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teslacoolguy

Sat Feb 09 2008, 03:58PM

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

i am planning to make a 2.5kj capacitor bank for a can crusher. the capacitence will be 7800uf at 800v and i cant find a formula to find how many amps that will be because i need to find an appropriate scr to withstand the surge and was wondering if anybody has a formula to calculate that.

TheMerovingian

Sat Feb 09 2008, 05:11PM

Registered Member #14 Joined: Thu Feb 02 2006, 01:04PM
Location: Prato/italy
Posts: 383

The surge current depends on 1) capacitor ESR 2) connections resistance and inductance 3) Coil resistance and inductance. You should get these values , use an RLC simulator and then get the peak current and overrate it of 50% to calculate the needed SCR. Also remember to use antiparallel diodes to protect your caps from reverse charging.

PS: this should be posted in HIGH VOLTAGE section

ramses

Sat Feb 09 2008, 09:24PM

Registered Member #1208 Joined: Thu Jan 03 2008, 05:30PM
Location: Chesterland, OH
Posts: 154

the diodes that they sell at radio shack can't take this kind of abuse. especially if they decide to conduct backwards. so buy yourself a bridge rectifier on ebay and use a leg, 2 or 4 of that instead. I found out the hard way

teslacoolguy

Sat Feb 09 2008, 10:41PM

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

im not worried at all about the diodes i bought a 1ka surge diode from newark that im going to use so i should not have any problems at all

Barry

Sun Feb 10 2008, 05:03PM

Registered Member #90 Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 301

A simple RLC calculator here will let you try some values. Spin the inductance dial until you get the timing you want, and assume near-zero resistance to find the theoretical maximum peak current for a given LC circuit. Now if only we could actually build a zero-resistance coil ...

Let me know if there are any technical problems in the simulator.
Cheers, Barry

teslacoolguy

Sun Feb 10 2008, 09:55PM

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

ok im having a bit of trouble. i have no idea how much inductance or resistence a couple turn coil will be for a can crusher and i am getting readings up to like 70ka and i know that is way out of the ball park so im a little confused

Barry

Tue Feb 12 2008, 03:41PM

Registered Member #90 Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 301

heh, welcome to the electromagnetic design process. You have the capacitor and you're trying to figure out the requirements for the coil and switch, both of which depend on each other's design.

Since you have some idea about what size coil to use, I suggest that you estimate its resistance and inductance using a coil calculator like this one.

This will get you started but it's pretty rough estimate -- your coil is only a couple turns and the other wiring can have a significant effect on the actual values.

Keep in mind that 2.5kj is a heckuva lot of energy into a few inches of wire that represents almost a dead short. So I would expect that 50 - 100kA is not unreasonable. Indeed it's the whole point of your circuit as you want as much surge current as possible in a can crusher.

Cheers, Barry
I'm thinking: What would Scooby do?

ramses

Tue Feb 12 2008, 08:08PM

Registered Member #1208 Joined: Thu Jan 03 2008, 05:30PM
Location: Chesterland, OH
Posts: 154

well good luck finding a 100ka SCR... for that reason, I would use a trigatron (triggered spark gap), or an open air mechanical switch. fastmhz's 24kj cap bank thread has a few links, as well as pix of the trigatron he is using. if at all possible, I would go for a slightly higher voltage like 1.2kv to achieve the fastest possible risetime and biggest peak current.

Quantum Singularity

Sun Feb 17 2008, 02:11PM

Registered Member #158 Joined: Sun Feb 12 2006, 09:53PM
Location: Central Ohio
Posts: 282

Has anyone successfully done a can crusher with lytics? I was thinking the pulse is just not fast enough but not sure. I tried it with my 3kJ lytic bank, ~6800uF, I think maybe up to 1-1.5kJ, and got nothing. Alternatively, my 120uF energy discharge cap charged at 1KJ put a serious hurtin on a can. IDK, maybe I just had a poor coil, I didnt play with the lytic can crushing much.

ramses

Mon Feb 18 2008, 06:18PM

Registered Member #1208 Joined: Thu Jan 03 2008, 05:30PM
Location: Chesterland, OH
Posts: 154

the guy from powerlabs did one, but it didn't work all that well... probably because he used a SCR switch with a 3ms turn on time... use a trigatron!

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