Discharge a huge voltage

Always apply a safety factor, I would use 800 and 30. Whether the 30 amp is needed depends on your duty cycle, if the dutycycle is low, 20 and even 10 might be enough. Just check the allowed peak current.

If he is saying

Shouldnt he be useing a 1kV-1.2kV fet or IGBT with maybe a drain clamp?

He knows best, just apply some safety factor over the highest expected values.

The capacitor can take up to 900v, but I am only charging it to 600v.. I thought maybe if I can get some transistors in parallel to drain the current, but using transistor rated below 600v. But I do not think it will work.

It will work ONCE. After that you have a permanent short.

When you say discharge at "0.01 seconds interval" do you mean discharge it *every* 10 milliseconds? What are you trying to do?

Yes, you need a transistor or mosfet rated for 600 (or 900?) volts and

The resistor will have to absorb a lot of energy.

John

This is the first step for ultimatly a tesla coil. In the first I would like to control the capacitor discharge. The circuit above I will create a gap for the spark. If I do not put the transistor then I cannot control the discharge and everytime a spark will occur, it will empty my capacitor. And to do this, it will take almost .5 seconds, which is too long.

JP

As others have said, you need a switch FET, parallel FETS, IGBTS or whatever more robust than 20A and 600 volts. Probably at least 30 A and 800 volts.

But, the time constant of 30 ohms and 900uF = 0.027 seconds. So, it's not quite clear what you expect to do every 0.01 sec, 10msec. In that time the cap will not be recharged or discharged hardly at all through a 30 ohm resistance.

Secondly if you are trying to make a Tesla coil circuit, you barking up the wrong tree. Tesla coil primaries require very high voltages, at least 10kV to be effective. The capacitor is in resonance with the primary inductance at or above 100kHz. The secondary is also in resonance at the same frequency with it's inductance and self capacitance. The primary capacitors are in the range of 100's of pico farads to several nano farads depending on the primary inductance they are intended to resonate with. The capacitor voltage has to be way in excess of the driving voltage typically 10kV to 20kV. Also the capacitor must be able to withstand high RF currents.

You mentioned a spark gap?? There is no way 600 volts will bridge a gap. That voltage is way too low. You have to get to several kV before and air gap is effective.

Good luck with your adventure.

Certainly not in parallel -- a parallel connection sees the full voltage across each device.

In theory you could connect transistors in series, with appropriate gate or base drivers. But it'd be a lot cheaper to just get a 900V transistor.

Were you discharging down to nothing I'd suggest an SCR (or stack thereof). But you're not...

You need to get a transistor that's rated for _more_ than 600V -- the voltage rating on any device is "don't ever exceed this, even once, or it'll die, you'll die, men, women and children will die, aughhhhhhhh!". Different companies and industries treat this absolute maximum rating with different attitudes -- electrolytic capacitors are generally rated much closer to the edge than semiconductors, for example.

I'd use a transistor that's rated for at least 800 or 900V, and if that

The current ratings that they put in big print on FET data sheets is based on a well heat-sunk transistor that's operating continuously at room temperature. You really need to do the thermal analysis for your particular situation to make sure that your power devices aren't going to go 'poof!'. When you're done you may find that you can get by with one rated at 20A (or less, depending on the surge or instantaneous current rating), or you may find that you need more.

Post a copy of your circuit someplace and folks will critique it for you.

What about a big assed 'puck' style SCR / thyristor? I don't know about how fast you can trigger the things, but don't they use them for big motor control apps? I know you can get them with hold off voltages in the 1000V+ range and dump hundreds of amps through them. Might get a bit hot, but what about the heat from the cap ESR???

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I do not want to discharge all the gap, Ok Bob mentionned the time delay, I may have to reduce the resistance, (increase the current) BUt I need a spark in the circuit. So capacitor, resistance, spark gap and transistor. Ok so I need several kV, lets say 3000v , but now lets say I use a 10 ohm resistance, that gives use a wapping 300 amp. Is there such a transistor rated at 3kV and 300 amp?

That was my original idea, but when I saw outstanding figures I decided to decrease the voltage and start low.

JP

Turn-off becomes an issue. I know that there are ways to turn off an SCR on a DC supply, but it's some wacky thing involving an auxiliary inductor that snaps the current off briefly so the SCR can turn off -- it all looks pretty smoky to me.

Maybe if you share _all_ of what you _really_ want to do?

There are IGBT modules (if not individual transistors) that do over 3kV, and if you can't find one that does 300A, you can probably gang them together if you can figure out the current sharing.

Just how you trigger the spark has a huge cost/precision tradeoff; knowing how tightly you need to control the spark makes a big difference to how you may want to drive it.

It's definitely getting into the realm of tubes (mostly thyratrons) ruling, though at high amperage the tubes are available, but do tend to cost a few thousand dollars. At the lower amperage ranges, if you stick to those tubes that are still commonly available they can cost a lot less than silicon to do the same job.

On the third hand, the application becomes highly suspect as I can think of a number of nefarious uses this might be aiming towards I wouldn't care to help with. Not quite as blatant as the person a few months ago that was evidently looking for help in designing a roadside bomb, but a spark gap creates an infernal amount of RF noise...

I am looking to make a unidirectional spark. I also thought of using like the old days a rotary spark gap, but with a 3600 rpm motor, even with 10 contacts around I get 3600rpm/60 =3D600rps with 10 contacts =3D

JP

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Ok here is the circuit I am trying to replicate. Some of you will say it is not possible, it is not concrete, but hey I am just trying it out. Like I said , I just want to make a simple circuit with a capacitor discharging through a spark gap for now, nothing else.

Also, in the schematic shown, there is a diode that seems to be on backwards, but some dud on the web site says it is not.. I still think it is on backward, can somebody has an explanation for that. The circuit is doable, the effect well not so much I think thanks JP.