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  • Stu WØSTU

Bleeder Resistors (G7A01)

The 2019-2023 General License question pool requires that you understand the purpose of a power supply bleeder resistor:

G7A01: What useful feature does a power supply bleeder resistor provide?

A. It acts as a fuse for excess voltage

B. It ensures that the filter capacitors are discharged when power is removed

C. It removes shock hazards from the induction coils

D. It eliminates ground loop current

Let’s start with a review of DC power supply components. A DC power supply is typically comprised of three parts:

1. Transformer: Shifts AC voltage from one value to another value. For example, 120 VAC may be transformed into an AC voltage nearer to 13.8 volts, the voltage value used by most common amateur transceivers.

2. Rectifier: Converts the AC waveform into a varying voltage direct current. Even though the voltage is provided in just one direction after rectification, it still will be of pulsing variability in voltage value over time.

3. Filter Circuit: Smooths the varying DC voltage into a less variable DC voltage over time, providing a more nearly constant DC voltage that is usable by the transceiver circuits.

In the simple half-wave rectifier depiction below, the diode serves as the rectifier and allows only the positive voltage half of the AC waveform to pass beyond the transformer circuit. This generates a pulsing DC voltage of variable value (and changed in peak voltage value in accordance with the windings ratio of the transformer in the power supply circuit).

a half-wave rectifying power supply showing full sine wave input and positive-only half sine wave output pulses.

Most electronic circuits cannot readily use such a pulsing DC voltage, so a filter is incorporated to receive the pulsing DC signal and smooth it to a more steady-state voltage.

Placing one or more capacitors across the transformer secondary beyond the rectifier will smooth the pulsing DC. The filter capacitor(s) will accept and store charge during the positive pulses and then release that charge during the intervening gaps. The output DC signal will still have a mild ripple, but with proper selection of filter capacitors this variability can be reduced to a level that is suitable for DC powering of the transceiver (or other components).

The output of a power supply's transformer with capacitor across the leads resulting in a smoothed output with ripple instead of a pulsed output.

However, when the power supply is disconnected and powered down, perhaps for a maintenance operation, the filter capacitor(s) will maintain a charge that can be a shock hazard. The filter capacitors may be discharged through a resistor over a short time period, converting the charge to heat as current passes through the resistor to ground. This is known as bleeding the capacitors, and the resistor through which the bleeding occurs is a bleeder resistor.

power supply diagram with bleeder resistor connected between the filter capacitor connection and ground

The answer to General Class question G7A01, “What useful feature does a power supply bleeder resistor provide?” is “B. It discharges the filter capacitors.”

-- Stu WØSTU


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