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NST death?

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oucivileng

Fri Feb 27 2009, 10:22PM

Registered Member #1772 Joined: Tue Oct 21 2008, 05:23AM
Location: Athens, OH
Posts: 71

I've built a very simple SGTC and have been rebuilding things and improving them as I learn more about this stuff but think I have a bit of a problem. I built a new safety gap yesterday and was trying to set it but am having trouble getting it to arc. I'm using three drilled and tapped brass balls on the ends of steel bolts for the safety gap. To set it, I'm connecting it by itself to the NST (15kv/25ma). To get any spark at all, I have to turn it on and off repeatedly (not rapidly, I wait a few seconds before i turn it on again). Most of the time I just hear a buzz and nothing happens. After about the tenth time I turn it on I'll get a spark. If I shut it off, I have to go through this all over again to get another spark. I know the balls are close enough that it should be arcing.

This has been a minor problem on occasion with the entire circuit connected but seems to be getting much much worse lately. I really haven't used the NST for very much although it was used when I bought it. Is this a sign that it's dying?

Arcstarter

Fri Feb 27 2009, 10:47PM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

Not necessarily dying, perhaps it has a built-in GFI? This would likely turn it off when overload and stuff like that are present. Take the case apart (depending on what kind) and if there is a circuit board, that is the GFI. Send pics if possible if you can take it apart.

oucivileng

Fri Feb 27 2009, 11:01PM

Registered Member #1772 Joined: Tue Oct 21 2008, 05:23AM
Location: Athens, OH
Posts: 71

I know there's no gfi. It's got a secondary midpoint ground. I'll see if I can get a pic.

oucivileng

Fri Feb 27 2009, 11:05PM

Registered Member #1772 Joined: Tue Oct 21 2008, 05:23AM
Location: Athens, OH
Posts: 71

pics...

Proud Mary

Fri Feb 27 2009, 11:44PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

You could start by testing all the windings with the ohmeter. Are the resistances between the ends of the secondary and the centre tap equal, or nearly so? Is there any leakage between primary and secondary?

Any faults detected by this simple test mean something serious has gone wrong. Unfortunately, it will only detect the more obvious kinds of faults which show up at low voltage.

If you could tell us what test equipment you have, we may be able to devise a much more thorough diagnostic procedure for you.

oucivileng

Fri Feb 27 2009, 11:53PM

Registered Member #1772 Joined: Tue Oct 21 2008, 05:23AM
Location: Athens, OH
Posts: 71

I have a multimeter and that's about it. I'm at school right now and most of the equipment I've been using is not here. I can probably use some of the university equipment but not until monday at the earliest. Is there anything I need to know about opening this thing up?

Proud Mary

Sat Feb 28 2009, 12:47AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Aha, a young man in a hurry!

You know it's not impossible that there is a thermocouple safety cut-out inside the case, which would completely explain the behaviour you've described.

But, frankly, without instruments, anything anyone says here is going to be guesswork.

Measurement is at the very heart of all we do!

oucivileng

Sat Feb 28 2009, 12:53AM

Registered Member #1772 Joined: Tue Oct 21 2008, 05:23AM
Location: Athens, OH
Posts: 71

So Harry, if the problem is a thermocouple, how can I identify it and what can I do about it? What instruments would I need besides my ohmeter?

Ultra7

Sat Feb 28 2009, 01:05AM

Registered Member #1157 Joined: Thu Dec 06 2007, 12:11PM
Location: Houston, TX
Posts: 307

If you have a chicken stick, attach a wire lead to one of the NST HV terminals and use the chicken stick to attempt to draw an arc to the case. It should arc to the case as close as 7 cm. If that checks out, then attempt to get it to arc to the opposite HV terminal. That arc should jump out as close as 15cm. if this all checks out, disconnect it and do the same thing with the opposite HV terminal.
It is possible that one of your HV windings has fried.
It is also possible that while the case shows no GFI circuit, there may indeed be one in there anyway.

Do you have photos of the safety gap?

Edit: Egad, Change all measurements of cm to mm, I suck at the metric system.

oucivileng

Sat Feb 28 2009, 01:43AM

Registered Member #1772 Joined: Tue Oct 21 2008, 05:23AM
Location: Athens, OH
Posts: 71

Ok, I was able to draw arcs with the stick without too much trouble. I did figure out that one of my lead wires might have some issues though. I swapped it out for another wire and things ran a little better but still not the way they used to. I figured out that I can draw an arc in the spark gap as long as i help it get started with a nail on the end of a stick but it bothers me that I didn't have to before.

No, I don't have a picture of the safety gap right now. It's just two steel bolts (+ and -) held horizontally by some acrylic with a vertical bolt to ground in between. Each bolt has a 3/4" solid brass ball on the end.

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