Safely testing 22 kV capacitors

Ignoramus27088 wrote:


Well, let's see. P = 22 kV squared / 1E6 = 484 Watts. Well, that isn't so amazing, but I don't know where you will find a 1 Meg Ohm resistor with a 500 W rating and a 22 KV voltage rating.
Jon
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
OBones wrote:

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.
--
--John
to email, dial "usenet" and validate
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
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 up.
You get away with it - smile - sell it - it blows up when used. Not a happy day.
Martin
Martin Eastburn @ home at Lions' Lair with our computer lionslair at consolidated dot net NRA LOH & Endowment Member NRA Second Amendment Task Force Charter Founder IHMSA and NRA Metallic Silhouette maker & member
Ignoramus27088 wrote:

-
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Martin H. Eastburn wrote:

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.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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.
r.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Ignoramus27088 wrote:

A FranceFormer is a neon sign transformer, and definitely AC.

Hmmm, better be sure that stick is dry, clean and free of cracks in the wood.

The boom might be quite impressive, as these are probably Marx generator pulse discharge caps, and may have a current capacity of tens of thousands of amps.
Jon
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Yes, they say "DISCHARGE CAPACITOR" on the dataplate. They probably were using them to power high power lasers. One of my acquaintainces worked there on lasers at some point.
i
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
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 kVAC?
thanks
i
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Ignoramus6399 wrote:

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.
-jiw
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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 these are.
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 or so.

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 themselves, LOL.
Tim
-- Deep Fryer: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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 enough?

Yes, tat would be very nice.
i
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

30-50kV PIV "diode", collectively. You can make that "diode" out of as many 1N4001 diode*s* as you need. :)
Tim
-- Deep Fryer: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Thanks, now I understand a little better. I will soon make a stack of, say, 30 1N4007 diodes. That would let me test the capacitors with 13 kV.
i
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Ignoramus27098 wrote:

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 enough.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Hm... Were you trying to rectify a 60 Hz sinewave?
i
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Fri, 31 Mar 2006 19:05:12 GMT, Ignoramus27098

Well, first thing you need is a big capacitor...

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Ignoramus27098 wrote:

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.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Rich256 wrote:

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.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
    [ ... ]

    You can make 30KV from a single 1KV 60 HZ transformer, with enough diodes and capacitors. The basic circuit of a voltage multiplier is:
(AC)---+--)|-+------+--)|--+-----(DC) | | | | \---/ --- \---/ --- \ / / \ \ / / \ --- /---\ --- /---\ | | | | (G)-)|--+-----+-)|---+------+
    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.
    Enjoy,         DoN.
--
Email: < snipped-for-privacy@d-and-d.com> | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Thanks DoN. I think that I will just make a 13 kV charger with my 9 kV AC source and a diode.
i
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here. All logos and trade names are the property of their respective owners.