Question
Which of the following is an appropriate power supply rating for a typical 50 watt output mobile FM transceiver?
Answer Options
- A) 24.0 volts at 4 amperes
- B) 13.8 volts at 4 amperes
- C) 24.0 volts at 12 amperes
- D) 13.8 volts at 12 amperes
Correct Answer: D
Explanation
Most modern amateur radio equipment, including mobile FM transceivers, is designed to operate on the nominal voltage of a vehicle’s electrical system, which is 13.8 volts DC. Using the power formula (P = E \times I), you might note that 13.8\text{ V} \times 4\text{ A} \approx 55\text{ W} and choose answer B. However, when calculating the required current (Amperes) for a radio, you must account for its efficiency (a 50W radio is usually about 50-60% efficient) plus headroom for everything else the radio is doing (speakers, display, other processing).
Given typical efficiency, a 50W radio will draw roughly 80 to 100 watts of input power. At 13.8V, the required current is approximately 100\text{ W} / 13.8\text{ V} \approx 7.2 amperes on peak transmission. Therefore, a power supply capable of delivering 13.8 volts at 12 amperes is an appropriate and safe choice, providing the necessary margin for varying efficiency.
It is OK for a power supply to be capable of supplying more current than a device is consuming.
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