Capacitor Shorting

[freeskier89]

I have a quick question. I tried searching for it but I got a lot of mixed results. Well anyway, I have a 200V 45000uF bank consisting of 45 200V 1000uF caps wired all together in parallel. I have used it for a coilgun, I was wondering if just shorting the caps over a small filament will damage the capacitors. Many people use a screwdriver to discharge caps, but supposedly that is not reccomended. The coil is already less than an ohm of resistance, but I am wondering if an even more direct short will damage them. Don't worry, I am/will be far away from the short/capacitors when the expirement is conducted . Oh, I am using a SCR rated for a ~12000A surge for the triggering.

Thanks a lot!
-freeskier89

 

[Someone Electro]

Yea caps are not suposed to be shorted.But in a coilgun its almost like shorting.

And dont use an screwdriver becose it will be destroyed.

I tryed conecting an pice of thin wire there the coil shod be.Wen i trigerd it the wire exploded in a big bang and spark.There ware also a few pecis the the orange hot wire tlying around.(Probobly not plesant to get on your skin)

I put it in a plastic box to keep the from flying evryware.(and hiting me)

Foil also goes of in a bang.

 

[freeskier89]

Thanks for the info . So you would you discourage the thin filament shorting expirements? Out of curiousity how many shorts (through a 2' wire) do you think the caps can take?

Thanks!

 

[Nigel Goodwin]

Thanks for the info . So you would you discourage the thin filament shorting expirements? Out of curiousity how many shorts (through a 2' wire) do you think the caps can take?

To discharge capacitors you should use a resistor, the value and wattage being determined by the amount of charge that needs discharging.

A light bulb is a pretty good 'resistor' for doing it, and it even gives a nice visual indication!.

Using capacitors in coil guns is rather abusing them, and outside their design specifications, so it's anyones guess how long they will survive, you could only test them to find out.

 

[Styx]

Well capacitors can actually be shorted as long as the voltage doesnt drop below 70% of its charged value (ie a momentary short)

We short out 30mF worth of capacitance (at 270V) for DeSat testing at work (only way to develope 3000A)

As long as the time between blaps is very long (order of minuts) and the capacitor is of relatively high quality with high ripple current capability (the caps we used were 3 in parallel with a ripple current handling of 100A)

It's usually the heat that is developed that causes the breakdown of the electrolyte that then blows the thing apart

 

[mstechca]

I normally conect the leads together to short my caps, but since I don't deal with HV, shorting might not be the best way to go.

Connect it to a resistor that can handle the power.

 

[audioguru]

Photoflash caps are specially designed for a coilgun type of application. Ordinary caps aren't designed to be shorted.
The resistance of an incandescent light bulb is 1/10th its hot operating value so I would use a resistor instead to slowly discharge your caps.

 

[freeskier89]

Thanks a lot for the replys. I ended up shorting them a few (15?) times just for the heck of it . Styx, most of the time they only dropped, 60% or so, but a few times it was a straight 170V discharge. The caps seem to be fine, except they seem to be charging slower (Could be my multimeter, but my guess is its the caps) . They still have a lot of power though :lol: . I might post some pics tomorrow.

Thanks again

Edit: I just was thinking, I had a heater plugged into the same extension cord as my bank. Maybe that was the problem with slower charge times. Your thoughts? I can't test it at the moment.

 

[Oznog]

It's hard to say. They definitely won't last as long when discharged like this. It's not only hard on the capacitor plates but also the dielectric between the plates.

But how much less? A cap commonly lasts billions and billions of AC cycles in normal use. So if one discharge were equal to the stress of a million AC cycles this wouldn't be a problem at all since the coilgun isn't going to be used thousands of times. Given how few cycles you will use this may not be a big deal.

Photoflash caps are a good idea though. A lot of the electronics surplus websites carry a lot of them for really cheap, sometimes like $0.50-$1.

 

[lord loh.]

Photoflash caps are specially designed for a coilgun type of application. Ordinary caps aren't designed to be shorted.
The resistance of an incandescent light bulb is 1/10th its hot operating value so I would use a resistor instead to slowly discharge your caps.

Sorry!!! I do not get this part... I measured the resistance of electric bulbs (the first resistances, I measured when I purchased my first multimeter )

The resistances ranged from 15 ohms to 60 ohms for 100W to 15W bulbs respectively.

