The W8EXI Wingfoot VFO Exciter
Bias Power Supply Schematic Diagram and Circuit Description:

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General Information On The Bias Power Supply:
The bias power supply provides a negative voltage with respect to ground that is used for grid block keying the transmitter. With grid block keying, a negative bias is applied to the keyed stages when the key is up, turning them off. When the key is down, the bias is removed, and the stages are turned on.

The bias power supply also provides power for the differential or timed sequence keying system. The differential or timed sequence keying system is a sophisticated circuit used in the Wingfoot VFO Exciter that controls when and how the bias is applied to the various keyed stages.

The bias power supply is straightforward. It consists of a power transformer and full wave bridge rectifier, followed by a simple capacitor input filter. Output is about -175V at a load current of 13mA.

Bias Power Supply
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Bias Power Supply:

Power Transformer: The power transformer is a Stancor Type P-3045. 117V AC is connected to the primary of the transformer via X1 and X2 when the power switch is turned on. The power transformer does not need to supply much current. The maximum current drawn from the bias supply is less than 20mA.

Bridge Rectifier: The secondary of the transformer feeds a standard bridge rectifier circuit. When the top connection is positive and the bottom negative, the upper left and lower right diodes conduct. When the top connection is negative and the bottom positive, the upper right and lower left diodes conduct. Thus, a full wave bridge rectifier conducts on both halves of the input waveform, and the output is a stream of DC pulses at 120Hz.

Filter Capacitor: The stream of DC pulses from the rectifier must be smoothed out. The filter capacitor is charged by the pulses, and fills in the gaps between them. Another way to look at it is that the rectifier output is DC with a large AC component added to it. The capacitor blocks the DC, but shorts the AC component to ground, eliminating it from the output.

Bleeder/Load Resistor: To prevent the filter capacitor from charging up to a relatively high peak voltage and to provide a minimum load for the power supply, a resistor is connected across the output. This resistor also bleeds away the charge on the filter capacitor when the power supply is turned off.

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If you have any questions or comments, you can send E-Mail to Dr. Greg Latta at glatta@frostburg.edu