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Old 27th June 2004, 07:52 PM   #11
Pjotr is offline Pjotr
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Hi,

Those transient suppressor diodes should not be used directly across the mains. They can stand a lot of current and are rated in watts, not in joules. They can actually absorb not many joules, so they can take the transient current only for a very short time. These diodes can only be used AFTER a proper mains-filter that has also a proper transient protection varistor that takes the main bulk of the transient energy.

Probably there was originally a mains input filter in the apparatus your PSU came from.

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Old 27th June 2004, 08:15 PM   #12
imix500 is offline imix500  United States
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Pjotr, there is a pretty extensive filter section before the device in question and the main rectifier bridge. 2 large inductors with dual windings that the ac passes through, a large ac cap, and 3 devices that are either MOV's or caramic disk caps or a combination of the two. I think it might be PF corrected, but I'm not sure. This supply is from 1996 and I don't know if that was the norn then as it is today. Thanks!
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Old 27th June 2004, 08:32 PM   #13
wrenchone is offline wrenchone  United States
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the component in question is most likely a bidirectional transient suppressor diode. These are fast-acting, but their energy ratings are pretty poor. They tend to fail short, and this would account for blowing the fuse. You could substitue an appropriately rated ZnO varistor for the supressor diode for improved ruggedness. this approach is pretty much industry standard these days. If there is a substantial amount of filtering ahead of the suppressor element, you don't need to worry so much about having a fast-acting suppressor, as the filter will blunt the rising edge of the transient. A 14mm varistor should do the trick, and it will have a much better energy rating. It will eventually fail short, too, but they are easy to find and replace.
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Old 28th June 2004, 04:47 AM   #14
imix500 is offline imix500  United States
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Thanks for all your help guys. I was gonna chuck this thing but I figured I'd throw what I knew up here and see what came back. This forum has saved me from spending considerable $- again. Wrenchone, is ZnO a certain type of MOV? I've been looking through data sheets but I don't see the juction material (I'm assuming thats what the metal oxide is) on them. I'm looking at littelfuses' UltraMOV line. Is this the type of MOV you were talking about? Thanks.
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Old 28th June 2004, 01:17 PM   #15
Circlotron is offline Circlotron  Australia
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Quote:
Originally posted by imix500
Pjotr, there is a pretty extensive filter section before the device in question and the main rectifier bridge. 2 large inductors with dual windings that the ac passes through, a large ac cap, and 3 devices that are either MOV's or caramic disk caps or a combination of the two. I think it might be PF corrected, but I'm not sure. This supply is from 1996 and I don't know if that was the norn then as it is today. Thanks!
It's a filter to stop any HF switching noise from the DC/DC converter section getting back out onto the mains and causing radio inteference. Must-have stuff nowadays at least.
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