Cactus Cap (MMC)

This is the new MMC I built for the 4" coil refurbishment project. The component caps are 68nf/1600V metalized polypropylene film capacitors with axial wire leads. I used 5 series strings in parallel with 10 caps per string for a total of 50 caps and 34nF. I chose 34nF for resonant operation with the intended power supply of two 9kv/60ma NSTs in parallel. Fabrication is always an issue with the MMC. How do I mount & wire all those darn caps? I took a novel approach with this cap and glued all the series strings to the outside of a length of 2" PVC pipe. I used sort of a wire-wrap technique to attach all the 10M, 1/2W bleeder resistors, and tacked them all together with a soldering iron. An important tip about MMCs--don't let anything touch. Notice on the Cactus Cap that none of the individual caps touch one another except at the leads, and none of the resistors touch any of the caps. If you crowd the caps and/or resistors too close together, you're inviting disastrous arc-over between components. Besides, the extra space no doubt assists with cooling.

After I got all the caps glued down and wired together, I noticed all the cap leads sticking out sort of looked like bristles. I was going to trim off the excess wire lead length until I thought to myself "this thing looks sort of like a cactus". The name stuck, and I left the extra lead wire sticking out. I know the cactus cap is weird looking, but it was simple & cheap to build and is quite serviceable. It lends itself to vertical mounting by simply standing it on one end, or horizontal mounting as I did using a couple of "L" brackets. Not everybody has perfboard, PC stock, or Plexiglas sheet lying around for MMC fabrication, but almost everybody has scrap PVC pipe in their junk pile. I can't imagine a cheaper or easier MMC mounting scheme.


The cactus cap goes international! Here's another neat execution of the cactus cap by UK coiler Colin Heath. Nice photo Colin. Good workmanship on your "cactus" too. Does it work as good as it looks?