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Registered Member #1253
Joined: Tue Jan 22 2008, 02:06PM
Location:
Posts: 39
I've come up with an idea for an MMC for my coil. Before I run out and spend my money, I just wanted to make sure it was "up to par". Here is a pic of my design. All the information is on it. If I am missing anything please let me know.
Thanks!
Mod Edit: the dilithium crystals can nae take more than 400 pixels
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
You might be missing a bleeder resistor across each cap. If you connect all 3 strings at all the nodes, then you only need one per 3 parallel caps.
If there's any unbalanced leakage on the caps, it prevents a steady build up of charge on one leading to over-voltage, and of course it runs the charge down at the end for safety.
Registered Member #160
Joined: Mon Feb 13 2006, 02:07AM
Location: Melbourne, Australia
Posts: 938
Your value is a bit too close to resonance. You should have about 1.4-2 times the resonant value otherwise the voltage rise on the bank may go to high exceeding the rating.
Registered Member #1198
Joined: Sat Dec 29 2007, 05:39AM
Location: Seattle, WA
Posts: 21
Generally people use the 942C series capacitor from CDE. They have better specs for use in TC service. Also using a higher value cap (.1-.15uF) will cut down on the number of caps you need (and generally the cost).
Registered Member #480
Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644
Remember that the peak output voltage of your transformer (if you limit the input voltage to 120) will be over 21,000 volts (15,000 RMS volts X 1.414). Your bleeder resistors will need to withstand this voltage, so use a resistor across each capacitor to allow the voltage to divide across the maximum number of resistors. Also, use special resistors rated for high voltage use. The Vishay VR-37 (.5 watt) metal film resistors are rated for 3500 volts, vs 500 volts for standard metal film resistors. The VR-37 resistors cost around $0.10 each, so the cost is insignificant.
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
Is that the idea for the bleeder resistors?
No.
The centre terminal of each cap pair is floating, so charge could build up on that. In addition, the ressitors need to withstand 2x cap voltage, so you need to start fiddling with series strings, or expenzive resistors.
Try this (the configuration, not the 1k/1n default values!). Even a value as high as 10meg would dissipate 1 watt at 3kV, so you might be better off with 22meg or 47meg. These still give seconds for dumping the charge when you've finished.
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MMC Design
