Question T0A11
From subelement T0 - T0A
What hazard exists in a power supply immediately after turning it off?
Why is this correct?
Filter capacitors store electrical charge like small batteries and retain this energy even after power is disconnected. Unlike transformers or inductors that stop being dangerous immediately when power is removed, capacitors can hold lethal voltages for minutes or hours. Options A, B, and C describe phenomena that cease when power is off, but capacitors are 'electrical grudge-holders' that maintain their charge until properly discharged.
Memory tip
Remember: anything that 'stores' energy (capacitors, batteries) stays dangerous when disconnected, while anything that just 'processes' energy (transformers, resistors) becomes safe immediately. This storage vs. processing distinction helps identify post-power-off hazards across all electronic equipment.
Learn more
Power supply filter capacitors smooth DC output by storing charge during voltage peaks and releasing it during valleys. Larger capacitors in higher-current supplies store more energy and take longer to discharge naturally. The bleeder resistors that safely discharge capacitors may have failed or been omitted in older equipment. Always verify zero voltage with a meter before touching power supply components, even in equipment that's been unplugged for hours.
Think about it
Why do you think capacitors in switching power supplies might be more dangerous than those in linear supplies, even at the same voltage rating?