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Large capacitor discharge rate?

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LAN_403

Microfarad

Thu Oct 18 2012, 08:36PM

Registered Member #4343 Joined: Mon Jan 16 2012, 01:45AM
Location: Florida
Posts: 10

Hi guys,
I've done a couple of projects in the past (coil gun and micro railgun) with photoflash caps, which have a very fast discharge rate, and I was wondering what he discharge rate of something like these would be?

It's 450V 10,000uf capacitor made by Cornell Dubiliar. If it would discharge fast enough, these perfect for a two-stage .50 caliber coilgun project I have in mind...
Thanks in advance.

Mods: if this would be better suited for a different subforum, please move it. Thanks.

Microwatt

Fri Oct 19 2012, 02:17AM

Registered Member #3282 Joined: Wed Oct 06 2010, 05:01PM
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Posts: 224

about 100ms

Andre

Fri Oct 19 2012, 02:37AM

Registered Member #6921 Joined: Wed Sept 26 2012, 07:47PM
Location:
Posts: 109

depends on the resistance,

Time = R Ã— C
Time = 100 ohms x 10,000uF
Time = 1 second

Pinky's Brain

Fri Oct 19 2012, 02:38AM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
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Posts: 837

Microwat, what happens if you try to discharge it faster? Does the electrolyte flash-boil? (The normal ESR at ~15 mOhm is not the limiting factor at 100 ms and I don't think parasitic inductance is either.)

Microwatt

Fri Oct 19 2012, 03:48AM

Registered Member #3282 Joined: Wed Oct 06 2010, 05:01PM
Location:
Posts: 224

why don't you look this information up i am sure the manufacturer has this information laying around.

Pinky's Brain

Fri Oct 19 2012, 04:57AM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
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Posts: 837

The derating curves don't go far enough for what coilguns put these things through :) Just interested in your or any other's experience ... what happens to the ESR during short circuit discharging of electrolytic capacitors? (Don't have a high current shunt or scope, so I can't try it out myself.)

Steve Conner

Fri Oct 19 2012, 09:23AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

As far as I know, the ESR stays constant, and it is in fact the limiting factor. In practice a large inverter-grade capacitor will just blow a chunk out of whatever you try to short it with. This has always been my experience: pieces of my circuit get scattered around the room with a huge flash and bang, but the capacitor doesn't seem at all bothered.

The short circuit current is determined by the ESR: 450/0.015 = 30kA.

The fastest possible discharge is also determined by the ESR. Short-circuit discharge time constant = RC = 0.015*10000e-6 = 150us.

In a short-circuit discharge all the energy is dissipated inside the capacitor. Stored energy = 0.5*10000e-6*450^2 = 1kJ. If we assume the capacitor innards consist of 250g of water (seeing as it is filled with water-based electrolyte) then the internal temperature rises by about 1 degree celsius. That explains why the capacitor doesn't boil or explode.

The maximum possible power output is 3.3MW into a 15 milliohm load, which explains where that chunk of your screwdriver went.

All in all, much more powerful/destructive than a couple of photoflash caps.

kilovolt

Sat Oct 20 2012, 07:11AM

Registered Member #2018 Joined: Tue Mar 10 2009, 09:56AM
Location: Switzerland
Posts: 74

Steve Conner, you're right, but normally the resistance of the "load" even if we always think it is a nice short circuit, will be higher than the ESR of the capacitor itself, because there are always contact resistances and inductance of the short bridge itself.

Best regards
kilovolt

Sulaiman

Sat Oct 20 2012, 09:42AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

don't forget esl,
for this size capacitor esr = esl at around 10kHz,
so a direct short-circuit across the terminals would have di/dt limited by the esl,
which would be about 20 nH at a guess,
20nH & 10,00uF gives a 'resonant' frequency of about 11 kHz
so 50 - 100 us seems about right.

I haven't any large Dubilier capacitors but quite a few Epcos,
which datasheets say are DISCHARGE-PROOF.

With 3/16" copper tubing formed into a single loop 12" diameter as a 'load'
the inductance would go up to about 20nH esl plus 812 nH loop inductance
... 1.75 kHz 'resonant' frequency .. around 300 us for 1/2-cycle.

For a coilgun the inductance of the coil
and the current carrying capacity of the switch/transistror/thyristor
will be the main determining factors for discharge rate,
not the capacitor durability, esr or esl.

Just remember to wear ear and eye protection ! ( 1kJ )

teravolt

Tue Oct 23 2012, 03:47AM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

the time to rise will be determined by the series inductance in your circuit and the max current will be determend by the total series resistance. So to get the moast energy to your load make you leads short and beafy as pausible.

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