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Question about DC resonant charging
Questions to you who use DC resonant charging.
Do you use a choke that is resonant with the tank cap at some
breakrate?
For a small coil that would make a very large choke, especially if it
were air core, which would be most desirable in my opinion. I got to
thinking about this when I was trying to figure out how to size the
current limiting inductor for a filtered DC supplied TC with a static
gap. I think now after thinking more about it, that I do not
understand what it means, this "DC resonant charging".
Help please.
For my application, I have a 6.5KV at open circuit DC supply mad from
a small single MOT of about 500 to 600 watts capacity judging by the
core size. The configuration is a full wave voltage doubler, two
MODiodes and two MOCaps with the core of the MOT connected to the
center of the two series capacitors. Each cap is about 0.9uf so total
capacity of the filter cap as seen by the output terminals
(+3.25KV,-3.25KV) is 0.45uf. 6.5KV is open circuit measured. As
current limiting is not efficient in a simple half wave rectified DC
source with a MOT, I forgo the short circuit current test. I guess it
would deliver an average charging current of about 100mA at 5000VDC,
or 500VA with no problem. I threw this PSU together from junk just to
stick on my "medicine coil" and see what it could do with some power.
I would think close to a foot would be easy. Of course that is the
height of the coil, so it couldn't go much beyond that without a
larger topload to do some field shaping. It runs with a 6.5KVAC 7.5mA
current limited transformer and rectifier out of an old copier HVPSU
charging a 14nf MMC. When the cap reaches firing voltage, it
discharges through the gap (two solid copper balls about 1 1/4" in
diameter with some airflow)and the primary (4 turns 10AWG solid THHN
red, helical, Dia. 3.25", Height 1") to ground. Transformer sec., tank
cap, pri. all use common ground point (third pin). Secondary uses the
ground rod in the middle of the floor. Secondary is 4.25" OD X 12"
long, has 440 turns of 26AWG magnet wire space wound at 40 TPI for 11"
winding length. Topload is either a 1.25" aluminum ball on the end of
a piece of 3/8 copper tubing about a foot above the coil, or a 1" X 7"
aluminum foil tape covered and smoothed Styrofoam disk with the edges
turned by scraping with a knife on a lathe. It works exactly as
planned, I couldn't be more pleased, but y'all know how it goes...
Gotta push it a little harder, then just a little bit harder, then...
So I grabbed the MOT and a couple of rectifiers and a couple of caps
and now I have a DC source that I can just slap on their and see what
happens with some juice- BUT WAIT! that wont work. As soon as the gap
fires, the diodes will blow. Don't have to try it to figure that one
out. Gotta limit the current on the HV side. So how much do I limit
it. And how do I control it. No variac, so got to get it right the
first time. The more I think about it, the more I think I am on the
wrong track. I try to use an inductance with a long enough time
constant with the tank cap to keep it from reaching firing voltage too
soon. But the numbers come out with an inductor that is just too darn
big.
Any insight would be most appreciated.
thanks,
deano