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Apparently around 20 microwave caps is enough to explode stuff?

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Arcstarter

Thu Dec 04 2008, 09:51PM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

Well, i have some large GE caps(only one is any good), it was on a thread here before, and i decided to charge it up and see if it worked. Well, the capacitance is only 12.5uf and max voltage is i think 7650 volts, which would be 365 joules. I decided to charge it with a mot instead of my pig to be sure it worked. That is about 25 joules. I just grabbed a microwave transformer and hooked it pretty much straight up, with a diode and two current limiting resistors, which are each 16kohms, in series for 32kohms and about 24 watts. It charges in about 4 seconds, and after a few charges and discharges the resistors get hot enough to boil any water on your skin :P.

I did a few discharges, they were pretty loud, but not requiring any ear protection. I decided to get some very thin(~36 gauge) wire and see if it would just blow the end off. Well, to my amazement it blew the whole thing up! And this is only 25 joules.. So i cut a few pieces of aluminum foil, each about 1/4 inch thick and ~3 inches long and it blew up too! It was fairly loud, so i put ear protection on :P. Well, i decided that was still not fun enough, so i cut some thin slices of pickle. They where about 3/8th inch, and they got a hole blown in them.

This is roughly equal to 12-20 microwave caps. The problem with this cap is that is weighs about 30-50 pounds. But, it is made for i think it was 150kvar. But that doesn't matter really. The inductance is probably low though, so microwave caps may not do quite as good. You could even take it a step further and get 20 microwave caps and put 2 in series and 10 of those strings in parallel for faster discharge times. If you had all the caps the same and 1uf, that would be 10uf and 4kv, which is 80 joules, much more than my cap at 2kv! But at 4kv my cap is 100 joules.

By the way, i measured my cap to be 12.5uf, not just something i think.

Be careful though!

Mattski

Thu Dec 04 2008, 11:16PM

Registered Member #1792 Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527

From what I've read, microwave caps will eventually fail when pulse-discharged and can explode. Here's an old forum post:

so use them at your own peril and take precautions.

And their ESL and ESR is definitely going to be worse than a real pulse capacitor, but they might be good enough to blow some things up. And remember: 2 in series by 10 strings in parallel is just 5uF because you divide capacitance when you put them in series.

Arcstarter

Thu Dec 04 2008, 11:39PM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

Mattski wrote ...

From what I've read, microwave caps will eventually fail when pulse-discharged and can explode. Here's an old forum post:

Haha oops, i see what i did. I knew that there was 20 caps so i multiplied .5 by 20... Well, 40 joules then.... And yes, i know that microwave caps would explode. Just saying :P.

LithiumLord

Fri Dec 05 2008, 12:06AM

Registered Member #1739 Joined: Fri Oct 03 2008, 10:05AM
Location: Moscow, Russia
Posts: 261

Well, MO caps will explode as an experiment I saw on YT proved. However you may get the current below the maximum pulse current for them by means of inductors. Bad that current is a mystery afaik :(

teslacoolguy

Fri Dec 05 2008, 03:23AM

Registered Member #1107 Joined: Thu Nov 08 2007, 10:09PM
Location:
Posts: 792

According to Steve Ward here

he was able to charge mo caps to 8kvdc each before they broke down and they took the breakdown very easily. I would go with a happy medium and charge them to max about 5-6kv depending on what they are rated. They are not the best solution for high power discharge but they will work to some point.

Mads Barnkob

Fri Dec 05 2008, 11:42AM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

A collegue from work tried with 18 MOCs and he would have to discharge them real fast in order to get some of the charge before it was burned by the internal resistors.

He charged the caps with around 4-5 kV and exploded more HV diodes than cans :) But he did get to crush some cans abit.

But its not worth the time getting them compared to finding some bigger lyttics or pulsecaps, atleast dont invest money in it.

Billybobjoe

Fri Dec 05 2008, 08:17PM

Registered Member #396 Joined: Wed Apr 19 2006, 12:55AM
Location: Pittsburgh, PA
Posts: 176

MadsKaizer wrote ...

But its not worth the time getting them compared to finding some bigger lyttics or pulsecaps, atleast dont invest money in it.

I disagree with that statement. Have you checked the prices on pulse caps lately (even on eBay)? Electrolytics are cheaper but then you need a massive freewheeling diode and they're slow, even compared to MO caps. See Steve's page here (again):

The ESL is pretty good considering the price.

Thanks to Steve Ward's idea I obtained 25 MO caps from a surplus site for $1 a piece. I'll get some pics of my setup later but it crushes cans and explodes wire like a champ. I've shot some 40 shots off with my bank (at between 6 and 7kv) and they are still holding strong.

Conundrum

Fri Dec 05 2008, 09:12PM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061

interesting. the problem would seem to be safety- all that energy being dumped into one cap would be a severe shrapnel hazard!

-A

Billybobjoe

Fri Dec 05 2008, 09:35PM

Registered Member #396 Joined: Wed Apr 19 2006, 12:55AM
Location: Pittsburgh, PA
Posts: 176

Conundrum wrote ...

interesting. the problem would seem to be safety- all that energy being dumped into one cap would be a severe shrapnel hazard!
-A

Definitely! I meant to cover that in my first post. Parallel capacitors are likely to destroy the one that fails . . . Mine are enclosed in a solid plywood box with a thick clear PETG top. Luckily, the internal resistance as MadsKaizer mentioned earlier happens to be a good safety feature. The caps tend to discharge very quickly without continuous charging - my charging circuit is designed (by the help of some forums members) to stay attached when the bank is fired. By themselves the caps will go from a full charge to empty quickly - maybe 10 seconds - and if you are doing that often be aware of power dissipation in the enclosed container. 25 capacitors in parallel seems to be the maximum for a 9/30 NST - any more and you'd need a current upgrade to reach 7 or 8 kv quickly (and when you're overrating the voltage like in this case you want to keep the time charged/charging down to a minimum).

EDIT - See here as well for some good information:

aonomus

Fri Dec 05 2008, 09:44PM

Registered Member #1497 Joined: Thu May 22 2008, 05:24AM
Location: Toronto, Ontario, Canada
Posts: 801

Don't most modern microwave caps have built in bleeder resistors? Could the bleeder resistor heating via continuous charge contribute towards the heating and eventual explosion, or is it just dielectric breakdown?

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