# Capacitor Discharge Resistor Calculator

This tool calculates the value of Resistance (Ω) required to discharge a capacitor in a specified amount of time. It also calculates the power requirements for the resistor (important for a practical circuit design)

Enter

• Final Voltage (V)
• Initial Voltage (Vo)
• Time (t)
• Capacitance (C)

## Formula

V = Vo*e−t/RC

t = RC*Loge(Vo/V)

R = t/(C*Loge(Vo/V))

P = Vo2/R

## Example Calculation

For an initial voltage of 10 Volt and final safety threshold voltage of 0.1 V, a capacitor of 10 uF. In this case to discharge this circuit in 100 millisecond, the calculator recommends a resistor value of 2.17 kΩ.

When designing this circuit, we have to consider the power rating of the resistor. The value is based on the maximum or initial voltage and given by the formula P = Vo2/R. The calculator recommends a value of 46 mW. However in a practical circuit, I suggest using at least twice that value or around 100 mW. This provides a bit of safety factor and takes into account resistor tolerance for instance.

## Background

The resistor in a Capacitor discharge circuit – also called a bleeder resistor is a safety component used in electronic circuits to discharge capacitors automatically after the power is turned off.

This prevents the cap from retaining a hazardous charge. It’s essentially a high-value resistor connected across the terminals of a capacitor or between the positive and negative voltage rails in a power supply circuit.