A bleeder resistor is an electrical component that absorbs electrical power in unregulated power supply outputs to improve voltage regulation. It is usually used in unregulated direct current (DC) power supplies. The bleeder resistor may be eliminated in regulated low-voltage power supplies, and is not needed in linear voltage regulators or switch-mode power supplies with fast duty cycle control to maintain a steady DC voltage. High voltages may be difficult to regulate using electronic regulators. A circuit with a flyback transformer may use a bleeder resistor to maintain a near constant high voltage for cathode ray tube (CRT) televisions and oscilloscopes.
Typical alternating current (AC)-to-DC power supply has a rectifier and filter. The rectifier accepts AC, in the form of a sine wave, to generate either a half-wave or a full-wave pulsating direct current output. Instead of a positive and a negative portion of the sine wave, the rectifier outputs a positive only version of the sine wave or even redirects the negative cycle to become another positive half-cycle in a process called full-wave rectification. The filter after the rectifier is usually a capacitor bank that is able to sustain load currents as needed. The bleeder resistor is needed to improve the voltage regulation of a power supply because it decreases the no-load voltage of an unregulated power supply, which makes the no-load output voltage closer to the lower rated load voltage.
The conversion from AC to DC transforms the voltage levels due to the capacitors in the filter and the loading. For instance, given a step-down transformer providing rated secondary voltage of 10 volts alternating current (VAC), which is root mean square (RMS) level, the DC output at no load is about 15.7 volts direct current (VDC), while the rated load voltage may be about 10 VDC. It can be observed that the difference in no load to rated load voltage is about 6 volts (V), which is high and a 38% change from no load to full load. An ideal change from no load to full load output voltage is 0 VDC.
The bleeder resistor lowers the no-load voltage. Instead of 15.7 VDC, connecting a bleeder resistor across the DC output may cause the no-load voltage to be 11 VDC. In this case, the no-load to full-load change is only about 9%, given a rated load voltage of 10 VDC.
Using a full-wave bridge rectifier instead of a single-diode half-wave rectifier improves the power supply performance. A single diode or half-wave rectifier results in a 50% less utilization of input AC power because the half-wave rectifier draws AC power only in either the positive or negative half-cycle of the input sine wave. The rectifiers and resistive components do not store energy, while the filter capacitor stores energy toward the peak of the AC sine wave. The stored energy helps maintain the DC voltage as constant as allowed by the load current.