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Spark gaps -- making and triggering

Eh, not really, consider the whole thing is over in a hundred microseconds, while a relay might take a whopping ten miliseconds to engage. For it to be a "relay that sparks a lot" in any meaningful way, you would have to close the gap before the sparks are done with.

Hum... figure it has to close the gap (sufficient to hold off the voltage mind you!) in 10us or so. Let's say a mass 0.l kg, distance 0.02 m and time = 10^-5 s. Average velocity must be 0.02/10^-5 = 2 x 10^-2 x 10^5 = 2000 m/s, breaking the sound barrier I believe! Assuming constant acceleration from an initial velocity = 0 to final velocity = 4000 m/s, that's dv/dt =

4000 m/s / 10^-5 s = 4 x 10^3 x 10^5 m/s^2 = um, 0.4 gig m/s^2, or roughly 40 million G's? F = ma so that's around 4 MN (mega newtons), or for N = G * 1 kg ~= 10 on Earth, roughly 400k kg, that is to say, 400 metric tons, sorry, tonnes. This force has to be applied with a rise time under say, 5 us, so inflating a wide piston with pressurized air isn't going to cut it (speed of sound as it is). A light gas piston might, particularly if pressurized greater, but it still begets fast risetime elsewhere to do it. An ignited H2/O2 gas mixture might do it. Other propellants may also work, considering the speed of sound is much faster with higher gas temperatures.

Oh, and a final kinetic energy of 1/2 * 0.1 kg * (4000 m/s)^2 = 800kJ ;)

The force and energy would of course be much smaller given a lighter contact, but that doesn't matter, your biggest concern (if it were a concern) is really getting things to actually move in the first place.

Heh.

Fortunate that gasses can ionize so quickly and completely, huh?

Though on the other hand, if you pressurize or vacuum the spark gap chamber, you can get shorter distances, requiring less robust drive train. 'Course, if you pressurize the chamber, you can also get a lot more ions in the spark! Come to think of it, that might be a good reason to use something easily vaporized, like zinc or aluminum...

Tim

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Reply to
Tim Williams
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Thanks... I went to farm supply and bought some 1/4" copper tubing, 1" by 1/8" aluminum flat to make a bus connecting several caps, 3/4" copper pipe and corresponding pipe caps. I will start putting a plan together.

i
Reply to
Ignoramus26024

You do need HV relays, which can be expensive. I get many of mine from eBay sellers. Manufacturers are Kilovac, Gigavac, and Jennings, which you can use in your search terms. You can expect to pay $75 to $250 for a 30kV vacuum relay, and perhaps more if I'm bidding against you. :-) I just won three Kilovac KC-38 25kV spst relays for $60 each, a better price than usual.

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Here's a nice 35kV spdt relay with cables and HV connectors.

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At modest power levels I use series stacks of many 2W carbon- composition resistors. These dissipate the energy throughout their bulk, rather than just in a thin wire, etc. For higher power levels I'm building up a collection of Advanced Power resistors, mostly taken from high-power IGBT snubbers, etc., but quite a few of these will be required in a series stack.

Thanks, Bert, that's a good lead, for repetitive applications. I've created simple heat-spreading spice models of transient- energy absorption in the 0.1us to 10ms timeframe, which I've vetted against microsecond-scale measurements. You know, cells of thermal-mass with thermal-conductivity connections. With this I was able to see how well-designed TVS parts are - I was impressed. I've also studied MOSFET avalanche with these spice models. We have a site-license for a good FEA program at the Institute, but I haven't taken the time to learn how to use it.

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Reply to
Winfield Hill

Thanks Win, I will keep an eye on eBay for HV relays, in fact I set up a snipe for one.

i
Reply to
Ignoramus30285

My 100uF 10kV and your 70uF 12kV caps each hold 5kJ at full charge. The largest HVR C-series resistor, C2654, about 1" in dia and 2" long, takes 5.6kJ for a 125C rise, and is rated for 22 to 44kV operation, depending on the resistance value.

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Here's one on eBay,
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Too bad he only has one resistor. But his is a pretty high resistance value anyway, 2.8k, which has a 280ms time constant with 100uF. HVR says you can use 5 ohms and still apply up to 22kV pulses. That would be a 500us time constant with 100uF. But 5-ohms would also create a 2000A peak current with 10kV, compared to a 400A typical max spec for Kilovac relays. 100 ohms would mean a 100A peak current and a 10ms time constant.

I'd rather use two C2654 resistors per HV capacitor. Two 1k in series would give a modest 5A peak/cap for long relay life. That'd be 8 resistors and 20A peak for a bank of four big caps and take one second for five time constants (10kV to 67 volts).

HVR's rod and tube style is attractive, the 10" ones handle 22kJ, so only one or two would be required for a full 4-cap bank. Actually, I have a modest selection of these in stock, from the Flea at MIT.

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 Thanks,
    - Win
Reply to
Winfield Hill

You can easily build your own as well. For example, here's a design from HV hobbyist Bob LePointe:

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You can also get some potential construction ideas by looking at some of the open frame HV relays made by Ross Engineering:

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Bert

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Reply to
Bert Hickman

b>>

Bert, thanks, Bob's relay looks very nice. If I do not win the relay that I set a snipe on (I usually lose on most of my auctions), I will make something like what he did. I believe that I have a 12v pull solenoid lying around somewhere.

I think that it makes sense for me to make a system where it is electrically impossible to have a power supply connected to the system, at the same time as the spark is triggered.

i
Reply to
Ignoramus30285

When I was playing with nitrogen lasers, hammering the spark gap with a screwdriver handle produced much better pulses than just letting the voltage increase until sparkoer (once every 10 seconds or so).

I suppose part of the charge went into corona discharge. This will matter less with these huge caps.

Thomas

Reply to
Zak

Or spoons? Ladles?

Thomas

Reply to
Zak

Just be sure the un-powered state shorts the capacitors.

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 Thanks,
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Reply to
Winfield Hill

There's alot of energy involved.

And lots of people don't know what they're doing.

Reply to
Cydrome Leader

When I was in High School, a friend had a "rechargeable cherry bomb". It was something on the order of 200 uFd at 10kV. We'd charge it up and fire it with an ignitron tube through a 1/2" gap. It made one heck of a bang.

The other fun thing was discharging a large HV cap in small containers of water. The resulting shock wave would throw water up in the air and shred the container. This was in the days of metal frozen orange juice cans, which his family would empty every morning at breakfast. When my friend got home from school, he'd shred the day's can with a smaller capacitor (2 uFd @ 20 kV?). The plan was to make a metal sculpture out of the results, but I don't recall that he ever got around to that part.

Doug White

Reply to
Doug White

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