NST showing some weird readings.

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Tyler_Johnson
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Joined: Tue May 21, 2024 9:25 pm
Real name: Tyler Johnson

NST showing some weird readings.

Post by Tyler_Johnson »

I am running tests with an EE on my NST before finishing my electrical system, and we have gotten some weird scope readings. With a multimeter, from either terminal to center tap, the circuit is open. The same is true for terminal to terminal. We get a voltage step-up on the scope, but both terminals are in phase (which I read on the FAQ’s is possible with a capacitor, but combined with the other factors seems strange). We are guessing there is a break in the secondary winding, but neither me nor the EE are exactly experts in NST’s so we figured we should ask here before declaring it dead. Is this normal, and if so, how does it work? Thanks!
Tyler_Johnson
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Joined: Tue May 21, 2024 9:25 pm
Real name: Tyler Johnson

Re: NST showing some weird readings.

Post by Tyler_Johnson »

Nevermind, we figured this out. Each winding has its own grounding wire, which connects inside of the SGFP to a yellow wire, which runs back through the tar and into ground. Since we took out the SGFP, they were no longer connected, causing the open circuit.
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Rich Gorski
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Re: NST showing some weird readings.

Post by Rich Gorski »

Tyler,

Glad you figured it out.

FYI, I have a 15000V, 60mA NST and I just measured the DC resistance between the HV terminals as 5400 ohms and between center tap and each side of ~2700 ohms. I don't believe my NST has a GFP inside. Its made by Magnetek / Jefferson. Got it for $40 from a neon sign repair shop a few years ago.

Rich G.
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Richard Hull
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Re: NST showing some weird readings.

Post by Richard Hull »

Back in the Tesla coiling days (1980's AND 90's) we located an old neon shop. There was a pile of dead transformers outside in a large pile. The owner said we could take all we wanted. We retrieved about 40 neon xfmrs. Most all had one half winding dead. We just took two of the same rating, grounded the cases and took full voltage from the xfmr's hot, working insulators. Lots of nice tesla coils were made from that zero cost batch.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Rich Gorski
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Real name: Rich Gorski
Location: Illinois

Re: NST showing some weird readings.

Post by Rich Gorski »

Tyler,

This link from RTF Technologies might be of interest to you. It shows images of disassembly of a NST to create a high voltage rectified power supply with a current monitor circuit. Note that the link also shows that the center tap has been disconnected from ground so that when one end of the secondary is connected to ground the full voltage can appear on the other end. Remember that when the center tap is ungrounded 1/2 the full voltage of the NST will appear at that connection so it must be well insulated from the case (the link talks about this).

http://www.rtftechnologies.org/emtech/nst.htm

Rich G.
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