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Registered Member #135
Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735
Okay, so I needed a transfromer immediately. I've been waiting for my ebay stuff, but I couldn't wait a moment longer. I had been put in the position of having to demonstrate my recent coil for CSULB Kaleidascope event, and wanted a crowd pleaser. So I swallowed the bullet and got a GFCI NST.
This was a big risk, because the manufacturer could have easily potted the whole thing under tar, or worse, resin or epoxy, perish the thought. So I took the gamble and it paid off.
Here are the conversion pictures, man, I thought I was going to break a sweat because it was sooo difficult. It was almost as hard as changing a light switch.
Note: The GFCI did not interfere with a Jachob's ladder or drawing an arc from midpoint to hot. The GFCI references chassis ground and midpoint gnd for its fault detection. It does not sense the hot leads from the transformer, just variations in gnd potential.
Note 2: I attached a resistor between chassis gnd and midpoint to make sure there was absolutely NO current loop. This is very important! There shouldn't be any current, but you never fully know what someone has done in design. Had there been a current loop, I would have had to figure out some way to maintain the two different gnd potentials, most likely with a capacitor or resistor.
Anyways, this is a Transco 12/60, cost $140. I got a look at the price guide while the girl was off somewhere. Franceformers are really expensive now! $240 for a 12/60 Yikes! So don't be afraid of the new ones unless you know its all potted together.
I also did a flame test on the potting material. It is soft and pretty flammable. I believe it to be a polyester resin. If the need arises, I will have to see about melting it out.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Well, I would still remove the GFCI module anyway, if I was using the NST with a Tesla coil. It's just one more thing to go wrong. I guess the only thing is, it might be useful as a canary. If it dies, it means there's a lot of RF around and your NST secondary might be next?
Why didn't you just solidly ground the midpoint to the chassis? That's how it's done in regular non-GFI NSTs
Registered Member #316
Joined: Mon Mar 13 2006, 01:30PM
Location: Marietta, GA
Posts: 212
I'm no expert, but I think if you have a line filter on your NST you may trip a GFCI because of the noise that gets filtered out and grounded. This happened with the GFI sockets in my garage a lot. It might have been something else, though
Registered Member #135
Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735
In the spec you're not allowed to gnd their midpoint. It's at a 50V potential above chassis. Grounding this would short their current detector and most likely blow out the GFCI from the current loop.
I did ground the midpoint to chassis after GFCI removal, Brown to Green.
Registered Member #127
Joined: Fri Feb 10 2006, 03:36PM
Location: Cincinnati, OH - USA
Posts: 44
I have a couple Transco NSTs I have been using for my TC. Both are GFCI protected and completely unmodified. I have never had them kick off (except when I deliberately shorted one HV lead to ground) despite considerable abuse (no filter for a while, no safety gap, etc.) But it is good to know what you did to rig it in case the need ever comes up. Thanks Hazmatt.
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New NST to Old NST
