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Is it possible to use an SCR to make a small solid state tesla coil? Answered

From what I understand, an SCR is like a diode that can either be turned on to act like an everyday diode, and it will only turn back off when the power is cut. With this understanding, will this circuit work? if not, can you explain? even if it doesn't work as a tesla coil, will it work as a fence charger (with a trigger transformer or ignition coil?)

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steveastroukBest Answer (author)2013-03-25

SCRs turn off when there is no current through them, or no voltage across them.

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-max- (author)steveastrouk2013-03-25

that's what I meant by "when power is cut" to them. well at least I understand there operation correctly.

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steveastrouk (author)-max-2013-03-25

That's not correct, they can be switched when power is still applied to them, but its tricky.

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-max- (author)steveastrouk2013-03-25

So then what am I misunderstanding, when power is cut to the SCR itself, when there is no voltage across it. (without the capacitor) and no current flows. I don't mean the entire circuit. if they can be turned off while current is flowing, how is it done? Just with a brief "blackout" of power?

I read somewhere where that older SCR's require backwards current at the gate to turn them off again. (also, are SCR's and Thyristors the same exact thing?)

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steveastrouk (author)-max-2013-03-25

Be careful talking of "power", SCR (same as a thryistor) are current controlling devices. If you can very briefly blip the anode to cathode current to zero, then they turn off. Its back in the mists of time I actually studied these things, but see if you can find schematics for the McMurray inverter, if you're interested in SCR commutation.

Steve

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-max- (author)steveastrouk2013-03-26

I took apart a switching power supply and inside was an MCR72. I played around with the connections and I had it wired up to an LED. I then touched the gate to +v rail, and it turns on, and when I touched it to ground, it turns off. I though it is not possible to turn these off. Con you explain this please?

Currently I am trying to make a low voltage version of the above circuit. only problem is that I need something that will only conduct after reaching the specified threshold voltage. Will a zener diode work?

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steveastrouk (author)-max-2013-03-26

You're only tickling it. That's an 8A device. Get some load on it, and try switching it off.

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-max- (author)steveastrouk2013-03-26

will it pop? I currently have nothing that draws that much amperage at 12v.

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steveastrouk (author)-max-2013-03-27

Keep it heatsinked, and keep the current say at 1A - try a car taillight

Steve

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-max- (author)steveastrouk2013-03-27

I tried out the SCR on 9V with a flashlight bulb, and I wasn't able to turn it off by touching the gate to ground. but shouldn't that allow electricity to flow from the first PNP transistor to ground instead of the second NPN transistor and thus break the clamping of the SCR, like I did with 4V?

what would happen if I replaced the neon with either a zener diode (in reverse) or to the proposed circuit above? and what are some real world useful applications for this component? I never see much circuitry using them.

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steveastrouk (author)-max-2013-03-28

It wasn't the voltage that let the circuit work, it was the fact that the load was so low it wasn't actually latched on properly.

The reason YOU don't see circuits using them, is that you don't deal in high power electronics like inverters, phase controlled bridges etc etc. They are THE goto method in high power systems.

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-max- (author)steveastrouk2013-03-28

that's why I used a bulb instead of an LED... fore more current flow. this explains why I see then in older electronics and in high power stiff (like where I got this one from.. a ~100W switching power supply) thank you for your reply and stuff. Im going to try to build the above circuit with a DIAC and see what happens. Thanks!

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steveastrouk (author)-max-2013-03-28

In a 100W switcher, its probably there as a "crowbar" circuit. Look that one up ;-)

There are GIGAwatt inverters that rely on thyristors that look like trashcan lids.

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-max- (author)steveastrouk2013-03-28

I built a circuit with a drill motor, where a 3000 uF capacitor charges up though some small resistors, then discharges into the SCR. This creates a significant delay from the time voltage is applied to when the motor turns on. only problem is that the capacitor remains charged for next use, so cutting the power, it turns on instantly.

Is there something along the lines of an anti SCR, where it is on until a voltage on the gate is applied, then latches off?

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steveastrouk (author)-max-2013-03-29

No, but there are tricks to turning SCRs OFF when you need to - like I said, see "Mcmurray inverter" for some concepts. Its pretty heavy electronic theory - I studied it for my degree, a long time ago.

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-max- (author)steveastrouk2013-04-15

Thank you for your help. I built it using an ignition coil and now I have a good fence charger like circuit. I can't seem to get it to work with higher frequencies because the SCR will stay latched. I will continue work with this and see how far I can take it!

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steveastrouk (author)-max-2013-04-15

That's the gotcha with SCRs.

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-max- (author)-max-2013-03-27

with either a zener diode or a DIAC or SIDAC? are SCR's, trisistors, triacs, diacs and TRIAC's and all part of the same family and often used together, (I often see them used in old-school technology)

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The MadScientist (author)2013-03-27

This kind of Tesla coil isn't solid state, it is called an off line Tesla coil (OLTC) I'm not sure if anyone has built one with a thyristor but it may be possible. (most are made with IGBT's, I'm building one at the moment.)

Here's a couple of links about them.
http://scopeboy.com/tesla/index.html
http://scopeboy.com/tesla/oltcfaq10.html

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-max- (author)The MadScientist2013-03-27

whats the difference between OLTC's and SSTC? other than that OLTC looks like a IGBT driver that is powered at mains?

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The MadScientist (author)-max-2013-03-28

A SSTC directly resonates the secondary coil by oscillating the primary coil at a set (calculated resonant) frequency.
Example: Say the resonant frequency of a secondary coil was 500kHz then a SSTC would oscillate the primary coil at 500kHz.
Whereas an OLTC will pulse the IGBT at most at 1kHz to simulate the effect of a spark gap on a traditional Tesla coil (but at a lower voltage), this means that an OLTC still requires the inductor/ capacitor resonant matching involved with traditional Tesla coils.

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-max- (author)The MadScientist2013-03-28

Yeah, I figured that out while looking down that article. what would it be called if the IGBT was driven at a variable frequency, that can be changed anywhere from 1khz to 500kHz?

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The MadScientist (author)-max-2013-03-28

The IGBT's are driven at a variable frequency but at 1Hz to 1000Hz, the idea is not for the IGBT to oscillate the primary coil but to let the tank capacitor discharge through it and let the tank capacitor and the primary coil do the oscillating like a normal Tesla coil but with the spark gap replaced.

Also I don't think there is any kind of IGBT that can oscillate at frequencies that high. There max is generally 2kHz (I think).

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-max- (author)The MadScientist2013-03-28

what I mean is if one were to build what I described, what would it be called?

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The MadScientist (author)-max-2013-03-28

I'm not too sure what NE2 is but if you were to drive the thyristor the way you describe and you make that capacitor tuned, then it will be an OLTC.

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-max- (author)The MadScientist2013-03-28

no, I meant the OLTC I described with the IGBT being driven at a very high frequency, with a tuned circuit. the NE2 is a neon light. They act like small capacitors until about 80V is reached when the neon will glow and act like an almost dead short.for a small circuit.

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iceng (author)2013-03-25
  • The capacitor gets charged
  • to the neon break-over and
  • drops into conduction with a lower voltage.
  • This difference is enough to fire the SCR
  • A Heavy pulse of current discharges capacitor through the primary.
  • Now the Resistor and SCR are chosen  so that the holding current is high and the resistor prevents the SCR from staying in conduction .... ready to fire again.
  • It id called a relaxation oscillator

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