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Re: [TCML] Single MOT Coil?
On 8/16/13 8:32 PM, Greg Peters wrote:
Hi Ted,
I just joined this list today (for the 3rd time in the last 15
years), and am in the process of building a 3 phase, DC powered coil.
I would be very interested to learn more about your 10kW power supply
and especially rectifier and charging choke specs. So you're not
using any balancing resistors or capacitors? That's very interesting
as it saves considerable expense and effort. Do share some more
details please!
equalizing resistors and caps were popular back in the day when
manufacturing wasn't as consistent, and the caps/resistors were cheaper
than just buying more diodes for the string.
The idea is that if one of the diodes in the string turned off faster
than the rest, it would take more than it's rating of the reverse
voltage. But that's for slow diodes (particularly for line frequency..
it's not like the voltage reverses that quickly at 50/60 Hz)
These days,
a) diodes are a lot faster.. they'll all have turned off before the
voltage has gotten very high
b) diodes are a lot more consistent
c) diodes have better & more consistent reverse leakage characteristics
If you are using diodes specifically designed for HV strings, this is
even more so. There's a Philips/NXP series of 6kV diodes designed for
use in things like CRT HV supplies (triplers and quadruplers) that is
specifically made to be strung in series. (And potted.. they're too
small to stand off 6kV around the package in air)
In fact, the latest recommendations (e.g. 20 years or so) is that
capacitors actually make for larger and more frequent failures, because
of the stored
energy, and their consistency and uniformity is a lot worse (20%
tolerance isn't unusual for the inexpensive ceramic disk type)
I use stings of 1N5403 (1kv 3A) overkill here is really cheap and
I've never lost a diode stack in either the 1kW or 10kW DC coils.
In my case I've used about twice as many diodes (no equalising R/C)
than the theory suggests is the bare minimum.
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