For some reason I'm not seeing the original post, so I'll tack this on
here--if their condition is unknown it would be a good idea to have a solid
barrier between you and the capactor when charging and discharging the
first time, and do one at a time..
Also, have good ventilation in the test area--if one blows you probably
don't want to breathe what comes out of it.
Discharging caps that way can ruin the cap. Just turn off the supply or
apply a resistor load. The internal plates violently move - ripping themselves
You get away with it - smile - sell it - it blows up when used. Not a happy
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Given that they were made by Maxwell and sold to
Fermilab, they are probably spec'ed for pulse
discharge. That said, I'd not want to be on the
same city block when iggy crowbars them.
Putting a hard metalec short across them will no
doubt blow half the terminal away, create a deafening
blast and generate a nice EMP pulse. There's no
way I'd do it.
I worked in a medical laser research group. We had 2u odd caps @ 10kv+ as
part of a home brew laser supply. Before working on it we'd double check the
caps were disharged in case the discharge resistors had failed.
A couple of PVC pipes 2m long with a 12" length of heavy neon sign cable
linking them, a couple of M4 screws were stuck through the ends.
When the caps were charged it was like a gun going off - very sharp bang, it
usually bought people out of offices all down the hallway. You never stood
behind whoever had the rods in case you got a reacting elbow in the face.
I have a bunch of 1N4007 diodes (1000v rated). If I put, say, 20 of
them in series, would that be sufficient to rectify 9,000 VAC safely?
I have read some articles discussing that since leakage amps are not
identical, that I need to put resistors in parallel with them. If so,
these need to also be 1kV rated resistors, right?
If not, would anyone have a suggestion for a 30 mA rectifier for 9
According to http://www.pupman.com/listarchives/1998/August/msg00143.html
"Use a MOT diode: typically they are 9kV 450mA and very cheap."
Apparently MOT means microwave oven transformer. Note, the 9kV RMS
AC of your Franceformer presumably peaks at about 13kV, so you would
need a variac on its input if using a single 9 kV diode. Also see
http://www.pupman.com/listarchives/1998/August/msg00309.html in same
thread, which implies that obvious tubes like 1B3 won't handle 30 mA.
Nah- 9kVAC is 12.7kV peak, call it 15kV peak. The rectifier has to stand
off full voltage on the backswing, so you need 30kV diodes, minimum. 40 or
50kV would be even nicer. ;)
If you make a doubler (which uses diodes of the same rating, but produces
twice the output voltage) you can test the capacitors at a bit over rated
voltage (maybe 26kVDC, 118% of ratings). All the caps I've bought are rated
for 150 or 200% of rated voltage for some time, though that doesn't mean
If you bring up the voltage slowly, through a resistor or variac perhaps,
you can monitor it (assuming you get a voltage probe) and stop right at 22kV
Yeah, but conversely, I recall reading an article which stated that modern
diodes are avalanche rated, meaning that if the voltage across one diode
increases to say, rated PIV, current starts going up (it looks like a really
high voltage zener diode), pulling it back into balance.
Capacitors across the diodes were also recommended, but today's diodes are
more rugged to pulse and avalanche conditions (we've come a long way from
"top hat" diodes!) so this isn't necessary either.
Could rip the diodes out of a few TV sets/monitors. Or use the flybacks
Deep Fryer: a very philosophical monk.
Tim, I lost you a little bit here, sorry. Are you saying that I need
single 30kV diodes and that putting 1 kV diodes in series is
unsuitable? Or are you saying that twenty 1,000 V diodes is not
30-50kV PIV "diode", collectively. You can make that "diode" out of as many
1N4001 diode*s* as you need. :)
Deep Fryer: a very philosophical monk.
Did something like that about 40 years ago. In our case we had to
bridge each diode with a small capacitor and large resistance resistor
because when seeing the reverse voltage, they did not all turn off fast
Both rectifying 400 Hz and use as a series diode in a resonant charging
circuit. Actually I was using 1 ampere silicon rectifiers which were
not fast enough. All it takes is one not turning off fast enough.
I don't remember just what we ended up using. I think it was about
.001u and 10 Meg across each diode.
Another problem was that a few diodes had bad junctions and couldn't
take the current. When you put 30 of them in series the probability of
getting a bad one is pretty high. Had to weed them out first.
You can make 30KV from a single 1KV 60 HZ transformer, with
enough diodes and capacitors. The basic circuit of a voltage multiplier
| | | |
\---/ --- \---/ ---
\ / / \ \ / / \
--- /---\ --- /---\
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That one is enough for a 4X multiplication of the voltage. Keep
adding pairs of diodes and capacitors and you keep adding voltage
multiplications -- and losing current capacity.
1 KV AC is 1.414 KV DC peak, times 4 gives you 5.656 KV just
with this simple circuit. At work, we used fairly small assemblies
potted in epoxy to get 45 KV (three 15KV taps from a 1KV P-P input.
But -- it might take nearly forever to charge your caps.
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