Enter V, C, and R to find discharge time — or V, C, and T to find the required resistance.
Bleeder Resistor Circuit
A bleeder resistor is connected in parallel with the capacitor in a power supply. After the supply is turned off, the resistor safely discharges the stored energy so the capacitor does not remain at a dangerous voltage.
The capacitor voltage decays exponentially — the bleeder resistor determines how quickly it reaches a safe level after power-off.
Bleeder Resistor Calculator
A bleeder resistor connects in parallel with a capacitor — typically the main filter capacitor in a power supply — to discharge stored energy after the supply is switched off. Without one, capacitors in high-voltage circuits can hold a dangerous charge for minutes or hours. The calculator above uses the RC exponential decay formula to find the required bleeder resistance for a target discharge time, or the discharge time for a given resistance.
Discharge Formula and Calculator Inputs
τ = R × C — time constant
T = −R × C × ln(Vsafe / V0) — discharge time to safe voltage
R = −T / (C × ln(Vsafe / V0)) — required bleeder resistance
Enter any combination of the five inputs — the calculator solves for whichever values you leave empty. V0 is the initial capacitor voltage. C is the capacitance (μF, mF, F, or nF). R is the bleeder resistance (enter to find discharge time, or leave empty). T is the target discharge time in seconds. Vsafe is the voltage considered safe to touch (defaults to 1 V; many DC safety standards use 50 V, so this is configurable).
The calculator returns seven results: bleeder resistance, discharge time, time constant (τ), number of time constants to reach Vsafe, initial discharge current at power-off, peak power the resistor handles at that first instant, and total energy stored in the capacitor.
Time Constant and Discharge Curve
The time constant τ = R × C defines how fast the capacitor voltage decays. After one time constant, the voltage drops to ~36.8% of V0. After five time constants, it is below 1%. The number of time constants needed to reach Vsafe depends on the ratio Vsafe / V0. For a deeper understanding of RC time constants, see our RC Time Constant Calculator.
Example: V0 = 400 V, Vsafe = 50 V → n = −ln(50/400) = 2.08 time constants
For the same 400 V supply with Vsafe = 1 V: n = −ln(1/400) = 5.99 — essentially six time constants. The lower your safe voltage threshold, the longer discharge takes and the more time constants you need.
Sizing the Bleeder Resistor — Worked Example
A switch-mode power supply has a 400 V bus capacitor of 470 μF. You want the voltage to drop below 50 V within 5 seconds of power-off.
R = −5 / (0.00047 × ln(50/400))
R = −5 / (0.00047 × −2.0794)
R = 5 / 0.000977 ≈ 5,116 Ω → use 4.7 kΩ (standard value)
With a 4.7 kΩ bleeder resistor the actual discharge time to 50 V is T = 4.7k × 470μ × 2.08 ≈ 4.6 seconds — within the 5 second target.
Initial Current and Peak Power
Ppeak = V0² / R = 400² / 4700 ≈ 34 W
Estored = ½ × C × V0² = 0.5 × 0.00047 × 160000 = 37.6 J
The 34 W peak power only lasts an instant — it drops exponentially as the capacitor voltage falls. But the resistor must survive that initial surge. A wirewound or thick-film resistor rated for pulse loads is appropriate here. The steady-state power draw while the supply is on (400 V across 4.7 kΩ = 34 W continuous) is the real sizing constraint — this is heat the resistor dissipates as long as the supply runs.
Continuous Power Dissipation While On
The bleeder resistor stays connected while the power supply operates, constantly drawing current from the supply. For the example above: P = 400² / 4700 = 34 W. That is a significant waste of power and heat. Increasing R to 47 kΩ drops continuous power to 3.4 W but extends discharge time to ~46 seconds. The trade-off is always between faster discharge (safer) and lower power waste (more efficient). In battery-powered or energy-conscious designs, the bleeder is sometimes switched — an active discharge circuit turns it on only at power-off. For power calculations, use our Electrical Power Calculator.
Safety Standards and Safe Voltage Thresholds
Different standards define different safe voltage limits. IEC 62368-1 (IT and AV equipment) requires capacitors to discharge below 60 V DC (or 42.4 V peak AC) within a specified time after disconnecting mains. UL 60950 uses similar thresholds. Some industrial standards use 50 V DC. The calculator defaults to 1 V (effectively full discharge) but lets you set Vsafe to match whichever standard applies to your product.
Choosing the Right Resistor Type
Power Rating
Size the resistor for the continuous power dissipation (V0² / R), not just the peak. Derate by at least 50% — a resistor calculated for 34 W needs a 50 W or higher rating in practice to handle ambient temperature and component ageing.
Voltage Rating
Standard resistors are rated for 200–500 V. If V0 exceeds the resistor’s voltage rating, use two or more resistors in series to split the voltage. Each resistor sees a fraction of the total, and the total resistance is the sum of the individual values.
Resistor Type
Wirewound resistors handle high pulse energy well but are inductive — fine for a DC bleeder. Thick-film and metal-oxide resistors work in lower-power applications. Avoid carbon-film resistors for high-voltage bleeder duty; they can arc across the spiral cut at voltages above their rating.
Frequently Asked Questions
Last updated: March 2026