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How to protect a TIG welder rectifier

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Daedronus

Mon Nov 12 2012, 12:33PM

Registered Member #2329 Joined: Tue Sept 01 2009, 08:25AM
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Posts: 370

For my flash lamp laser starter I use this:

and a couple of small HV capacitors.

Essentially it was the same thing as a TIG HF, it injects a HV pulse in a low voltage circuit.

Rick

Mon Nov 12 2012, 08:22PM

Registered Member #4220 Joined: Sun Nov 20 2011, 12:11AM
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Steve: You suggest a cap across each diode, whereas most schematics I've seen only show one across the rectifier output. What would be the advantage of the four caps?

Daedronus: It sounds like the your system creates a one-time spark, not a current, like a propane ignitor. I wonder if that would suffice for my application- just to initiate the arc? It'd certainly be much simpler.

Ash Small

Mon Nov 12 2012, 08:32PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Personally, I'd just wind a choke on the core you have and test it, along with your capacitor, to see if it blocks the HF. (or most of it)

This can be done without the rectifier being connected.

Caps across the diodes will just transfer the problem elsewhere, into the buzzbox or whatever.

TVS's across the diodes 'may' save the diodes in the event of capacitor failure.

Steve Conner

Mon Nov 12 2012, 08:51PM

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

Ash, if you wind the choke so the fields cancel, that's a common-mode choke. Useless for stopping differential-mode noise (the kind that the HF arc starter generates) because the choke's inductance also cancels as far as the differential mode is concerned. You'd be as well using an air-cored choke, because the magnetic core never sees any of the differential-mode noise.

A capacitor across each diode won't really do anything that a single capacitor across the output wouldn't, except maybe make you feel better.

Ash Small

Mon Nov 12 2012, 09:11PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Steve, It's ten years since I wound mine, but at the time (before 4HV) I did A LOT of research on the internet, especially into chokes.

Any inductor will impede a change in current, by generating a magnetic field.

As there is a simultaneous pulse on each winding of the choke, these can be wound to either add, or cancel.

Do you have any reliable links on this subject, as I wouldn't want to give Rick any misleading information?

(I built mine with a lot of 'trial and error'. It worked, so I left it as it was. I would like to understand more about the theory. The beauty of this circuit, to me, is that it has both the high pass filter (the air core xformer) and the low pass filter (the choke).

2Spoons

Tue Nov 13 2012, 12:47AM

Registered Member #2939 Joined: Fri Jun 25 2010, 04:25AM
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- a common mode choke has differential impedance too, as there is inevitably leakage inductance - sometimes designed in on purpose.
- referring to Ash Small's schematic : the 200nF cap is essentially a short at HF, so what the choke sees is largely common mode signal. This same approach is commonly used in EMI filters for mains.

Ash Small

Tue Nov 13 2012, 01:03AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

2Spoons wrote ...

- a common mode choke has differential impedance too, as there is inevitably leakage inductance - sometimes designed in on purpose.
- referring to Ash Small's schematic : the 200nF cap is essentially a short at HF, so what the choke sees is largely common mode signal. This same approach is commonly used in EMI filters for mains.

It is the EMI filter for mains that I based my design on.

Steve Conner

Tue Nov 13 2012, 07:59AM

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

Well, Rick is complaining of excessive size and weight. I'm saying that the magnetic core in your choke serves no purpose. 2spoons' point backs this up: the paths taken by leakage flux are outside the core, so removing it doesn't really affect the differential mode inductance.

I don't believe common-mode noise is a problem, at least as far as the diodes are concerned.

Ash Small

Tue Nov 13 2012, 11:28AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

So you are saying the choke does nothing.

If the 200nF cap wasn't there, would the choke still do nothing?

Does it not impede the HF at all?

Steve Conner

Tue Nov 13 2012, 12:46PM

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

I'm saying that the choke would probably work just as well, if not better, if you used the same two coils, but uncoupled, and without the big heavy iron core.

I'd also question whether the choke is even needed in the first place, since Miller don't use one.

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