A bleeder resistor is used in unregulated DC power supplies to improve voltage regulation, but is not needed in low voltage regulated power supplies or linear voltage regulators. It is used in circuits with a flyback transformer to maintain a constant high voltage. A full-wave bridge rectifier is more efficient than a single diode half-wave rectifier.
A bleeder resistor is an electrical component that draws electrical energy into unregulated power outputs to improve voltage regulation. It is usually used in unregulated direct current (DC) power supplies. The bleeder resistor can be eliminated in low voltage regulated power supplies and is not needed in linear voltage regulators or fast duty cycle controlled switching power supplies to maintain a constant DC voltage. High voltages can be difficult to regulate using electronic regulators. A circuit with a flyback transformer can use a bleed resistor to maintain a nearly constant high voltage for televisions and cathode ray tube oscilloscopes (CRTs).
A typical alternating current (AC) to DC power supply has a rectifier and a filter. The rectifier accepts alternating current, in the form of a sine wave, to generate a half-wave or full-wave pulsating direct current output. Instead of a positive and 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 capable of carrying the load currents as needed. The bleeder resistor is needed to improve the voltage regulation of a power supply because it reduces the no-load voltage of an unregulated power supply, which makes the no-load output voltage closer to the lower rated load voltage.
The AC to DC conversion transforms the voltage levels due to the capacitors in the filter and the load. For example, given a step-down transformer supplying a nominal secondary voltage of 10 volts alternating current (VAC), which is level root mean square (RMS), the no-load DC output is approximately 15.7 volts direct current ( VDC), while the rated load voltage can be around 10 VDC. It can be seen that the difference between the no-load voltage and that at full load is about 6 volts (V), which is large and a 38% change from no load to full load. An ideal transition from no-load to full load output voltage is 0V DC.
The bleeder resistor lowers the no-load voltage. Instead of 15.7V DC, connecting a bleeder resistor on the DC output can cause the no-load voltage to be 11V DC. In this case, the variation from no load to full load is only about 9%, given a nominal load voltage of 10 VDC.
Using a full-wave bridge rectifier instead of a single diode half-wave rectifier improves power supply performance. A single diode or half-wave rectifier results in less than 50% usage of the input AC power because the half-wave rectifier draws AC power only in the positive or negative half-cycle of the input sine wave. Rectifiers and resistive components do not store energy, while the filter capacitor stores energy towards the peak of the AC sine wave. The stored energy helps keep the DC voltage constant as the load current allows.
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