What is a capacitor safety discharge calculator?
Charged capacitors can store significant energy and remain at dangerous voltages long after power is removed. A capacitor safety discharge calculator helps you choose a bleeder resistor and estimate how long it will take for the voltage to drop to a safe level.
By entering the initial voltage, capacitance and either a discharge resistor or a target discharge time, the calculator can determine:
- The time required to reach a specified safe voltage
- The resistor value needed to discharge within a given time
- Peak discharge current and resistor power dissipation
Capacitor discharge basics
When a capacitor discharges through a resistor, its voltage decays exponentially:
V(t) = V0 × e−t / (R C)
- V0 – initial voltage
- R – discharge resistance
- C – capacitance
- t – time
The product R × C is the time constant (τ). After one time constant, the voltage has dropped to about 36.8% of its original value. After five time constants, the voltage is less than 1% of the original.
Solving for time or resistance
To find how long it takes to reach a target voltage VS:
t = −R C × ln(VS / V0)
To find what resistor is needed to reach VS in time t:
R = −t / (C × ln(VS / V0))
Stored energy and resistor power
The energy stored in a capacitor is:
E = ½ × C × V02
During discharge, this energy is mostly converted to heat in the resistor. The initial discharge current is:
I0 = V0 / R
and the initial resistor power is:
P0 = V02 / R = I02 × R
The calculator estimates peak power so that you can select a resistor with adequate power rating and, if necessary, short-term surge capability.
Example – discharging a 470 µF, 400 V capacitor
- C = 470 µF
- V0 = 400 V
- Target safe voltage VS = 50 V
- Chosen R = 100 kΩ
Time constant:
τ = R × C ≈ 100,000 × 470 × 10−6 ≈ 47 s
Time to reach 50 V:
t = −R C × ln(50 / 400) ≈ 47 s × ln(8) ≈ 98 s
Initial current:
I0 = 400 / 100,000 = 4 mA
Initial power:
P0 = 400 × 4 mA = 1.6 W
A resistor rated for at least 3–5 W is recommended due to the initial power spike and heating during repeated discharge cycles.
Design tips for safe capacitor discharge
- Define “safe” voltage: Many standards consider below 50 V DC as a significantly safer level, but your application or regulation may require a lower value.
- Use permanent bleeder resistors: High-voltage power supplies often include a permanently connected resistor so capacitors discharge automatically when power is removed.
- Account for tolerance and temperature: Resistor and capacitor tolerances affect discharge time; design with margin rather than assuming exact values.
- Label and verify: For service personnel, indicate discharge times on the equipment and confirm actual behavior during testing.
This calculator is intended as a design aid. Always follow relevant safety standards and lockout procedures when working with high-energy capacitors and power electronics.
