Bleeder resistors for cap array (tesla coil mmc)

Discussion in 'The Projects Forum' started by Undead Toaster, Mar 6, 2015.

Mar 6, 2015
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I'm building a spark gap tesla coil for a school project and I'm realizing that I've gotten in over my head somewhat, but having already spent money on capacitors and a transformer, I'd like to try to do it anyway. My electronics knowledge extends slightly beyond high school physics.

What value resistors should I be getting to make sure that the capacitors (in a multiple mini array for the primary cap) discharge in a reasonable amount of time (under 10 minutes or so) when the coil is not in use? What I've read indicates that I should be using resistors in the neighborhood of 5 MΩ, but I don't know what power rating to get (or if it even matters) and want to be sure. I'm using 13 parallel strings of 15 0.01 µF, 3 kV (DC) capacitors for 45 kV and 8.67 nF total. The peak voltage from the transformer should be around 21 kV (it's a 15 kV neon sign transformer, times sqrt(2)), so there should be a max of about 1.4kV across each capacitor, I think.

I looked over the textbooks, but I don't think I have enough background to understand how to do this. I am slowly making my way through them, but that'll take time... Thanks!

2. DickCappels Moderator

Aug 21, 2008
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Let's see: You have about .01 uf charged up to 45 kv. A good target for safety and comfort is to have the voltage down to under 40 volts within 10 minutes.

That can be quickly re-arranged to give:

τ = T/[Ln (Vc/Vs)]

In which τ is the time constant, RC.

Plugging in
T = 600 seconds
Vc = 40
Vs = 45 kV
T = 40

We get τ = 85.4 (the time constant needs to be 85 seconds or less.

85 seconds / .01 uf = 8.9 x 10^9. I would use a 10 gigohm resistor and make sure that it or the series of resistors are rated to at the least 130% of the maximum applied voltage.

With only one bleeder resistor, the total voltage for the capacitor network will discharge to below 40 volts in about 10 minutes, but it might leave some individual capacitors in the network charged. If this is a concern, rearrange the network to be one string of 15 sets of 8 capacitors in parallel and then put a bleeder across each parallel set of capacitors, figuring 45 kV/15 for each set of 8 capacitors and their respective bleeders.

45 kV can hurt you. Keep everything clean, keep yourself well away from the live parts and you might want to use a grounding hook to discharge things and to assure that things stay grounded when you are working on the high voltage section.

You can make a grounding hook with some heavy wire and a length of PVC pipe. Works fine in spite of the low cost.

Where τ (Tau) is the time constant = RC

3. alfacliff Well-Known Member

Dec 13, 2013
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how long will the caps take to discharge when ac is the source? my tesla coils never retained a charge when using ac, only dc.

Mar 6, 2015
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I went with 10 megaohm resistors, since I got impatient (probably not a good idea for an ameteur working with high voltages...) and that is pretty standard for tesla coils of the size I'm building, as far as I can tell. There will be one across each capacitor. Obviously that is quite a bit lower than what you calculated, DickCappels (thanks, by the way!), but it should not be a problem, right? Other than decreasing the efficiency of the tesla coil? Assuming a max voltage in the bank of 22 kV, it would take roughly .6 seconds to drop each cap from 1.4 kV to 3 V (entire MMC from 22 kV to 40 V).

5. DickCappels Moderator

Aug 21, 2008
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"...since I got impatient (probably not a good idea for an ameteur working with high voltages...)"

The take just as long to discharge from a given voltage regardless of whether they were charged by DC or AC (as in the spark ending when the input AC voltage is above zero.)

You've got that right! Slow and steady is the only safe approach.

Don't know whether using 10 Meg instead of 10 Gig will be a problem -have you calculated the RMS voltage across each resistor and from that the power?

6. alfacliff Well-Known Member

Dec 13, 2013
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what will the actual voltage be across the caps? it depends on how your tesla coil is arranged. is the spark gap between the caps and the primary coil, or in parallel? if in series, the voltage drop of the gaps is how much charge you might get if turned off at peak voltage. if in parallel with primary, the caps will discharge through the coil as soon as power is removed.

Mar 6, 2015
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The gap will be in parallel. I figured the RMS voltage across each cap should be 1.4 kV (15*[2^(1/2)]/15). Assuming P=V^2 / R, P = 2 / 10e6 = 0.002 W. My resistors are 0.5 W

Mar 6, 2015
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That should have been 2000 / 10e6

Mar 6, 2015
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sorry for not posting this all at once...
But won't a dangerous voltage remain once the air in the spark gap stops conducting?

10. alfacliff Well-Known Member

Dec 13, 2013
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the voltage across the caps is rf or ac, you cant charge a cap with ac or rf, only if you stop the supply during a voltage peak. also, the voltage will be whatever the voltage of the conducting spark gap is, less than the peak voltage of the supply transformer.

Mar 6, 2015
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So I could leave off resistors entirely, and use a grounding hook like DC mentioned to ensure the caps are discharged after I run the tesla coil?

12. alfacliff Well-Known Member

Dec 13, 2013
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thats the way I do it, never seen a spark when discharging either.

13. DickCappels Moderator

Aug 21, 2008
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After power is switched off, before you touch the capacitor with your hands or hand tools, please carefully short each one individually to assure that that none of them has a surprise in store for you.