Deep Dive: T0A11
The correct answer is D: Charge stored in filter capacitors. A hazard that exists in a power supply immediately after turning it off is charge stored in filter capacitors. Large filter capacitors can store significant charge (hundreds of volts) for minutes or even hours after power is removed. This stored charge can deliver a dangerous or fatal shock. For amateur radio operators, this is a critical safety concern. Always discharge capacitors before working on power supplies, and wait for them to discharge naturally or use a discharge tool.
Why Other Answers Are Wrong
Option A: Incorrect. Circulating currents in DC filters aren't a hazard after power-off - there's no power source to create currents. The hazard is stored charge, not currents. Option B: Incorrect. Leakage flux in transformers isn't a hazard - it's a design characteristic. The real hazard is stored capacitor charge. Option C: Incorrect. Voltage transients from kickback diodes occur during operation, not after power-off. The persistent hazard is stored capacitor charge.
Exam Tip
After power-off = charge stored in capacitors. Think 'C'apacitors = 'C'an 'C'ontain charge. Large filter capacitors store dangerous voltage for minutes/hours. Always discharge before working. Not about currents, flux, or transients.
Memory Aid
After power-off = charge stored in capacitors. Think 'C'apacitors = 'C'an 'C'ontain charge. Large filter capacitors store dangerous voltage. Always discharge before working on power supplies.
Real-World Example
You turn off your power supply and immediately start working on it. The large filter capacitors still hold 400 volts of charge. You accidentally touch the capacitor terminals and receive a severe shock. The capacitors can hold this charge for 30 minutes or more. Always discharge capacitors with a proper discharge tool or wait for them to discharge before working on power supplies.
Source & Coverage
Question Pool: 2022-2026 Question Pool
Subelement: T0A
Reference: 2022-2026 Question Pool · T0 - Safety
Key Concepts
Verified Content
Question from the official FCC Technician Class pool. Explanation reviewed by licensed amateur radio operators and mapped to the T0A topic.