Author Topic: Are bleeder resistors necessary if I can easily discharge my capacitor?  (Read 932 times)

Offline jpb133

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I'm working on a mmc bank for my dual mot tesla coil, is it absolutely necessary to add bleeder resistors if I can easily discharge the capacitor? The terminals are close enough together that I can easily put a metal rod across them with the help of a well insulated pole. And wouldn't they discharge across the secondary coil of the transformer anyway? Can I simply short out the ends of the capacitor bank or must I discharge each capacitor individually? Thanks in advance for any information.

Offline profdc9

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I'm working on a mmc bank for my dual mot tesla coil, is it absolutely necessary to add bleeder resistors if I can easily discharge the capacitor? The terminals are close enough together that I can easily put a metal rod across them with the help of a well insulated pole. And wouldn't they discharge across the secondary coil of the transformer anyway? Can I simply short out the ends of the capacitor bank or must I discharge each capacitor individually? Thanks in advance for any information.

I made a dual mot tesla coil, and I discharged my capacitors with a rod across the safety spark gap.  However, if you forget to do this, and don't have bleeder resistors, the charge could easily kill someone.  Also, discharging the capacitors this way is hard on the capacitors, and occasionally even high quality Cornell-Dubilier capacitors have been known to explode if accidentally shorted.  I included twelve 33 megaohm resistors in series across each metal-oxide varistor as a bleeder.   I also had to include about 1000 pF of capacitance @ 20 kV for RF protection as well.

I have to say having built both a rotary spark gap MOT tesla coil and a DRSSTC, the DRSSTC is much more fun, and in some ways, easier to build.  Getting the rotary spark gap to work reliably was very hard.  The spark gap wheel is very hard to balance.  But I am happy to share details of how I put it together.  The schematic of my dual MOT tesla coil is attached.

Dan

Offline jpb133

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Thanks for your help, it sounds like it would be worth it to add them in for safety and for the longevity of my capacitor bank. Can I use regular 10m ohm resistors either 1/2watt or 1 watt?

Offline MRMILSTAR

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All energy discharge capacitors have a maximum current rating. As an example, I use a bank of 6 Maxwell 37667 capacitors in series-parallel for my spark gap coil. Each capacitor has a maximum current rating of 25,000 amps. This sounds like a lot but if you discharge directly across the terminals you can easily exceed this possibly resulting in damage. I found this out the hard way. My first capacitor bank had the bus bars too close together. During operation I would get an occasional very loud bang caused by flash-over between the bus bars. After about 5 of these flash-overs, the case of one of my capacitors cracked under the mechanical stress and it started leaking oil. I then re-designed the array to move the bus bars further apart. I also use a bleed resistor across each capacitor.
Steve White
Cedar Rapids, Iowa
Retired electrical engineer

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