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Old 24th September 2004, 08:15 PM   #1
Prune is offline Prune  Canada
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Question Rectifier ringing snubbers

I know this has been mentioned here before:
http://www.hagtech.com/pdf/snubber.pdf

The problem is, how to guesstimate values for the snubber for those that can't measure transformer and diode parasitics because they don't have a scope?
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Old 24th September 2004, 08:35 PM   #2
sam9 is offline sam9  United States
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I suspect that the performance difference between "optimum" and "approximate" is not large. Facing the same problem as you I've used C=.01uF and R= 1kR -to- 2KR. I may be totally wrong but I think these values will get one close to ideal under a wide range of circumstances and in any case much better than no snubbing or even C-only snubbing.
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Old 24th September 2004, 09:13 PM   #3
Prune is offline Prune  Canada
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On a 40 VA transformer, my C was similar to yours, but the R was more like 100 Ohms...

However, now I'm faced with very different iron -- 1.5 kW microwave oven transformer with a rebuilt core (and removed shunts), and I have a feeling it's different enough that these values are useless.

Wouldn't there be any way to perform a measurement to see if given values are helping without an oscilloscope or inductance/capacitance meter?
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Old 24th September 2004, 11:09 PM   #4
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Prune,

You can buy an o-scope good enough for what you are wanting to do on ebay for $20. Try Vancouver surplus electronics stores. Is Rendell Parret Electronics still there on 4th ave? I lived there in the 60's to early '70's. There was also The Ham Shack just south of the Granville St. bridge on the east side. Better yet go to a Hamfest. There must be lots of those in the area. Do a google on "Hamfests British Columbia" or "Hamfests Vancouver" to find one.
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Old 24th September 2004, 11:29 PM   #5
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Default Rendell Parret

Rendell Parret is now RP Electronics and had a shop at 4th and Arbutus. The parking around there is horrid and they do not have anywhere as much surplus as before. They also have a shop near BCIT, near Lougheed Highway and Winnlingdon. I've moved to LA now and since I moved, I heard they may have moved the whole operation to the new site.

The surplus business in Canada has never been the same since Vanguard in Edmonton closed in 1979.
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Old 25th September 2004, 01:51 AM   #6
Prune is offline Prune  Canada
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Speaking of which, I've been looking at scopes on eBay, but my problem is that I don't know which ones are any good. For example, if I'm dealing with digital audio also, how many MHz do I need? S/PDIF is between 12 and 13 MHz, so would a 20 MHz scope be any good? What sensitivity in mV, and what other features/specs should I look for, or what kind of probes? For example, I've seen a 20 MHz with 5 mV sensitivity and dual trace sell for $100, but I don't know if this is good enough for audio work or whether the price is any good.
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Old 26th September 2004, 01:00 AM   #7
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Buying a scope on Ebay is a very good idea, but beware. Get one from someone that says it works as it should, not I don't know how to test it, but it makes this line.

For digital, you may want to look at something 100mhz. Also beware. probes can cost $50 each, so be sure to factor that in on your bidding. I would budget $200 for what I think you need.
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Old 26th September 2004, 01:36 AM   #8
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Prune - a slightly simpler way of looking at snubbers is to realise that in essence, it's an AC-coupled resistor. All the damping comes from the energy of the ringing being dissipated in the resistor. The cap functions to reduce losses in the resistor at the line frequency - you could use a resistor alone, but imagine the waste heat...!

Obviously, the bigger the cap, the lower the frequency the damping is effective from. 100ohms is about right for the resistor, so you are left guessing the strays to work out how big to make the cap. If you're winding your own secondary on a big core, I'd make the guess conservative and go larger with the caps, assuming the ringing could be at a lower frequency than your previous experience; try 0.047uF maybe.
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Old 26th September 2004, 04:14 AM   #9
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My secondary is the MOT output, 2.1 kV AC RMS, into a bridge rectier. Would the values mentioned above make sense for this situation too?
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Old 26th September 2004, 01:06 PM   #10
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Help! At those sorts of voltages you're going to *really* need to know what you are doing. Even a small cap will be storing a lot of energy; probably need parts rated in excess of 4KV as well. That said, current is going to be low, so maybe you don;t need snubbers at all.

Sorry, but it's well outside my experience.
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