bridge mode, to have a go at measuring the capacitance and inductance of the transformer.

The author of SW claims it's good for measuring coils and caps, and the article does mention using an impeadance jig (at least I think it did)...... Could be the way to go.

Tony.
 
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