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I got a NST! Answered

I recently bought a 11kv, 30mA Neon Sign transformer. I can get nice arcs off of it and everything (maybe about 2 inches? when rectified it makes shorter high pitch arcs), but I'm wondering about protecting it. after a couple hours of playing with it there was a loud pop in the case and it stopped working. I opened it up (it was held together by rivets, I just drilled the rivets out) and it's a switched mode neon sign transformer. I thought it was a bit light! So I found a (transistor? To-220 case) that had it's face blown off, luckily I found the other piece and I'm going to order the component and replace it. So, is there a cheap easy way of preventing this happening? While opening the case I added a number of saftey features, a saftey switch, and a "ON" indicator.


Here are some of the inside Pics you can kinda tell where something is missing the left side of the circuit board is kinda messy since I added a few saftey features, such as a status light and a saftey switch (before it was just a pull light switch, and you didn't know if it was on or not until you plug it in, scary!)


Nice! The transformer looks almost like a flyback transformer. And the circuit is interesting, it uses a GDT, what all solid state tesla coils uses.

thanks, the only reason I think this blew is because I used no NST protection, so I think switch mode neon sign transformers are fine, and the same thing would have happened to a regular one, it's just that this is cheaper to fix. I think the huge inductive kick back just blew the mosfet. Hopefully when replaced it'll work, I'm going to put in RF filters and a saftey sparkgap.

Yeah, you might be right about the inductive kick back theory... Good luck making a filter for your NST! (BTW, what type of filter are you going to use?)

I'm just going to use 2 inductors and a spark gap, any advice on the coils?

any advice on the coils? I don't know how to make a HV inductor...

It is really easy to make HV inductors... You wrap a few hundred turns of enameled wire on a 1 inch diameter and a few inches long PVC pipe. It is really like making a tiny tesla coil.

the transistor is getting 10 cousins from singapore, riding third class!

well, replacing the transistor didn't fix anything... sigh

Are there any other transistors in the circuit? If so, maybe one of them might have blowed.

Or even worse, did the transformer short out? Check with your mulitmeter... I really hate it when my flyback transformers short out... :-(


9 years ago

Offhand, I'd guess that arcing through air is passing more current than the neon gas... Maybe limiting the duty cycle will help.

like by using a huge resistor?

Or a hv capacitor in series with the output? Or can I do something to the input of the transformed to limit the current?

to limit the output voltage, your best option is and inductive load. that way, you control the current without losing voltage, like in a resistor. ps: so lucky you have a nst pps: this new keyboard sucks :-(

seems easy enough, just have to find some HV wire or something

You can use enameled wire, I use that type of wire for most of my high voltage projects.

A resistor would certain limit the current (but lessen the arc length?) I was thinking you just held the arc too frequently and too long, and the transistor overheated and died. Beats me if rectifying it might contribute to the problem....

another peculiar thing that I might need an answer to. The transistor that blew was an IRF830 There are 2 mounted side by side. The left one blew Should I replace both? Their drains arn't connected to anything, only their gate and source are connected to the circuit board, and I know it's not connected through the tab because it's mounted on mica between it and the heatsink. Is that right!?! btw, I found a good price on them, so yay, ordering

Replace the transistor (It is actually a MOSFET) that blew up, the other one should be fine. You don't have to replace both of them, unless they are both blown.

According to the datasheet, the lead that you think it is "drain" is source.

Are you sure that the tab is insulated by mica from the heatsink? Did you check the resistant between the MOSFET's tab and the heatsink with a ohm meter?
If the resistant is Infinite, then there must be a wire connected to a screw and the nut.

Also, does the MOSFET's get very warm when in operation?

I'll post pics later, but here are some arc pics that I snapped on my phone it was working for about 2 hours then I got a huge spark, then shortly after a huge pop in the case


Can you also please post some pictures of your switch-mode NST, I would like to see the inside! :)

I want an NST so badly. But only a transformer, not a switching one

I thought it was just a transformer when I bought it, but meh, it was only 30 bucks (and that's including shipping!)

Okay, I am so jealous. You don't know how bad I want a NST...

Oh, wait a minute, did you said it is a switching type? Oh, no, I am not jealous, I have one of those (A homemade one, I call it a flyback transformer driver. I want a "real" NST.)

Hey, guess what?! I don't have any idea what an NST is!!!! :D