How do you safely discharge a capacitor?

In summary, capacitors can be discharged with a resistive load, depending on the voltage it's charged to. Capacitors can also be dangerous, so be sure to understand the risks before discharging them.
  • #1
AJKing
104
2
I hear of people discharging them with screwdrivers, but is that the best call for 10000+μF capacitors?
 
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  • #2
I would think you can discharge them with a resistive load depending on the voltage it's charged to.
 
  • #3
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
 
  • #4
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?
 
  • #5
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!
 
  • #6
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.
 
  • #7
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
 

FAQ: How do you safely discharge a capacitor?

1. What is a capacitor and why does it need to be discharged?

A capacitor is an electronic component that stores electrical energy. It needs to be discharged in order to release this stored energy and prevent any potential danger or damage to other components or individuals.

2. How do you know if a capacitor needs to be discharged?

If you are working with a capacitor, it is always best to assume that it needs to be discharged. However, you can use a multimeter to check if there is any residual voltage present.

3. What are the steps to safely discharge a capacitor?

1. Turn off the power source and unplug the device if it is connected to a power outlet.2. Use insulated tools, such as pliers or screwdrivers, to touch both leads of the capacitor at the same time.3. Hold the tools in place for a few seconds to ensure that all the stored energy is discharged.4. Double check with a multimeter to ensure there is no residual voltage present.

4. Can you discharge a capacitor without a multimeter?

Yes, you can use a resistor to discharge a capacitor. Connect a high-resistance resistor (at least 1kΩ) between the two leads of the capacitor and leave it in place for a few minutes to allow the energy to dissipate.

5. Are there any risks involved in discharging a capacitor?

Yes, there is still a risk of electric shock when discharging a capacitor, even though it has been disconnected from a power source. It is important to use insulated tools and protective gear, such as gloves, to minimize the risk. Additionally, make sure to discharge the capacitor slowly to avoid any sparks or explosions.

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