Brian,
You didn't mention the size of your variac, but my guess would be that this incident had nothing to do with your Hairpin circuit, but was much more likely due to heavy inrush currents from powering up your variac at just the right (or wrong) time along the 60 hz sine wave of your mains electrical service. Until the core of a variac (or any other inductive load, like a transformer or a motor) is magnetized to the point of developing the appropriate CEMF due to inductive reactance, the very low DC ohmic resistance of the copper wire coils inside the variac are essentially a dead short and it can take a few milliseconds for the core to magnetize and the inductive reactance to "kick in". I often trip my 20 amp service breaker on initial power up of a 20 amp or larger rated 120/140 volt variac, regardless of what the variac dial is set at or whether there is a load connected to the variac or not. From my experience, this doesn't seem to be an issue with
smaller variacs, say < 15 amps rating, probably because the smaller guage wire in the coil has sufficiently more DC ohmic resistance to prevent the inrush currents from momentarily rising to sufficient levels to trip the service breaker. You can verify the very low DC ohmic resistance by simply placing leads of a DMM, set in resistance mode, across the input to your variac, as the resistance will read < 1 ohm, which would translate to hundreds of momentary amps if the variac is energized at or near the peak of the input sine wave of the mains supply. To remedy this problem, you could add a resistive load (like a 100 watt light bulb) in series with the input to the variac at power up, then sidestep the light bulb resistor with a straight conductor after initial power up. I've just been too lazy to build a setup with a relay or contactor to do this automatically ;^)
Also, as a side note, a 15 amp breaker sounds more like a lighting circuit than an outlet circuit, as outlet circuits are usually on 20 amp breakers, at least in most electrical code jurisdictions of the US. But a 15 amp breaker, as opposed to a 20 amp breaker, would only worsen this problem.
David
On Wednesday, December 31, 2014 3:28 PM, Brian Hall <brianh4242@xxxxxxxxxxx> wrote:
After drawing a few arcs from my simple desktop SGTC, I hooked up the traditional hairpin circuit (as seen at the top of this page http://www.transformacomm.com/en/tech/tesla-hairpin-circuit.htm but with just a short copper bar across the top, there were no bulbs to light)
Basically I added another cap in parallel, removed my primary coil and then where the primary coil connections would be, I connected a 1/2" O.D. 2' long copper tube to each side. To connect the tubes at a right angle I used two 90 degree elbows and an 8" length 'stub out' copper tube, also at 1/2" O.D., at the other end of the 2' long tubes, for essentially a long inverted U shape. They were just press fit, no soldering of any kind.
The spark gap was set to trigger at about 80 volts on my variac, which I used to control power to my 10kv 23ma OBIT. The two caps were each 10kv at .01 microfarads.
Now the freaky part - the variac is both unplugged and switched off for safety in rewiring from coils to bars, along with of course being at 0 volts, and the instant I plug it in... I noticed a light go out on the far side of the basement - of course I have not yet mentally connected the dots. The spark gap which worked minutes earlier has not a single spark fired as it had when I was using this primary part of the circuit for my SGTC, which I also tried with the two 10kv caps in parallell. Still with nothing happening, I switch on the variac and turn it up, and no sparks were in the spark gap when there ought to be - something's off. I turned down the variac and switched it off, unplugged it, and was quite puzzled and went to turn on the overhead light - to find it didn't work. The 15 amp circuit breaker had been tripped!
After resetting the breaker, everything else still works thankfully - what on earth could have happened that my variac, even it it was switched on, was at 0 volts and still that triggered the 15 amp breaker? One thought is that instead of going through the spark gap, the current just went through the caps and copper bar (even with 0 or less than 10 volts applied?) and back fed through the transformer and variac into the wall, triggering the breaker instantly - but if that was the case, then swapping out the bars for my primary coil (which is exactly hot it was earlier) would have done the same thing, right?
What do you think happened? I am hesitant to repeat the conditions because my furnace may be on the same circuit, and it is going to be very cold the next few days. I disconnected the caps and just used the variac and transformer to arc across the spark gap, that part still works fine. 15+ amps going through all that in what was probably less than 1/2 second would be... very strange?
---------------------------------- Brian Hall
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