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Rectifying my NST?

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Dr. Slack

Fri May 16 2008, 09:09AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

$8 a packet for 100

exactly, which is why their sharing specs are non-existent, and why it is feasible to derate their reverse withstand voltage so much and still have an economic solution

Is there a limit as to how many you can wire in series

theoretically yes, as dropping a forward 0.7v each, you will eventually run out of output with a long enough string, but it's silly to build a string that long. OTOH, what can happen is that you may grab one of these 50-long strings for a quick experiment with rectified mains, and then the forward drop is going to lose a significant fraction of your output voltage. Moral, only use high voltage strings for rectifying high voltages.

A figure of merit for a high voltage rectifier diodes is reverse withstand / forward drop, so it doesn't matter much whether you have 500v/0.7v or 400v/0.7v, you are still losing waaay less than 1% of your output voltage to diode drop.

When a few of the diodes have failed short circuit due to turning off before their string-mates and seeing the full string reverse voltage for a moment, the efficiency will improve. That's the beauty of their usual short circuit failure mode, you don't know they've failed (if you start with a long enough string), and you don't need to know.

Do what you can to match them when building a string. If the diodes arrive bandoliered, that's good, they're very likely from the same batch. Never mix diodes in a string from different manufacturers, or if their markings are different types.

Steve Conner

Fri May 16 2008, 09:22AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

That looks like one of the resin potted NSTs, they're popular in Europe, in fact I have a similar one. They're made by companies like Masonlite and F.A.R.T (seriously...

On my Masonlite transformer, the (e) terminal is the centre tap of the HV winding, as far as I know. It's hooked up to the ground fault protection module.

lpfthings

Fri May 16 2008, 09:28AM

Registered Member #1361 Joined: Thu Feb 28 2008, 10:57AM
Location: Cairns, Australia
Posts: 305

Yeah its a F.A.R.T resinblock. Do they have built in ground protection modules?? I removed the OCM when i got mine, but thats about it.

Steve Conner

Sat May 17 2008, 08:17AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Mine had a little circuit board under the lid, connected by a bunch of screws. I assumed it was a ground fault monitor because of how it was connected.

I left it in place, since I was actually running a neon sign (counts as abnormal use of a NST around here :P) but it would be easy to remove. I don't think they'd embed a protection circuit into the resin or anything.

What is an OCM anyway?

lpfthings

Sat May 17 2008, 10:39AM

Registered Member #1361 Joined: Thu Feb 28 2008, 10:57AM
Location: Cairns, Australia
Posts: 305

Open Circuit Monitor. How did you get the lid off? it looks like its all just been molded together?

Backyard Skunkworks

Mon May 19 2008, 12:38AM

Registered Member #1262 Joined: Fri Jan 25 2008, 05:22AM
Location: Maryland, USA
Posts: 451

You can get the 1N4007s WAAY cheaper then $8 for 100, I got 500 for $8 from mouser. Radiosh** charges like $0.25 each or something...

HV Enthusiast

Mon May 19 2008, 02:54AM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

Center taps of those NSTs are typically tied to earth ground (case). If that is the case, than you won't be able to get your 15kV rectified output properly. You'll only get 7.5kv rectified to ground.

lpfthings

Mon May 19 2008, 05:47AM

Registered Member #1361 Joined: Thu Feb 28 2008, 10:57AM
Location: Cairns, Australia
Posts: 305

Yeah, i found that out kinda. I put the 2 strings of 48 diodes on each of the HV terminals, and it arcs, and its diffinately 25hz, but i dont get a full arc length. I need to get these diodes mounted on something, they keep breaking at the joins >.<

Dr. Slack

Mon May 19 2008, 07:15AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

you won't be able to get your 15kV rectified output properly. You'll only get 7.5kv rectified to ground.

Yes, well, 10.5kV peak rectified from 7.5kV rms.However, if you've got both a +ve and a -ve output from the rectifiers, you will have 21kV between the two outputs.

I need to get these diodes mounted on something, they keep breaking at the joins >.<

I'm not sure I'd recommend this unreservedly as it takes a lot of time, and might require stuff you don't happen to have to hand (stuff that once you've got it to hand, you'll wonder how you did HV without it). Put a generous dob of hot-melt glue on each solder joint, then slip the whole string inside some heatshrink tube, best to use transparent. Starting at the bottom, shrink the tube and melt the glue. Move up slowly, letting the shrinking tube squeeze the melted glue into all the voids - gotta judge the amount fairly finely. The result is fairly robust, insulated and corona-suppressed, and less messy than the oil-filled plastic tubes I have seen suggested.

BTW, when using diodes on a transformer, make sure you draw both -ve and +ve half cycles from any output terminal. A single polarity of diode on one terminal will draw DC from it, and will saturate the trasnformer.

Steve Conner

Mon May 19 2008, 09:02AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

wrote ...
How did you get the lid off? it looks like its all just been molded together?

These NSTs have a plastic lid that covers up all the terminals, held on by a couple of screws. There are two metal blades sticking out of the lid, that go into those switch thingies in the middle, isolating the transformer from the line when you take the lid off.

You probably never got the lid with your NST, and there is no way that I know of dismantling it any further. The resin potting won't melt in an oven like you do with the traditional American tar potted ones, and it's pretty damn tough too. I tried once, and couldn't get it off even with a blowtorch and a 10lb sledgehammer :(

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