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Re: [TCML] Cap bleeder resistors



DC Cox wrote:

"Here's what happens: As long as the neon transformer is connected to the cap the charge will migrate back thru the center tap winding (usually around 70K Ohms impedance) and go directly to ground."

This is NOT true with an MMC. Although the net voltage across the ends of the MMC may be zero, individual capacitors in the string may retain a significant charge if bleeder resistors are not used. This can be very easily demonstrated with any MMC that does not have bleeder resistors, by measuring the residual voltage across each cap with a HV probe immediately after power has been switched on, then off. The tendency to retain a charge is influenced by the voltage that was present across the cap when input power to the transformer was interrupted (ie. near the peak of the sine wave, or near the zero crossing).

Since Andrew's original question related to bleeder resistors on an MMC, the conservative recommendation for maximum safety would be to ALWAYS use bleeder resistors across EVERY capacitor that is potentially charged to high voltage. Since special HV-rated resistors (Vishay VR-37 series, 1/2W, 3.5kV) cost less than $0.10 each, including bleeder resistors has little effect on the overall cost of a coil.

Incorporating bleeder resistors across every HV capacitor is a very basic design rule for any type of electronic equipment.

(Since the tank capacitor is usually connected directly across the ends of the HV transformer's secondary winding, I don't see where the presence or absence of a secondary center tap plays any role in capacitor discharge at all.)

Regards,
Herr Zapp



----- Original Message ----- From: "DC Cox" <resonance@xxxxxxxxxxxx>
To: "Tesla Coil Mailing List" <tesla@xxxxxxxxxx>
Sent: Wednesday, June 03, 2009 7:58 PM
Subject: Re: [TCML] Cap bleeder resistors


Technically, any cap should have a bleeder across it, especially in an
equi-drive circuit where the cap does not have a ground to discharge thru.

I have never used them in over 45 yrs. of Tesla coil building.  I have
handled a lot of coils with MMC and pulse type HV caps.

Here's what happens:  As long as the neon transformer is connected to the
cap the charge will migrate back thru the center tap winding (usually around
70K Ohms impedance) and go directly to ground.

If you have a circuit that is disconnected after coil operation, then
definitely use the bleeders.  If not, don't bother spending the
extra money.

Dr. Resonance




On Wed, Jun 3, 2009 at 7:45 PM, Andrew Robinson <teslamad@xxxxxxx> wrote:

Now that I think of it, I've never used bleeder resistors in a MMC
capacitor. Unless my physics are incorrect (someone please correct me)
bleeder resistors are not necessary in a tesla coil capacitor. I am under
the impression that they are used to safely discharge the cap after power
has been shut off, but if you look at a simple tesla coil schematic, a
complete circuit always exists with the capacitor, preventing charge
separation... the cap is always in a state of equilibrium when power is off. Does the cap staying charged have something to do with the back EMF from the primary transformers secondary winding? In all my experience, ive never had
a cap retain a charge after shutdown. Can someone shed some light on this
theory. Do you really need bleeder resistors? Thanks
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