So the hot operating resistance of the 15W bulb would be 600 ohms. Should that be considered less to discharge the capacitors?

And what is a coil gun?

 

[Someone Electro]

The caps will discharge slower and so whith less curnet.

 

[lord loh.]

And what is a coil gun?

And what is needed? To protect the capacitors by discharging them across a long time or to deliver a large current by discharging them quickly? An optimum of both I guess....

 

[audioguru]

And what is a coil gun?

It is a plastic tube wrapped with many turns of thick wire making a high current coil. An iron projectile is placed in the tube a small distance from the coil. Then a high-current-rated high-value capacitor is charged to a few hundred volts DC and is discharged into the coil. The enormous magnetic field in the coil pulls the projectile towards it then if the timing is correct, the capacitor is discharged when the projectile is in the center of the coil so it doesn't get slowed down.

And what is needed? To protect the capacitors by discharging them across a long time or to deliver a large current by discharging them quickly? An optimum of both I guess....

You should use capacitors that are rated for high currents (like used in photoflash circuits) for projects that discharge them with high currents. :lol:

 

[jimbob jones]

cap shorting

I dare you to short it out with a screwdriver blade. Go on'or else get someone else to do it for ya.

 

[Styx]

Re: cap shorting

I dare you to short it out with a screwdriver blade. Go on'or else get someone else to do it for ya.

been there done that, got the dent in the screwdriver

 

[jimbob jones]

Shocking***********************one liner.

 

[awright]

Capacitor Discharge

There is no general answer to the question about how long your capacitors will last under shorted pulse discharge conditions because capacitor life depends strongly upon the type of capacitor you are using, the voltage stress, the discharge current and energy, and the nature of the external curcuit. Certainly, the first thing to do is select capacitors rated for pulse discharge conditions (like photoflash and laser energy discharge capacitors).

In general, capacitors designed for power supply filtering applications are not well suited to pulse discharge conditions because, among many design factors that are beyond me, the internal connections between the capacitor plates and the terminals are not designed for the high currents that occur during quasi-short-circuit pulse discharges. If they don't fuse instantaneously (which, obviously, yours have not), they will go through damaging thermal cycling leading to premature failure. They may also have (relatively) high internal resistance or inductance leading to peak current limiting or pulse lengthening.

Capacitors rated for higher ripple are not necessarily suited to pulse discharge applications (although they may be better than those not rated for ripple current) because the ripple rating has more to do with the losses within the bulk of the capacitor and thermal transfer out of the bulk of the capacitor, not with the instantaneous peak current capability of the internal connections and punch-through resistance of the dielectric

If you can find it, a fascinating series of books is the 4 volume set, "High Speed Pulse Technology," by Frank Frungel (Academic Press). All 4 volumes would be very interesting to this crew, but Volume III, "Capacitor Discharge Engineering," is directly applicable to this question.

One of the critical factors affecting capacitor life is the presence or absence of voltage reversal (even of extremely short duration) across the capacitor dielectric resulting from oscillatory discharge (which is not trivial to detect or avoid). Exact external circuit characteristics, controlled by stray inductance, wire length, etc., strongly influences the oscillatory nature of the discharge circuit. The existance of oscillatory conditions may not be obvious in the absence of careful evaluation in the lab with a sampling scope. Discharging with a hard short (like a screwdriver) is highly likely to result in oscillatory discharge due to the relative absence of an energy dissipating element in the discharge loop, along with the presence of stray inductance.

Have fun, and wear safety glasses when playing with capacitor discharges.

awright

 

[freeskier89]

Alright I finally got around to posting some pictures. Nothing spectacular, but it is interesting :lol:.

 

[Someone Electro]

Also try shorting it acros an thin pice of coper or iron wire.

In my case the wire exploded in a very bright flash.(You beter not look at it wen fired)

I tryed to put it in an box an a small pice of paper on top.The paper got lifted about 2 cm.

 

[Someone Electro]

Here are some photos.

This is just after fireing.The flash totaly blinds the camera(whole screen goes white) so i rater post pohots of pices of hot wire flying around.

1st pic is thin coper wire and the 2nd is alu foil (seams to give an longer lasting flash).