G7A: Power Supplies and Symbols – Ham Radio General License Study Guide
G7A covers how power supplies convert AC line voltage into usable DC for radio equipment, and how to recognize common electronic schematic symbols. Five of the thirteen questions in this group refer to Figure G7-1, a diagram containing numbered schematic symbols that you must identify by name.
The exam draws from topics including the purpose of a bleeder resistor, what components make up a power supply filter network, what rectifier circuit uses two diodes and a center-tapped transformer, what makes a half-wave rectifier different from a full-wave rectifier, what portion of the AC cycle each rectifier type converts, what the output waveform of an unfiltered full-wave rectifier looks like, how switchmode power supplies differ from linear supplies, and which symbols in Figure G7-1 represent a FET, a Zener diode, an NPN transistor, a solid core transformer, and a tapped inductor.
Power Supply Stages
A linear power supply converts AC mains voltage to regulated DC through several stages working in sequence:
- Transformer — steps the AC voltage up or down to the required level
- Rectifier — converts AC to pulsating DC using diodes
- Filter network — smooths the pulsating DC. The filter uses capacitors and inductors: capacitors store charge and fill in the gaps between pulses; inductors oppose rapid current changes, further smoothing the output
- Regulator — holds the output voltage constant as the load changes
A bleeder resistor is connected directly across the output of the power supply (in parallel with the load). Its purpose is to discharge the filter capacitors when power is removed. Without it, the large filter capacitors can retain a dangerous charge for seconds or minutes after the supply is turned off — a shock hazard when working on the equipment. The bleeder also provides a minimum load that helps the regulator maintain a stable output.
Rectifier Types
The rectifier converts AC to pulsating DC. Three common configurations are tested on the exam:
| Rectifier Type | Diodes Required | AC Cycle Converted | Notes |
|---|---|---|---|
| Half-wave | 1 | 180° (one half-cycle) | Simplest; higher ripple; lower output current capability |
| Full-wave | 2 + center-tapped transformer | 360° (both half-cycles) | Requires center-tapped transformer; output ripple at 2× input frequency |
| Full-wave bridge | 4 | 360° (both half-cycles) | No center-tap needed; most common in modern supplies |
The unfiltered output of a full-wave rectifier is a series of DC pulses at twice the frequency of the AC input. Because both half-cycles are converted, the output pulses appear at 120 Hz for a 60 Hz AC input. This higher ripple frequency makes filtering easier — the filter capacitors have less time to discharge between pulses.
Switchmode Power Supplies
A switchmode power supply (SMPS) operates by rapidly switching the input DC on and off at a high frequency (tens or hundreds of kilohertz) rather than simply transforming 60 Hz AC. The key advantage is that high-frequency operation allows the use of smaller transformers and filter components — the higher the frequency, the smaller the components needed for the same performance. Switchmode supplies are lighter and more efficient than equivalent linear supplies, which is why they are used in most modern commercial transceivers. The tradeoff is greater potential for conducted and radiated RF interference from the switching action.
Schematic Symbols (Figure G7-1)
Five questions in G7A ask you to identify numbered symbols in Figure G7-1. Study this figure carefully — the exam will present the figure and ask which symbol number corresponds to a named component.
| Symbol Number | Component |
|---|---|
| Symbol 1 | Field effect transistor (FET) |
| Symbol 2 | NPN junction transistor |
| Symbol 5 | Zener diode |
| Symbol 6 | Solid core transformer |
| Symbol 7 | Tapped inductor |
G7A Practice Questions
Check Your Knowledge
G7B: Amplifiers and Oscillators →
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