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Discharge those capacitors

vk2fro

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While tidying up today I decided to put together the parts I have in a plastic tub, for the multiline ion laser I'm buying off Things (I need to build a power supply).

Picked up the transformer, put it in the box.

Picked up the capacitor, sat it upright in the box.

Tossed in a couple of heatsinks for the passbank transistors.

*BANG*

bloody capacitor had recharged itself. Scared the crap out of me.

The non adonised heatsink had bridged the capacitor terminals.

That cap now has a clip lead across it.

Reminded me of the time I had a photo flash unit in a cassette case - charge it up to 350v, and find the right style of street sign pole and short it out - it'd make a loud resounding bang :)

its funny how caps seem to recharge themselves over time.
 

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I have a 4KV cap sitting around I keep forgetting to jump. When I first got it, it was on the floor and my cat started rubbing on it. :eek: It now sits high on a shelf.

Static electricity is the main culprit of recharging.
 
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Meatball

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I'm curious though why there would be a net charge developing from static... from in the air? Or rubbing some glass or rubber and cat fur together?

If it sits there, why should one terminal gain more electrostatic charge than the other terminal, only a few cm away?
 
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I dont know physics, its just what I've been told. Something obviously happens. Maybe its from moving it around, or its placement.
 
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It's not actually static that's causing the recharging.

Capacitors (especially non-electrolytic) suffer from a 'memory' effect, where, even when discharged, they will slowly regain their charge, due to charge-induced polarisation in the dielectric material.

As the dielectric material reverts to it's original, unpolarised state, a charge is formed.

This is the reason why all large or high-voltage capacitors must be stored with their leads shorted, even when confirmed to be fully 'discharged'
 

ped

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The Capacitor.

Catching out weary technicians since 1746 :san:
 

ped

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Haha

I forget how many times iv'e discharged a 400v mains smoothing electrolytic through my in built zero ohm link (thumb) !
 

Meatball

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Try taking one of those 12V 1F capacitors for car amplifiers, charge it up, and grab yourself a large screwdriver.
 

HIMNL9

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Why limit our creativity with small capacitors ? ..... get a screwdriver big enough for short out this one :p :D



A capacitor bank – designed by RWM experts in close cooperation with scientists from the Rossendorf laboratory – supplies the energy (current pulses) required to produce the pulsed high magnetic fields. In order to carry out tests of this type, 50 megajoules (MJ) of electromagnetic energy and a peak current of several hundred kiloamperes (kA) are needed to generate a magnetic field of 100 Tesla (three and a half million times the earth's magnetic field).
"uhm, let me dischar..." *pseudo-nuclear KABOOM* :p :crackup:



EDIT: if instead you prefer low voltages, you can still play with 100 Farad ones :p ;D
 
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00Giorge

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I used to charge up a capacitor out of small engine ignitions when I was in school and set them on a table or desk and see how many people got bit by being curious and picking them up. I also killed my Fluke meter by accidentally touching a lead to the wrong terminal while trouble shooting some equipment and got a 75,000 zap to the meter. I go bit by the same one, you know what, it really hurts!
 
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When I was about 12 or 13 years old, I was working for my dad's construction company and had just started a diesel engine using its gasoline starter engine. When the diesel started I was looking for the metal shorting tab used to short out the spark plug and kill the gas engine. When I found that it had been broken off, one of the workmen told me to just grab the spark plug wire and yank it off to kill the engine. :oops:

That was the first and only time that I ever grabbed a spark plug wire on a running engine. At least I supplied the amusement for the guys on that day.:crackup:

Ed
 

millirad

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One nearly fatal experience I had with capacitors was the first time I attempted to look at the tubes in a JCM800 Marshall amp that I owned. It had been unused for about a week and I was just doing the "first step" to access the inside of the chassis. I had one hand touching the metal part of the chassis and then touched a Philips head screwdriver to the first screw. *WHAM*, I was knocked to the floor, blacked out for a few seconds and my head hurt from the shock. The "first step", is always to touch the chassis with a non-conductive shaft and a probe with a conductive strap/wire tied to ground. :)
 
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EDIT: if instead you prefer low voltages, you can still play with 100 Farad ones :p ;D
I recently built this small cap bank for a solar light, although I wouldn't dare short the thing out. 16v 500farads.

I also read somewhere that shorting caps out with a screw driver sometimes causes internal damage, and to use a resistor or load to drain it. I'm actually curious to what would happen if a 2.7v3000f shorted, or over-charged.
 
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HIMNL9

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^ about the 3000F one, if you mean the small "button-type" ones, it probably don't blow the cap itself ..... those small low voltage supercaps are designed as memory backup units in place of batteries ..... they have a very high capacitance, but also high internal ESR, so i doubt they can give you high currents at all.

About the other, instead, i don't suggest to short it with a screwdriver :whistle: :p
 




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