Balance Resistor

When capacitors are connected in series — usually to reach a higher rated voltage than a single capacitor permits — small differences in capacitance and leakage current cause the voltage to redistribute unequally. The capacitor with the lowest leakage or smallest capacitance sees the highest voltage, which can exceed its rating and trigger cascading failure. The balance resistor forces the DC voltage to divide according to the resistor ratio (which is tight) instead of the capacitor leakage ratio (which is loose).

Design rule: the resistor current must be ≥ 5× the worst-case leakage current of the capacitor it parallels. For a 470 μF / 450 V aluminium electrolytic with 1 mA leakage, a 90 kΩ / 2 W resistor draws ~ 5 mA at 450 V — adequate balancing with manageable steady-state dissipation. Foil capacitors in HV DC links use 100 kΩ – 1 MΩ depending on leakage spec.

Balance resistors live across the capacitor for the equipment's entire operating life, so long-term stability (drift < 1 % / 1000 h), high working voltage rating, and low VCR are essential. They double as bleeder resistors after power-off, discharging the capacitor bank to safe voltage within a defined time per IEC 60204 / IEC 61851. Construction is typically axial wirewound or thick-film HV cylindrical.