Capacitor Discharge

Large capacitors in power supplies can hold a dangerous charge for minutes or hours after the power is off. This tool finds how long a resistor takes to discharge one, or which resistor to choose to do it in a set time.

The theory

A capacitor discharging through a resistor falls along an exponential curve:

V(t) = V0 × e^(−t ÷ (R × C))

Solving for the time to fall from V0 to a safe voltage Vsafe gives t = R × C × ln(V0 ÷ Vsafe), where ln is the natural logarithm. The product R × C is the time constant (τ). After one τ the voltage has dropped to 37 %, after five τ to under 1 %.

Two other numbers matter:

• Energy stored = ½ × C × V0², in joules. 37 J is enough to be dangerous.
• Starting power in the resistor = V0² ÷ R. It is highest at the start and falls quickly.

Step by step

1. Open the Capacitor Discharge tab.
2. In I want to, choose Find the discharge time (you have a resistor) or Find the bleeder resistor (you want a set time).
3. In Capacitance (F), type the capacitor, for example 470u.
4. In Starting voltage (V), type the voltage it is charged to, for example 400.
5. In Safe voltage to reach (V), type the level you consider safe, for example 50.
6. For time mode, type the resistor in Discharge resistor (Ω, time mode), for example 100k. For bleeder mode, type your goal in Wanted time (s, bleeder mode), for example 60.
7. Read the time or the standard resistor value, plus the starting power and energy stored.

Worked examples

• 470 µF at 400 V through 100 kΩ: it takes about 98 seconds (1.6 minutes) to reach 50 V, and 235 seconds to be nearly empty. The resistor starts at 1.6 W. The capacitor stores 37.6 J.
• Bleeder for a 60 second discharge: the tool calculates 61.4 kΩ and suggests the next lower standard value, 56 kΩ, which discharges in about 55 seconds and starts at 2.9 W.

Things to watch

• Use a resistor with a voltage rating high enough for the starting voltage, and a power or pulse rating for the starting power. A bleeder resistor that stays connected all the time wastes power continuously, so size it for the idle loss too.
• Safety: a bleeder is a backup, not a guarantee. Always measure the capacitor with a multimeter before touching the circuit. Discharge large or high-voltage capacitors through a proper discharge tool, not a screwdriver.