Safely discharging a capacitor

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Going to attempt to change the capacitor on my tumble dryer... Its a 8 micro-Farad one.
I think its dead anyway but I'm assuming I can check with a multimeter? What sort of voltage/current is one of these likely to have?
Assuming it is still ' live' how do I discharge it to make it safe?
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paulfoel wrote:

Does your meter support capacitance? if not you're wasting you're time, these are only a few pence anyway in a maplins outlet.
-- Sir Benjamin Middlethwaite
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Most likely it is discharged through the motor windings unless the switching is such that it gets disconnected, although relying on the motor windings being intact is not smart when the thing is known not to be working properly. Secondly, it probably has a bleed resistor built in, although those can fail too. I would probably just short it out with a screwdriver, although that can in theory damage the capacitor. Leave it for at least 5 minutes since last powered up, and then short out, and you'll be fine. Making up a proper resistive discharger for this one occasion just isn't worth it.
--
Andrew Gabriel

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I was in the local Maplin shop the other day and saw that you can purchase 1 farad (huge) capacitors for power supply smoothing chav's in car audio systems.
Must be interesting when you accidently short one of these!
http://search.ebay.co.uk/search/1-farad-capacitor_W0QQfclZ4QQfnuZ1
David
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Nah - that's nancy stuff for kids
.... real man stuff here!
http://www.amasci.com/amateur/capexpt.html
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Or how about this :-
http://www.electricstuff.co.uk/marxthree.html
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snipped-for-privacy@hotmail.com writes

STOP IT STOP IT STOP IT
You're giving me ideas
--
geoff

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I've got this terrible urge to dig out some perspex sheet I've got left over in the garage...
I built a Van de graff generator when I was at school, but that was rather disappointing. (On later reflection, I'm not sure the belt material I used was a good enough insulator.)
Then I built a high voltage generator using a car ignition coil, EHT valve rectifier, and a capacitor made from kitchen foil and a large roll of cellulose acetate overhead projector film. I got really 'cracking' 4"-5" sparks from it for a day, until the cellulose acetate broke down.
--
Andrew Gabriel

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'DISCLAIMER: the experiments described below are fantastically dangerous, and they are described without reference to the many precautions needed to guarantee the experimenter's safety. Accidentally discharging these capacitor banks through your body can not only kill, but can explode flesh and bone.'
Followed by
'PARENTS: I supply no detailed plans for reproducing these experiments. Also, these experiments require large and expensive lab equipment which is not obtainable by children. (And the plans for an atomic bomb are safe for children too, because kids can't afford to buy kilograms of Plutonium!) If your kids have access to 5,000 volt high-current power supplies, then they are already in great danger, whether or not they read about my capacitor discharge experiments below.'
Has my respect!
--
Andrew Sinclair http://www.smellycat.org

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A capacitor with a digital readout? Now they've gone over the top! :)
--- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/ Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/ +Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm | Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html
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They went over the top years ago, those capacitors have been available for a while.
Just try to find a car CD player anymore that looks at home in anything but a gaudy racer boy Japenese compact. Seems like they're all fugly and bubbly, loaded with useless distracting blinky lights and buttons so small and jumbled it's impossible to operate them safely while driving.
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On Wed, 21 Dec 2005 19:59:50 GMT, James Sweet wrote:

Thats because youy get a really good one bult nto any car that chavs don't buy.
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Would be nice, but I'm afraid back in '84, '87 and '88 respectively, cars didn't come with CD players of any sort, hence my need to install aftermarket units in mine.
Not to mention I'm still not aware of any OEM units that will play MP3 discs, an essential feature to me that made all earlier CD players virtually obsolete. 10 hours of music on one disc, no more fumbling with CD's in traffic.
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Sam Goldwasser wrote:

I think that you need to understand the market they are trying to sell into. When you see the claim "A CAPACITOR stores current." you see what level they are aiming at!
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vortex2 wrote:

to one's alternator?
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CJT wrote:

Car wiring is radial from the battery to alternator and from the battery to load. The impedance of the battery is much lower than that of the alternator radial circuit - thus transient currents will be met almost entirely by the battery.
You may like to think what effect the starter motor has on the alternator.
Also, the energy stored in a capacitor depends on the capacitance and on the square of the voltage. Rather than have a 1F capacitor on the 12v line, it would be much better to go to the higher voltage power rails of the amplifier and stick beefy capacitors there.
--
Sue

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Palindr☻me wrote:

While this is true, don't forget that the starter motor will never be operating under load while the alternator is producing any current unless you're jump starting another car.
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On Wed, 21 Dec 2005 18:37:10 GMT, CJT wrote:

its nowhere as bad as what a statrter motor does...alternators are limited anyway.
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Does the motor have brushes and a commutator? If so, change the brushes before you start looking elsewhere, they're seldom expensive and are a consumable item. Capacitors may or may not last the life of an appliance but brushes are certain to wear from day one and are usually the first suspect in a misfiring/intermittant motor.
With brush type motors, the brushes wear down and consequently less pressure is applied by the spring, causing a higher resistance/poorer connection between brush and commutator. An arc is caused, which heats the brush and can deform the casing causing the brush to stick, making the problem increase exponentially. It also causes the commutator to become blackened which makes matters even worse.
I'd take out the brush carriers and check the brushes move freely and that there's plenty of length left on them. Also check the commutator is clean.
If the motor does not have a brush/commutator arrangement, I'd check all connections are secure and check the control board for dry/cracked solder joints before suspecting component failure.
Dave
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