The small caps are 3300 picoFarad at 10KV, the larger caps at the bottom of the stack are 4800pF at 12KV; each diode is 500mA at 15KV (high speed). The new design will incorporate 2 sets of 3 caps in parallel, in series with each other, for each stage (see images).
Using these values with a 40KHz supply:
Edrop = I /(f*C) * (2/3*n^3 + n^2/2- n/6)
Edrop = 1/(40000*4.95e-9) * (18+4.5-.5)
= 111111 volt drop per amp or 111 volt drop per mA (ignoring diode losses)
Before I go ahead and build this, I would like some suggestions / criticism on the design.
A couple of years ago someone from this forum made the following suggestions:
1) place the capacitor columns in a triangle
2) include protection resistors in-between the capacitors
3) place equipotential rings at each stage
4) put the large caps on the outer columns for more current
5) use a large metal sphere or torus on the output
I have implemented suggestion (1), capacitor columns in a triangle, into the design.
Since I have a large ballast resistor on my fusor (50Kohm 1000W) and the diodes are over-rated, I intend to not implement (2), the protection resistors, because they will complicate an otherwise robust construction.
Would like to implement 3), but I don't know at which points in the circuit the equipotential rings connect.
Would like to implement 4), but don't have sufficient large caps, so they are going on the first stage for ripple.
Will implement 5), a metal sphere on the output, when everything else is done.
The brass plates in the image will be waterjet cut, and their corners rounded off.
Cheers,
Jon
Doug Coulter on 2011-16-08 09:36 (from fusor forum)
This looks real nice. You connect the rings at the DC output of each stage, and can use one at the top as well - they want to be fairly large in diameter (cross section) so they aren't corona points themselves.
I wouldn't use resistors in series myself. You could make a case for having them in series with diodes, but they'd have to be able to stand off full supply volts without arcing across them -- that's hard to achieve. And of course, they add even more series impedance. To the extent you want some (I believe you do) -- get it by using smaller caps on the AC part of the multiplier.
If you have more than a single cap size, you'd put the larger ones on the bottom. But! This loses the equal voltage division in a quick discharge (arc) and causes diodes to have to carry potentially nasty peak currents in that case. If you find yourself killing diodes in that case, this would be the first thing to look at. Perfect prevention of any arcs can be a real challenge.
You can "get away with" a fairly "squishy" supply for a fusor, and it can even be desirable as a sort of built-in current limit that protects the driver stuff in the case of a too-heavy load. That is, as long as you can hit your target voltage under the desired load. It might even be desirable to have a fairly high output impedance so you don't need as much fancy protection in the electronics. You will surely let in too much gas at some point and create a situation where the thing will act as a 600 volt (or so) zener diode, or what amounts to a short across the supply. Voltage drop across the series AC caps can be a big advantage in this case as it doesn't make heat anywhere, and limits peak driver current.