# Value of Resistor to discharge a capacitor

Thread Starter

Join Date: Apr 2019

Location: Your heart

Posts: 3

**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.

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.

Join Date: Jun 2009

Location: UK

Posts: 66

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

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

Join Date: Nov 2015

Location: Farnham, Surrey

Posts: 1,152

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.

PDR

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.

PDR

Join Date: Feb 2001

Location: UK

Posts: 459

Join Date: May 2002

Location: UK

Posts: 165

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

Join Date: Apr 2019

Location: Here

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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.

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.