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Re: Neon sign transformer power: a new look

Posted: Fri Jan 10, 2020 1:22 am
by Joshua Guertler
Greetings,

If I find an NST that is secondary midpoint grounded, would I be able to cut the ground on the secondary to make it operate in a way more desirable for this set up? Thank you.

Re: Neon sign transformer power: a new look

Posted: Fri Jan 10, 2020 5:27 am
by Rich Feldman
We don't know enough about "this set up" to know. What do you think would be gained by disconnecting center tap, if that were possible?
You could learn a lot about NST's by searching and reading for a few hours. This forum site is a good place to start.

Re: Neon sign transformer power: a new look

Posted: Fri Jan 10, 2020 9:16 pm
by Richard Hull
To remove the center tap, you must tear the case apart, but first, short the two knobs together and turn the transformer on for about 1 hour. This will heat the transformer tar enough so that you can rip the metal case apart completely. you will wind up with a gooey tar lump that can, with a lot more effort than it takes to say, be removed from the transformer. Dip the black, filthy, tarless transformer into a 5 gallon pail of kerosene over night. Wipe it off carefully and once you examine the frame and windings, you will see two braided wires off of the two HV coils going to a riveted lug on the core's frame. (these ground each winding) take them loose and insulate them well and join them together, well insulated from the core. They want to arc to the core as they are near the core, (outside HV windings, near the core), because they are at half the potential of the total voltage. The entire job should take about two days with a number of cuts from sharp jagged metal tearing the steel case off the tar block and just might work without arcing. Do this outside and buy a lot of hand cleaner, bandaids and bactracin..... it is a filthy business. Finally the whole transformer must be placed in oil in a suitable plastic tub prior to powering up.

I have done this to about 6 large 60 ma transformers in my Tesla coiling days. I was younger and eager then, and this entire effort is a fools errand. It can be done, but only by the bold and stupid. See how easy it is to lift the grounded center tap from the core? it's a snap!

Richard Hull

Re: Neon sign transformer power: a new look

Posted: Mon Jan 13, 2020 10:26 pm
by Joshua Guertler
Greetings,

I recently got a 9 kV 30 mA NST with an internal resistance of 9.07k Ohms (not running). If I use a similar set up, what voltage output should I be expected, assuming that the current is roughly 20 mA? Thank you.

Sincerely,
Joshua Guertler

Re: Neon sign transformer power: a new look

Posted: Mon Jan 13, 2020 10:50 pm
by Rich Feldman
Read Richard Hull's "neons" document, in a recent FAQ.
Or read this thread and scale the numbers for your NST nameplate. Too bad if pictures can't be seen without opening them. Check images with IV in name, near top of second page (?).

Or make a kV meter and 20 mA dummy load, which you need anyway, and measure it yourself. Get hands dirty, the sooner the better.

Plain old NST's are an exception to general rule: "avoid intentional arcing with HV power supplies, unless arcs are your main objective".
Arc at will, starting with a wire well attached to NST case, brought to touch one HV terminal and then backed off.
Next exercise: add a sense resistor in series, and measure the AC current under short circuit and arc conditions. You will need to do that sort of thing anyway.

Re: Neon sign transformer power: a new look

Posted: Tue Jan 14, 2020 2:01 am
by Richard Hull
With the full wave circuit Rich shows, you might expect about 5.6kv or less, at 20ma. Just a guess of course. Doing the math....1.414 X 4.5 = 6.36kv ideal, no load and filtered. This ideal value will drop under load.

Richard Hull