How do you safely discharge a capacitor?


by AJKing
Tags: capacitor, discharge, safely
AJKing
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#1
Feb11-14, 07:18 PM
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I hear of people discharging them with screwdrivers, but is that the best call for 10000+μF capacitors?
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slakker
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Feb11-14, 07:33 PM
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I would think you can discharge them with a resistive load depending on the voltage it's charged to.
davenn
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Feb11-14, 08:19 PM
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yes a resistive load discharge is much better
a few k Ohms say ~ 4k7 and 1/4 Watt would be ideal
will give a slower discharge than just shorting the terminals

cheers
Dave

axemaster
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#4
Feb11-14, 09:33 PM
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How do you safely discharge a capacitor?


The answer would depend on exactly what the situation is with the capacitor. There are a few questions you should answer first:

1. What type of capacitor is it? I'd guess it's polarized electrolytic since it's 10000uF.
2. What is its rated voltage, and what voltage are you discharging it from?
3. What are its internal resistance and ripple current ratings?

And even before we discuss anything else, I will warn you that capacitors can be extremely dangerous. So I have one final question to add to the list:

4. How much do you know about applied electronics / physics? What else have you done before now?
AlephZero
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#5
Feb12-14, 06:38 AM
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Ideally, you have a discharge resistor permanently built into the circuit, so the problem doesn't exist. But check the time constant of the discharge circuit. a 1K resistor and a 10,000 μF cap has a time constant of 10 seconds, and will take about 5 times that long to fully discharge.

Just to add that large caps can "hide" some of their charge in the dielectric material, so even if you think you have fully discharged them, they don't necessarily stay that way.

The "screwdriver method" is definitely NOT recommended, though we all learn some things by experience!
phinds
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#6
Feb12-14, 07:54 AM
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Another problem with the screwdriver method is that you'll likely get a spark that will corrode the cap terminals and probably the screwdriver as well. This happens because the discharge starts as soon as there is the tiniest contact between the screwdriver and the cap terminal and the small area isn't enough to handle the resultant current (this is assuming a pretty large cap). I've seen this happen.
maltman
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#7
Feb12-14, 03:15 PM
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A proper circuit with large capacitors and high voltages should have a designed in bleeder resistor.

When I have worked on my linear amplifier - which uses 4 cx350a tubes at 1500V ( it has a built in bleeder at 470K ohm but it takes a while to bleed down )
So to speed it up I use a short 15KV insulation rated cable with well insulated alligator clips with a 4.7K ohm 1/2 watt resistor connected between one of the clips and the wire. Clip on and wait a minute.

no big sparks as the resistor limits the inrush current and safely bleeds the capacitors in the power supply


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