Lucky find at scrap yard

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John Taylor
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Lucky find at scrap yard

Post by John Taylor » Fri Jun 24, 2011 4:52 pm

While searching through scrap metal for a suitable chamber, I came across an amazing high voltage transformer. I was hoping to find some stainless material for chamber construction and, since I am still in the beginning phase, I need basically everything to get a working fusor. After discussing this transformer and what I wanted it for, the salvage yard gave it to me for nothing. It is a 1:120 or 1:200 ratio depending on which taps are used and is rated at 1750 VA with 500kv breakdown. I am so excited. I will feed it with a 0-280VAC from a variac, rectify it with a half-wave rectifier (until I can afford more Hi-V rectifiers for a full-wave bridge). Thanks to Richard Hull for his wonderful FAQ on current and voltage metering, I will end up with a full-fledged and adaptable Hi-voltage power supply that should be good for any undertaking I have.
Sorry about the length of this post...DID I MENTION I AM EXCITED ABOUT THIS FIND?
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Doug Coulter
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Re: Lucky find at scrap yard

Post by Doug Coulter » Fri Jun 24, 2011 6:51 pm

What I think you've scored is a "potential transformer" usually used as a step down in a substation for either measuring the HV on the main lines, or as a convenience power source. We have one too, it's a very fun thing indeed and very robust. You can't push the volt ratings too far, because the core will saturate if you try (which in the original use, tends to protect things from lightning spikes).

The main trouble with these as a DC power supply is the filter caps -- they have to be large to cut ripple at 60hz. They will however, often run up to about 2khz off a powerful audio amp (which we do here, those are surprisingly cheap at Musician's supply for example, and have current limits and other protection built in) so you can use smaller capacitors. Ours has good enough secondary isolation to allow it to be used in a bridge rectifier arrangement, which needs less filtering, and which can be cascaded just like CW multipliers can, even without a centertap on the secondary -- just takes more diodes (4 per stage).

These are very robust in general, and beware, they can make huge currents that can easily fry an unprotected variac on the input side.

For low frequency use, it shouldn't be real hard to just buy a ton of 1kv 1 amp to 6 amp diodes and series them up to make each desired diode - they are only pennies each in 100's, not bad.
Why guess when you can know? Measure!

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John Taylor
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Re: Lucky find at scrap yard

Post by John Taylor » Sat Jun 25, 2011 2:37 am

It is a potential transformer and I had thought about stringing a bunch of 1N4007 rectifiers to get the needed PIV. I hadn't even thought about the idea of using higher frequency. I may actually try this, but for now, I'll use a variac, but will have to figure out some kind of current limiting. Thanks so much for all the advice.

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Doug Coulter
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Re: Lucky find at scrap yard

Post by Doug Coulter » Sat Jun 25, 2011 5:46 pm

We've found a good way to start out is to use a big light bulb in the primary to limit current. Say from a few hundred watts up to a kw or so. If something is wrong, it'll just light up, no harm done. Remember, incandescents have about 1/10 the resistance cold that they do hot, so the voltage drop is not bad till they start to glow.
Why guess when you can know? Measure!

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Re: Lucky find at scrap yard

Post by DaveC » Mon Jun 27, 2011 9:29 pm

John - that is a nice find! Looks to be in the 23kV - 33 kV range, a "PT" ( as they're known in th' biz). The 500 kV part of the breakdown voltage, recognizes that lightning and switching surges (mostly lighting) can reach about 10X peak line voltage, in ditributions circuitry, hence the high limit.

Be sure to check out the actual connections on the secondary side. The two insulators suggest a floating secondary, as would be used in a phase to phase potential measuring transformer. Single phase to ground PT's usually have only one HV terminal, since the other end is at ground potential.

Dave Cooper

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