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Can I run Steve Ward's Mini SSTC without an interrupter?

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Cheezey

Sat Jan 11 2014, 08:37PM

Registered Member #28483 Joined: Fri Jul 12 2013, 09:00PM
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Posts: 29

I haven't built an interrupter yet because I just want to know it runs before I build it. I just tried plugging it in though, and it tripped the breaker. I inspected the power section and couldn't find any shorts (maybe I didn't look hard enough) and I started wondering if I do need the interrupter.

Could that be the problem?

Sigurthr

Sun Jan 12 2014, 06:22AM

Registered Member #4463 Joined: Wed Apr 18 2012, 08:08AM
Location: MI's Upper Peninsula
Posts: 597

You most certainly run it CW (uninterrupted). I don't remember it off the top of my head but you may need to make some modifications; the UCC chips have internal pul up resistors on the Enable pins but if both enable pins are tied together the internal pull ups aren't enough to allow a Logic HIGH, so you'll need to connect the enable pins to a +5V (or +12V) rail.

However, if you're tripping the breaker and there are no obvious visible faults, what is most likely happening is that you didn't phase the GDT properly and thus you're getting full cross-conduction (both FETs are ON at the same time, presenting a dead short to the input power).

If you don't have a scope to check the phasing of the Gate drive then check my video (it's in a thread in the TC section) about how to phase a GDT without a scope.

Mads Barnkob

Sun Jan 12 2014, 06:54AM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

On the SSTC5 I made, the CW mode was just a switch cutting off the interrupter signal, so there was no input to the UCCs, just left it floating.

Cheezey

Sun Jan 12 2014, 05:59PM

Registered Member #28483 Joined: Fri Jul 12 2013, 09:00PM
Location:
Posts: 29

Sigurthr wrote ...

You most certainly run it CW (uninterrupted). I don't remember it off the top of my head but you may need to make some modifications; the UCC chips have internal pul up resistors on the Enable pins but if both enable pins are tied together the internal pull ups aren't enough to allow a Logic HIGH, so you'll need to connect the enable pins to a +5V (or +12V) rail.

I had a sneaking suspicion I might have to do this, but I wasn't sure. Thanks.

Sigurthr wrote ...

However, if you're tripping the breaker and there are no obvious visible faults, what is most likely happening is that you didn't phase the GDT properly and thus you're getting full cross-conduction (both FETs are ON at the same time, presenting a dead short to the input power).

If you don't have a scope to check the phasing of the Gate drive then check my video (it's in a thread in the TC section) about how to phase a GDT without a scope.

That's a helpful video; thanks. I'll check my transformer.

Cheezey

Tue Jan 14 2014, 03:37AM

Registered Member #28483 Joined: Fri Jul 12 2013, 09:00PM
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Posts: 29

I checked and rewound my GDT and tried switching the wires on one of the secondaries - still tripping the breaker. I did connect the place where the interrupter should be to 12V though.

I'll try checking it with my scope tomorrow.

Sigurthr

Tue Jan 14 2014, 03:58AM

Registered Member #4463 Joined: Wed Apr 18 2012, 08:08AM
Location: MI's Upper Peninsula
Posts: 597

Your primary (of the tesla coil, not the GDT) could just be too low of impedance, what is your primary inductance, and what is the resonant frequency of your secondary?

Dr. Dark Current

Tue Jan 14 2014, 10:25AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

It may be the filter capacitor(s), you probably need a soft start device.

Sigurthr

Tue Jan 14 2014, 06:58PM

Registered Member #4463 Joined: Wed Apr 18 2012, 08:08AM
Location: MI's Upper Peninsula
Posts: 597

Yep surge current could do it too, though usually only initially and then it won't trip the breaker again. By the way, what size breaker are we talking about? I've scaled up this basic half bridge design to over 3KW, which on 120v service is more than 20amps draw. That just goes back to my suggestion of primary impedance.

Cheezey

Wed Jan 15 2014, 01:05AM

Registered Member #28483 Joined: Fri Jul 12 2013, 09:00PM
Location:
Posts: 29

Sigurthr wrote ...

Your primary (of the tesla coil, not the GDT) could just be too low of impedance, what is your primary inductance, and what is the resonant frequency of your secondary?

My primary impedance is about 2.1Î©. I might need more turns; I only have 4. I don't have any means of measuring primary inductance or fres (I don't have a function generator...

)

Dr. Dark Current wrote ...

It may be the filter capacitor(s), you probably need a soft start device.

You mean the caps draw tons of power at first? I also don't have a variac to soft start it... I need to start buying more expensive electronics equipment :P
EDIT: I found a cheap motor soft starter. Would that work? Is it overkill?

Anyway, thanks for all your help. I really appreciate it.

With the amount I'm tripping the breaker maybe I should attach one to my coil board so I don't have to go into the garage every time.

Sigurthr

Wed Jan 15 2014, 02:05AM

Registered Member #4463 Joined: Wed Apr 18 2012, 08:08AM
Location: MI's Upper Peninsula
Posts: 597

Yeah no wonder you're tripping the breaker! That primary will draw >50A! (though I'm wondering how you calculated the primary impedance if you can't measure frequency or inductance). I don't have a LCR meter either, but modeling in JavaTc does a good job of determining rough low frequency inductance, and from there you can calculate impedances at various frequencies; Xl = 2PiFL.

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