Draining a capacitor

[Coytee]

When you crack open the box (after unplugging of course) I understand that you need to still be concerned about the capacitors since they might still be holding a charge.

(saw this proved years ago when the tech opened up my old Yamaha receiver and while dinkering around inside, slipped. his screwdriver shorted one out and sparks flew & he had a melted notch in his screwdriver)

Anyways... is there some way you can drain the capacitors?

I once hit the power switch on my system and was really intrigued that it did not quit with the flip of the switch (like my ss did), it more "coasted" to off. Can I presume that while it was coasting to off it *might* be draining the capacitors??

I'm not planning on poking around inside, I'm just curious.

[KT66]

Good engineering practice, not always adhered to, would place a bleeder resistor of high value across the electrolytic cap(s)to completely discharge them in a minute or so after the amp is powered down. Use a voltmeter to dispel any doubt. Check each cap individually.

As you can not always be sure of this practice - after checking with a voltmeter- you can use a crock-clip wire with a 1K 1W R in series to discharge the capacitor(s) to ground. Once discharged you can leave the crock-clips in place,while you are poking around, as some charge buildup will result after initial discharge (due to dielectric absorption).

Remember to remove the wire before powering up again !!

Needless to say be careful with those high voltages in tube amps but equally with the high current discharge

in both tube & SS amps that will make you jump when you hear the loud crack of a spark caused by an accidental short!

Don't poke around unless you know what you are doing

[Audible Nectar]

Question related to this: How long does it typically take for caps in tube amps to completely discharge after power off?

[KT66]

up to a few minutes if bleeders are fitted.

if not caps will still discharge quite a bit as tubes will conduct until filaments cool down enough but it not possible to say how complete the discharge is. So best to check with a volt meter

[yaffstone]

We always left a shorting wire accross caps in the lab because they can charge back up left on the bench. Once shorted, they're safe. One way to do this is to make a wire with an alagator clip on one end and attach that to ground. Then hold the insulated part of the wire and touch the metal to the cap lead on the high voltage side.

[Erik Mandaville]

Without a bleeder resistor across the output of the power supply, a filter capacitor can hold a painfully strong charge for days. Be careful, especially if you use a bleeder resistor with alligator clips. As KT66 says, this is absolutely the right way to perform a discharge, but be extremely careful with what you're touching in making those connections. If in doubt or unsure, seek assistance from an experienced technician.

[WMcD]

I don't disagree with any of the above.

In ham radio days of vacuum tubes, the ARRL always suggested bleeder resistors built in. Maybe it is not followed on SS which typically uses lower voltage.

I'm taken by the use of the term Crock Clips by our buddy in the UK.

Some years ago I was reading a publication out of the UK where the author refered to crocodile clips. I thought the Brits were one up on us Yanks and they had something really wonderful where we had only . . . allegator clips. In a few seconds I gave myself a head slap.

It makes sense. Elton John's Crocodile Rock would not have sounded as good with lyrics saying Allegator Rock.

Someday, we'll have Camen Clips, too. Smile.

Gil

[Erik Mandaville]

Awe, c'mon, Gil -- just use one of these 'cute' things!

[3dzapper]

Until I went to Electronics School, I always thought they were "roach clips"!

[IAmtnbikr]

A friend of mine who I worked with years ago when still wrenching on cars was quite the prankster. He took a wooden block, and milled out the insides. Cut 6 metal plates to sit on each side, and put a cap inside hooked to two of the plates, which he charged up with a GM HEI coil, lol. It made for great fun when someone would inevitably grab it when seeing it sitting on his shelf. We also wanted to do the same with an old worn out air impact gun.....you know how everyone always picks up your tools and plays with them at the dealership.....we had a switch set to the trigger but never did finish the assembly of the impact...too bad! He told me a while back his brother made a similar toy with the cap from a flashing light used on a school bus.....I imagine that would straighten your hair!

[WMcD]

3dzapper,

That is very funny. There is some movie subplot in there. A slightly stoned EE student is in the lab and describes to the prof how he is hooking things up with roach clips.

Smile,

Gil

[KT66]

Crocodilus Maxilla aka caiman klip ?

or alligator if you are in the Florida swamps ?

BTW, many years ago I discharged a 8uF PIO filter cap (1.5KV !) in a 1Kw broadcast transmitter PSU with my right arm - hand to elbow. The experience and spirit of the moment is difficult to convey with justice other than by saying I was nearly a gonner. I quietly wished bleeders had been used by the bugger that wired the bleeding thing!

So it was both hands in my pockets from then on..

[D-MAN]

When I was in the Navy, we would get a hand-crank phone, charge up the cap with the wires bent to form a U-shape and put it in someones "rack" preferably down at the foot end, and then wait for the scream late at night.

Gee, those were the days.

DM

[Guest Anonymous]

Afternoon-

When I was younger, me and my buddies would take the biggest caps that we could find and take about 30 feet of wire and used to reverse the polarity and blow them up. Not very safe or smart, but it did provide cheap fun for an afternoon.

Best-

George

[Olorin]

Coytee,

My favorite way (for caps that are large enough for this to work) is to take an old CPU fan or power supply fan and alligator clip it to the legs of the cap. When the fan stops spinning, the cap is discharged.

[KT66]

I suppose drilling 1/16" hole in the can would drain the electrolyte.

Forget it.

[fini]

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On 4/12/2005 4:48:03 AM KT66 wrote:

I suppose drilling 1/16" hole in the can would drain the electrolyte.

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With a valve, no doubt...

[KT66]

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On 4/21/2005 7:40:04 AM mdeneen wrote:

Uh, don't forget when selecting a "bleeder resistor" that WATTAGE is rather crucial here. A capacitor can hold an enormous amout of energy.

For example: If the cap was on a 400V supply, and you connect a 1,000 ohm resistor to ground, what happens? Well, 400V across 1,000 ohms will create a brief current of 400ma. 400ma X 400V = 160 WATTS across your 1W resistor. Smoke.

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Uh .. not too relevant.

A 1 watter will do but you should use a higher value for mechanical robustness during repeated handling on the bench say a 10W ceramic would do nicely

A review of circuit theory is in order here. Remember that resistor ratings are continous ratings in the case at hand the short pulse dischage represents highish instantaneos power but minute average power. I would be more concerned with voltage breakdown of the modern miniature 1 W metal film resitors about 6mm long - the older carbon comp types were longer and safer in this respect.

I agree with mouse trap only in my case the plates were on roof beams 20ft high

so I always wondered wahat killed the rats : electrocusion or a broken back when they recoiled falling 20ft !

[JJkizak]

You could have a grounding bar hanging near the amp to place on the cap continously as they required in the military. Could maybe dress it up a bit to be fashionable with the living room decor.

JJK