Bleeder Resistor
A permanently connected resistor that drains residual energy from capacitors after power-off, ensuring the DC bus reaches a safe touch voltage (typically < 60 V) within a defined time per IEC 60204 / IEC 61851-23.
Definition
Large DC-link capacitor banks in inverters, EV chargers, BESS converters and traction drives can hold lethal energy for hours after the equipment is powered off — a 5 mF bank at 1000 V stores 2.5 kJ, enough to weld a wrench. The bleeder resistor provides a deterministic discharge path so the bus drops to a safe voltage within a specified time, protecting service technicians.
Safety standards dictate the rule. IEC 60204-1 (industrial machinery) requires < 60 V within 5 s of disconnection, or < 60 V within 30 s if a warning label and tool-access enclosure are provided. IEC 61851-23 (EV chargers) requires < 60 V in 5 s for the DC output, < 5 J residual energy in 1 s for accessible connectors. IEC 60664 limits the touch voltage class. Sizing the bleeder from V_t = V_0 × e^(−t/RC) gives R ≤ −t / [C × ln(60 / V_0)]; for 1000 V → 60 V in 5 s with 5 mF, R ≤ ~ 980 Ω, dissipating 1 kW continuously while energised — a job for an aluminium-housed power resistor with thermal switch.
Bleeders are paired with explicit pre-charge resistors and balance resistors in HV systems. They should be redundant where possible (two parallel resistors) and monitored for open-circuit failure, since a failed bleeder leaves a charged bus with no visible indication.
Related terms
Pre-charge Resistor
A current-limiting resistor placed between a DC source and a large DC-link capacitor bank during startup, charging the capacitors slowly through the resistor before the main contactor closes and bypasses it.
Balance Resistor
A resistor connected in parallel with each capacitor in a series-connected capacitor bank to equalise the voltage across each unit and compensate for capacitance and leakage tolerances.
Aluminum-Housed Resistor
A power resistor in which a wirewound element is potted with silicone gel inside an extruded aluminum housing, designed to be bolted to a heatsink for high-density continuous power dissipation from 5 W to 1500 W.
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