[TCML] my Pig Terry Filter
FIFTYGUY at aol.com
FIFTYGUY at aol.com
Mon Aug 18 00:47:00 MDT 2008
FWIW I added a pic of the Terry Filter I made for my pig, and some pics
of my control panel:
The Terry Filter definitely cured one problem that I noticed, where the
control cabinet actually "zapped" to my knee a little during runs. This
seemed like a Very Bad Thing, since the cabinet is solidly grounded to house
ground at a receptacle directly under the main panel, which has a typical ground
rod nearby. Furthermore, the pig has a floating primary and secondary -
neither are connected internally to the case, although I'm sure the core is.
I ran a wire from the star-connection point of the RF ground under the
coil back to the Terry Filter, which bolts to the pig case. The MOV's are 8.4
kV MCOV distribution arrestors.
Since I was pushing a lot of current with a pig, I waned to keep the
resistance down. So I used two 68 Ohm resistors in series in each leg, one
mounted on the filter itself, the other under the coil. Consequently, I had to use
a much higher capacitance to maintain a reasonable cutoff frequency of the
filter. Six caps in each leg, .036 uF 2500 V (Vishay MMKP 383 series). Nice
pretty blue, and yes, there are resistors mounted under each one.
I've only seen the safety gap fire once while running, just a very small
spark when I pulled in the power contactor to the pig. But it instantly
extinguished, with no climbing arc. I put the gap closest to the pig, so the
primary tank caps would have to go through the resistors to discharge (a little
easier on them, and less likely to power-arc at the safety gap).
I also wonder about all the fuss to avoid filter chokes on the high
voltage side, since wirewound resistors have non-negligible inductance (>100uH
each according to the manufacturer, and I measure around 70uH each on mine).
Everything's mounted on backboard made of a couple sheets of 1/8"
"fishpaper" glued together, with an angle iron support bracket that also provides
ground connection points. That's over $120 worth of insulating sheet according
to the Federal Stock Number, but surplus it's only a buck for a 2'x4' sheet.
Acrylic and Polycarbonate have gone through the roof - so if I don't need to
see through it or bend it, I'm gonna be using the fishpaper wherever I can!
The control cabinet pics are just to show how much stuff you can
cram in one small box!
Center for the Advanced Study of Ballistic Improbabilities
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