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Anonymous Poster #1

Capacitor Discharge Standard

04/10/2013 12:14 AM

Is there a standard (IEEE, IEC, ANSI etc) that dictates a method ( & of course guaranteed to be safe) to discharge a capacitor??? if there is, which standard is it an which number?

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#1

Re: Capacitor Discharge Standar

04/10/2013 12:39 AM

How about putting a one-watt resistor of V2 Ω across it until V < 10?

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#2
In reply to #1

Re: Capacitor Discharge Standar

04/10/2013 3:40 AM

All hail the New Standard <bows reverently>. GA

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#3
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Re: Capacitor Discharge Standar

04/10/2013 3:54 AM

Not bad for just wingin' it, eh?

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#4
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Re: Capacitor Discharge Standar

04/10/2013 4:05 AM

There's a can of fresh tuna in the post. Del sent it earlier.

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#5
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Re: Capacitor Discharge Standar

04/10/2013 4:08 AM

I dibs my own number: BS7672.

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#6
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Re: Capacitor Discharge Standar

04/10/2013 4:11 AM

Wonderful!

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#7
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Re: Capacitor Discharge Standar

04/10/2013 7:10 AM

Cool. Well handy 'til you get to big 'F'ers at big 'V's (if you follow my drift).

F'rinstance, 150μF at 100V would take about 1.5 sec, but 1F at 100V would take well over 2 hrs.

[Edit - reckon this is where the standards go from formulae to charts and tables ]

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#8
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Re: Capacitor Discharge Standar

04/10/2013 7:30 AM

That's where we graduate to light bulbs, heating elements, load banks, and other higher-wattage resistors. Where there's a will, there's a way!

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#9
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Re: Capacitor Discharge Standar

04/10/2013 8:08 AM

Agreed. Electric kettles/urns are handy - a 3kW one (230V) is about 20Ω. Even better if there's a sink & tap handy so you can plumb it in for continuous use.

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#10
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Re: Capacitor Discharge Standar

04/10/2013 9:13 AM

Beats 2ft of 6SWG fence wire...

(Don't try this at home, children.)

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#15
In reply to #8

Re: Capacitor Discharge Standar

04/11/2013 7:24 AM

Maybe use the capacitor as a power source to supply a switch mode (or other) current regulator connected to ground?

This would speed up the latter end of the discharge while preventing excessive current when first connected from damaging the capacitor.

The switch mode option would discharge the capacitor safely and quickly, without generating excess heat.

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#11

Re: Capacitor Discharge Standard

04/10/2013 9:34 AM

Vide the Indian Electricity Rules, 1956, Rule No. 70: "Suitable provision shall be made for immediate and automatic discharge of every static condenser on disconnection of supply".

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#12

Re: Capacitor Discharge Standard

04/10/2013 11:43 AM

Shorting probe...

ANSI C2, NFPA 70E, and maybe 29 CFR...those come to mind.

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#13

Re: Capacitor Discharge Standard

04/10/2013 12:35 PM

Don't know about any standards, but we had some high voltage transmitter capacitors and used a large wire wound resistor (maybe 200 watt with resistance value to give about 1 minute discharge time). The resistor was put in series with a grounding stick and you could draw a nice arc for a long period of time.

The technician brought the arc too close to the capacitor case (single bushing capacitor) one time and there was an explosion. The technician was not hurt and the capacitor seemed to be OK.

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#14

Re: Capacitor Discharge Standard

04/11/2013 1:12 AM

It may be wise to use the method used in spot welders. This way, a semiconductor switch (it's an SCR) is the trigger, so you don't need to be in charge of the heavy discharge current, only the trigger current.

Small hobby spot welders use a T 70 RIA 40 semiconductor (SCR) and you can scale up to the size used in the auto industry.

If you want a quick discharge, then use the SCR to trigger the capacitor across a suitably rated low resistance.

If you're happy with a slower discharge you may use a lower current-rated SCR and a higher discharge load resistor.

It's wise to check the capacitor maker for permissible discharge current, because some capacitor constructions will rupture under the high electrostatic forces generated during rapid discharge.

Summary: Check the maximum acceptable capacitor discharge current and design the discharge resistor to suit it. Select an SCR designed for spot welding applications and stay within its voltage and current ratings.

Mark Bingham
Relativity PL

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#16

Re: Capacitor Discharge Standard

04/11/2013 9:04 AM

We have had cases where a capacitor bank needs to be discharged quickly (for safety reasons). When you do this you may find that the discharge resistor explodes because it can't handle the peak power. Film and wire wound resistors are especially vulnerable to this kind of failure. In these cases we often use solid cermet resistors.

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#17

Re: Capacitor Discharge Standard

04/11/2013 10:16 AM

Then there is the Florida shade tree method. Run a large diameter wire from capacitor to nearby body of water.

Throw switch.

Run

Have fish fry!

Not found in your recommended practices eh? LOL

Not to be taken seriously either.

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