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Trouble scoping smps hot side
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sflsan



Joined: 15 Jan 2009
Posts: 6

PostPosted: Thu Jan 15, 2009 5:02 am    Post subject: Trouble scoping smps hot side Reply with quote

I have a peculiar problem that I have not come across before. When I power on the smps power supply and try to tie the scope ground to the hot filter cap negative to get the readings on the hot side, it sparks and blows the fuse. I am running through an isolation transformer.

The power supply started with a shorted fet and bridge rectifiers, but have been replaced. I have metered across the power leads and do not show a short.

I have used this method multiple times before with no problem, and this is the only method I know of to correctly scope the hot side, so I am curious as to why I am blowing fuses when connecting the scope ground to the hot ground (neutral). Also, the smps is auto sensing on the input if that matters.

Thanks for any help you can give.
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jts1957



Joined: 24 Nov 2008
Posts: 2472
Location: Far, Far Away

PostPosted: Thu Jan 15, 2009 5:48 am    Post subject: Re: The "hot side" Reply with quote

I've seen a similar effect twice. One tech had cut off ground pin of scope's power cord and had plugged it in "backward," and another tech had his scope and the item under test plugged into the SAME isolation transformer.
One of the "Variacs" we have at the shop is NOT isolated (must use a seperate very large isolation transformer).
I'm presuming appropriate types and mounting on replaced part(s).
Of course, there still could be a bad part in SMPS.
If I knew what you were working on (Brand/model, etc.) I might be more help.
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sflsan



Joined: 15 Jan 2009
Posts: 6

PostPosted: Thu Jan 15, 2009 3:26 pm    Post subject: Reply with quote

Thanks for the quick reply, that's all good info. The scope's ground plug is still intact, the mains wiring is correct, and it is not plugged into the same transformer as the power supply. That still doesn't mean I'm not doing something wrong though. I will still double check my setup just to make sure.

The power supply is out of a Sylvania 20" LCD TV, LC200SL8. The replacement parts were pretty much an identical match and are mounted and tied in correctly. The fet is in a full plastic casing, so no chance of an insulator problem there.
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torbjorn



Joined: 07 Jun 2007
Posts: 370
Location: Sweden

PostPosted: Thu Jan 22, 2009 1:42 pm    Post subject: Reply with quote

I have come across some isolation transformers where the secondary is not floating, but it has a mid tap that is connected to ground. Beware of those transformers! They are not intended for use in repair shops.
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sflsan



Joined: 15 Jan 2009
Posts: 6

PostPosted: Thu Jan 22, 2009 3:12 pm    Post subject: Reply with quote

The transformer I use lists 100% isolation under it's specifications. I have seen this spec on some before: Secondary neutral-to-ground bonding eliminates common mode noise, providing an isolated ground reference for sensitive equipment.

I'm assuming that's what you are talking about? If that were case, could the ground bond be safely disconnected?
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torbjorn



Joined: 07 Jun 2007
Posts: 370
Location: Sweden

PostPosted: Fri Jan 23, 2009 10:24 am    Post subject: Reply with quote

Yes, if you only are to use the transformer for repair shop/laboratory work, then it is ok to disconnect the ground bond. But in cases where a such isolation transformer is used to protect, for example, the power supply for a server room from transients, then the ground bond should be in place.
It might be prudent to mark the transformer with a big sign stating that the ground bond is removed.
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sflsan



Joined: 15 Jan 2009
Posts: 6

PostPosted: Thu Jan 29, 2009 1:22 am    Post subject: Reply with quote

That did the trick! Not sure why it wasn't listed in the specs, but once I cracked it open I noticed the ground bond, so I disconnected it and it worked perfectly. I'm the only one who uses it and only use it for troubleshooting in my shop, so it shouldn't be a problem leaving it disconnected. Thanks for all your help.
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jts1957



Joined: 24 Nov 2008
Posts: 2472
Location: Far, Far Away

PostPosted: Thu Jan 29, 2009 2:07 am    Post subject: Reply with quote

What happens to the next guy if you die?
I think that's why it might be good idea to label.
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sflsan



Joined: 15 Jan 2009
Posts: 6

PostPosted: Thu Jan 29, 2009 5:05 am    Post subject: Reply with quote

First off, how am I going to die?

And second off, who ever said I wasn't going to label it?
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Justmanuals



Joined: 21 Aug 2004
Posts: 1949

PostPosted: Thu Jan 29, 2009 6:14 am    Post subject: Reply with quote

sflsan wrote:
First off, how am I going to die?

And second off, who ever said I wasn't going to label it?


Well, how about if you're out sick one day and some unsuspecting geek comes along and uses it?
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