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-   -   Value of Resistor to discharge a capacitor (https://www.pprune.org/engineers-technicians/620480-value-resistor-discharge-capacitor.html)

Danial9874 13th Apr 2019 08:44

Value of Resistor to discharge a capacitor
Hello ,

To discharge a capacitor we normally use resistor
But what is the normal value of resistor to discharge the capacitor ( NO MATTER HOW MUCH IS THE CAPACITANCE )
Can anyone verify this
Thank you.

wyrwb1 13th Apr 2019 12:19

What voltage is the capacitor?

Jetstream67 13th Apr 2019 12:58

If you have no training or experience in this please leave large charged capacitors alone !

The resistor and the capacitance together determine the rate of discharge:

A slow discharge rate may be adequate in some circumstances which leads to a high resistance value and lower wattage resistor being suitable. This also disturbs the circuit less

If (e.g. for safety purposes) a rapid discharge is needed consider lower value resistors, resulting in greater heat dissipation and greater effect on the circuit if permanently in circuit. If connecting a resistor specifically to discharge consider sparks and potential for the resistor leads to weld on.

A common error in high voltage systems is to forget that resistors also have a breakdown voltage rating that may be surprisingly low in some applications

If you multiply the capacitor value in Farads by the resistor value in Ohms you get the 'Time constant' which is roughly the time in seconds to discharge the capacitor by 60% (the discharge rate getting slower and slower as it continues)

From this you see that any resistor value will discharge the capacitor eventually (assuming it doesn't burn out along the way) but for practical use you need to include the time you have and the capacitor size vs resistor power you see to find if there might be a suitable 'all purpose resistor in your work

PDR1 13th Apr 2019 14:13

The value of the resistor you might use depends almost entirely on the maximum voltage you would expect to be present in the capacitor and the maximum current you want to discharge it at.

This question sounds very strange - if it actually is a real question we need to know more about what the application is and why it is desired to discharge it.


Danial9874 14th Apr 2019 07:00

But is there a all purpose resistor that can discharge any capacitor no matter how much is it resistance ?

Golden Rivet 14th Apr 2019 19:27


happybiker 15th Apr 2019 13:25

Originally Posted by Danial9874 (Post 10447114)
But is there a all purpose resistor that can discharge any capacitor no matter how much is it resistance ?

It depends what you want the circuit to do and any resistor can discharge a capacitor. The value of the resistor will determine the time taken to discharge the capacitor. A large value of resistance will take longer to discharge the capacitor than a small value of resistance.

More detailed information can be found here: https://opentextbc.ca/physicstestboo...nd-capacitors/

TheOtherDutchman 15th Apr 2019 21:22

Use the standard T=5RC formula - use this to calculate the power rating of the resistor (P=I*I*R)
Standard PSU circuits use about 3 to 5 minutes to discharge to a safe level.
Anything above 40V DC is considered dangerous for human consumption.

Other things to think about...
Discharging a Cap too fast is like a battery short circuit = it will 'explode' in your face.
There's a lot of energy in capacitors... be careful, take your time.;)

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