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Old Jan 19, 2011, 04:31 PM
Wayne Giles is offline
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Rugby, UK
Joined Feb 2007
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Originally Posted by akschu View Post

That sounds like a good idea. I'm assuming that they would be put in like this:

GND -----i>| ----- DC -

How about this one?

I'm not sure how to size these things, I'm thinking it would probably burn out of there was a transformer failure, but perhaps not before the power supply shuts down.

Can you recommend one that will work?

I am going to order a few power supplies and build a new updated version for myself and a few extras for others that want them. But before I place my parts order, I would like to nail down exactly which diode to use.


Yes that one (33V) or the 28v version should be OK and I agree with connecting it across the 24V rail and retaining the output earth connection on the lower unit. Or you could put a 15V one across each 12V output.
You obviously understand the situation, so even this is not a 100% fix, it must depend on the resistance of any fault path. The lower the resistance, the bigger the zener capacity you need.
If you have an earth leakage trip on the mains input it does not need to be larger, but with a hard short and no trip who knows? We are talking about remote chances but that's what safety is about. There is no guarantee; specifically from me! But it would make it safer.
Just retaining the output earth on the lower unit would help; it's having completely floating outputs that would make me uneasy.

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