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Physics Junkie

Thu May 02 2013, 01:00PM

Registered Member #7267 Joined: Tue Oct 16 2012, 12:16AM
Location: Detroit, Michigan
Posts: 407

I discovered a 50Hz 220V 30A breaker that was mislabeled in my breaker box and so I got a line wired in my garage, it's wired in its own world and has no other load anywhere in the house. I want to use this for powering my bigger Tesla coils instead of using the 60Hz 120V 15A outlets. My setup now consists of 120V 10-15A outlet -> 140V 1.4kW variac -> 240/120 step down transformer 1.5kW wired in reverse to step up -> DC power supply/voltage doubler on the bridge. So I have a couple questions about properly powering my coils from this new 220V line.

-how would you people recommend I ballast the line before just simply plugging it into my rectifier? I've read around about using lights in series with the load.

-A 240 30A variac is out of the question now, too expensive, but I hope to get one in the future

-Any safety reasons why I cant just throw a larger fuse in my 140V powerstat variac and use that?

-any suggestions and recommendations are welcome.

HV Enthusiast

Thu May 02 2013, 02:30PM

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

Sounds like when you say ballast you are referring to surge / inrush protection?

If you have a variac in line, which it looks like you do, you can softly charge up the capacitors in your rectifying doubler circuit. Therefore, there is no need for inrush protection.

However, if you are just going to bypass the variac and jam the break on into a capacitor bank, then you will need some sort of inrush circuit.

Physics Junkie

Thu May 02 2013, 03:36PM

Registered Member #7267 Joined: Tue Oct 16 2012, 12:16AM
Location: Detroit, Michigan
Posts: 407

Yeah that's what I meant by ballast, is that not the correct term?

What are the hazards of using a 140VAC 10A variac on this line though? Say If I put like a 15A or 20A fuse in or whatever.

Steve Conner

Thu May 02 2013, 04:24PM

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

If you live in the US, a 240V line is actually two 120V lines 180 degrees out of phase, so it needs two breakers, one per line. Using just one breaker is a safety hazard: there's nothing to limit the current if the line without the breaker shorts to neutral or ground.

Applying 240V to a 120V variac will saturate the core and burn it out.

Physics Junkie

Thu May 02 2013, 05:04PM

Registered Member #7267 Joined: Tue Oct 16 2012, 12:16AM
Location: Detroit, Michigan
Posts: 407

Steve Conner wrote ...

If you live in the US, a 240V line is actually two 120V lines 180 degrees out of phase, so it needs two breakers, one per line. Using just one breaker is a safety hazard: there's nothing to limit the current if the line without the breaker shorts to neutral or ground.

Applying 240V to a 120V variac will saturate the core and burn it out.

It is two breakers. Thanks.

What would be an alternative to charging the caps? Or using incandescent lights as a resistor (that's just what I read online) in series with the hot line to the load(caps)? What about a power resistor?

HV Enthusiast

Thu May 02 2013, 10:21PM

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

I believe several 4hv'ers have built solid state variacs. You might find some of their designs / details by searching through the 4hv archives.

Physics Junkie

Fri May 03 2013, 02:27AM

Registered Member #7267 Joined: Tue Oct 16 2012, 12:16AM
Location: Detroit, Michigan
Posts: 407

Dang. Was hoping there was a simple way. Thanks

Steve Conner

Fri May 03 2013, 09:25AM

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

Arc Attack use a "pre-charge" circuit on their big coils. The mains connects to the rectifier through a big high-powered resistor, which charges up the filter capacitors slowly. Meanwhile the gate drive is disabled so the coil can't operate, as it would burn the resistor.

After a few seconds delay, a relay shorts out the resistor and the gate drive is enabled.

This works just fine, and is also used on industrial motor drives, electric cars and so on. A few light bulbs in series, or electric cooker elements or whatever, makes a perfectly good resistor, and you can use a hand-operated switch in place of the time delay relay.

However it's only for use once you're confident with the reliability of your electronics. You can use a lower powered setup, like your variac and step-up transformer, to prove them out before going over to raw 240V with no current limiting.

Physics Junkie

Fri May 03 2013, 06:59PM

Registered Member #7267 Joined: Tue Oct 16 2012, 12:16AM
Location: Detroit, Michigan
Posts: 407

Thanks Steve. That's more along the lines of what I was thinking. Trying to keep it simple, but at the same time, low cost. High power resistors and high power variacs can be costly, but indeed a necessary item for this application. I might experiment with the light bulb thing since that would be the most probable in my situation. Solid state variacs seem to be much less cost wise but more complex. Guess I just gotta pick and choose. Thanks again

HV Enthusiast

Fri May 03 2013, 09:17PM

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

PJ -

If you search for inrush circuits, you can find quite a bit of relay based inrush circuits as Steve described. Ham radio guys use these quite a bit in their linear supplies they use for their homebrew amplifiers.

Even high powered stereos use a similar circuit. Ever hear a relay kick in a second after you power up a Home Stereo Receiver? Thats the same thing going on.

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