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Marko

Thu Feb 09 2006, 10:07PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

In my school I just got six 21000 uF 100V electrolytic capacitors, scrapped from some (relatively) old supercomputer power supply, each stores around 100 joules (when almost full)
They seem to be in very good shape, im only wondering could they be of some use for small railgun, coilgun or just exploding (mini).

As mains rectified gives around 300V so I cannot charge them directly.

This could be easily done with few small transformers, but I want ot ask experienced ?!

Is it beter to charge whole bank parallel at ~100 volts or connect them 3 vs 3 in series to get 200V but double less capacitance?

Maybe to go all for voltage and series them all (need multiplier) :D

Most that matters is switch I could use, buyng monstrous SCRs is what I would avoid if possible, and voltage is too low for triggered gaps, some crude mechanical switch will weld every firing but his seems only solution.
Any other suggestions?? !lol

Michael W.

Thu Feb 09 2006, 11:04PM

Registered Member #50 Joined: Thu Feb 09 2006, 04:07AM
Location: Vernon, B.C, Canada
Posts: 324

The SCR - Watch ebay, sometimes theres good deals, I think the mechanical switch would be the only way to go.....

Simon

Fri Feb 10 2006, 02:28AM

Registered Member #32 Joined: Sat Feb 04 2006, 08:58AM
Location: Australia
Posts: 549

I'd go series - if they work at all. In parallel you've got a lot of capacitance (long pulse) and not much voltage.

Part Scavenger

Fri Feb 10 2006, 03:07AM

Registered Member #79 Joined: Thu Feb 09 2006, 11:35AM
Location: Arkansas
Posts: 673

Go for the most voltage and here's why: Wired in parallel, the caps will have the exact same joule capacity as in series. But, the higher the voltage the more amperage flows per fixed resistance. (Ohms law)

Be careful to research seriesing electrolytic capacitors, as I hear this can lead to problems. I have never had trouble, but I have never used 'lytics at that power level before. ;)

BTW, You could use a starter solenoid for the switch.

Pcnerd

Fri Feb 10 2006, 06:16AM

Registered Member #88 Joined: Thu Feb 09 2006, 02:37PM
Location: The Netherlands
Posts: 29

if you want to go in series or paralels is for yourself, do you want to keep your voltage low, paralel, en do you want high voltage in series. But, the energy stored will rise when put in series, not much, but mostly good for about 10-120&% I think.
be aware of charging, mostly a transformer is not enough, use a ballast, can be anything from a resistor to a lightbulb.

Steve Conner

Fri Feb 10 2006, 10:17AM

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

How can the energy stored be more when the caps are in series? Surely it should be no different.

Marko

Fri Feb 10 2006, 01:31PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

you want high voltage in series. But, the energy stored will rise when put in series, not much, but mostly good for about 10-120&% I think.

Stored energy is always the same, seriesing or paralelling cannot give more joules in any way.
capacitance is 21000uF = 0,021F and voltage 100V (round)

series

E = (0.021F/6)*600V^2 = 630 J

paralell

E = (0,21F*6) * 100V^2 = 630 J
(few volts lesser and we can circle 600 joules)

Matches laws of physics excellently.

Also voltage logic is questionable with elecrtolytics, that have relatively high internal resistance (compared to plate caps where 'outside' resistance is more limiting)

If I series them all I have more voltage but max current is exactly samo of max current for 1 capacitor. Voltage is high but limited to this current. (ohms law again)
Probably thats why is spoken that they are not good for seriesing (as plate caps are)
Thats why im asking about series or paralell connection..

In paralell max current is multiplied by 6, but voltage is low.

%-6

100V would be safest,
but to connect 3vs3 for 200V, and I can simply rewire it to 100V if problems occur.

Switch is far bigger problem, I dont like to go arcwelding so mechanical switch cannot be just a solenoid pushing some metal conductor, that will surely weld and dissipate far more power than intended target.
Solenoid is simple and good idea for triggered gaps, but not for direct contact switching.

conclusion: I must buy SCR.

If I dont find something in junk again !dodge

Pcnerd

Fri Feb 10 2006, 02:15PM

Registered Member #88 Joined: Thu Feb 09 2006, 02:37PM
Location: The Netherlands
Posts: 29

whoop's, I miscalculated, thought that 2 cap's in series was just the same, forgotten that that was half of the capacitance, stupid me, i'm sorry :s.

Self Defenestrate

Fri Feb 10 2006, 02:16PM

Registered Member #87 Joined: Thu Feb 09 2006, 01:36PM
Location: San Jose
Posts: 191

Definetly go for SCR switching. I'd build a bank with three paralled sets of two caps in series. Whats your line voltage?

Part Scavenger

Fri Feb 10 2006, 03:18PM

Registered Member #79 Joined: Thu Feb 09 2006, 11:35AM
Location: Arkansas
Posts: 673

If I series them all I have more voltage but max current is exactly samo of max current for 1 capacitor. Voltage is high but limited to this current. (ohms law again)
Probably thats why is spoken that they are not good for seriesing (as plate caps are)
Thats why im asking about series or paralell connection..

I don't really know what you mean by this. The higher voltage will allow the capacitors to discharge more quickly.

To demonstrate:
126000uF, 100V(the caps connected in parallel) shorted through 1ohms will discharge in .5-.63sec at peak amperage of 100A. Joule rating is .5C(V)(V) = 630J.
3500uF, 600V(the caps connected in series) shorted through 1ohms will discharge in .014-.018sec at a peak amperage of 600A. Joule rating is still .5C(V)(V) = 630J

Thus, you can see that in the first selection, 630J is discharged in .5-.63sec, but in the second selection it is discharged more than 30x faster! Plus, you have a higher peak current rating.

Here a good link to read:

I'd wire them up in parallel and see what happens, then if you're feeling lucky, wire them in series. Note series will pose a higher stress on your caps because of discharge time. And I have to agree with getting an SCR. I would not be very hard to find a SCR with a pulse rating of 600A, the ones ForceLabs bought handle 1600A. You can fry your SCR by exceeding the pulse current for longer than the rated pulse time. You can figure out how long the capacitor takes to discharge by 4RC, where R is the resistance you are dicharging the capacitance through. It actually takes longer than this, 98% actually discharges in 4RC, but by that time the current will be minimal.

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