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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.

Discussions

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guyfrom7up
guyfrom7up

11 years ago

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!)

100_4449.JPG100_4451.JPG100_4452.JPG100_4453.JPG100_4455.JPG100_4458.JPG
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Plasmana
Plasmana

Reply 11 years ago

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.

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guyfrom7up
guyfrom7up

Reply 11 years ago

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.

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Plasmana
Plasmana

Reply 11 years ago

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?)

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guyfrom7up
guyfrom7up

Reply 11 years ago

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

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Plasmana
Plasmana

Reply 11 years ago

I think it is good enough.

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guyfrom7up
guyfrom7up

Reply 11 years ago

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

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Plasmana
Plasmana

Reply 11 years ago

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.

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guyfrom7up
guyfrom7up

Reply 11 years ago

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

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guyfrom7up
guyfrom7up

11 years ago

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

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Plasmana
Plasmana

Reply 11 years ago

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... :-(

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gmoon
gmoon

11 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.

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guyfrom7up
guyfrom7up

Reply 11 years ago

like by using a huge resistor?

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guyfrom7up
guyfrom7up

Reply 11 years ago

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

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tech-king
tech-king

Reply 11 years ago

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 :-(

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guyfrom7up
guyfrom7up

Reply 11 years ago

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

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Plasmana
Plasmana

Reply 11 years ago

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

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gmoon
gmoon

Reply 11 years ago

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....

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guyfrom7up
guyfrom7up

11 years ago

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

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Plasmana
Plasmana

Reply 11 years ago

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?

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guyfrom7up
guyfrom7up

11 years ago

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

0919081852.jpg0919081902.jpg
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Plasmana
Plasmana

11 years ago

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

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LinuxH4x0r
LinuxH4x0r

11 years ago

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

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Plasmana
Plasmana

Reply 11 years ago

Yeah, me too! :(

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guyfrom7up
guyfrom7up

Reply 11 years ago

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

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Plasmana
Plasmana

11 years ago

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.)

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GorillazMiko
GorillazMiko

11 years ago

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

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Plasmana
Plasmana

Reply 11 years ago

NST stands for "Neon Sign Transformer" !

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westfw
westfw

Reply 11 years ago

"Neon Sign Transformer" !