Advanced Qualification · A-004 Power supplies · A-004-002-011
In a properly designed choke input filter power supply, the no-load voltage across the filter capacitor will be about nine-tenths of the AC RMS voltage; yet it is advisable to use capacitors rated at the peak transformer voltage. Why is this large safety margin suggested?
- A Under no-load conditions and a burned-out bleeder, voltages could reach the peak transformer voltage
- B Resonance can be set up in the filter producing high voltages
- C Under heavy load, high currents and voltages are produced
- D Under no-load conditions, the current could reach a high level
Correct answer: A — Under no-load conditions and a burned-out bleeder, voltages could reach the peak transformer voltage. From the official ISED question bank, verified word for word.
Why
Normally, a choke input filter keeps the voltage steady at about 90% of the AC RMS value, but this relies on current flowing through the circuit. The full explanation — and one for every question — is in Pro.
Practise this section
Power supplies: rectifiers, filtering, regulation, switching supplies and their failure modes.