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?

In a choke-input filter power supply, the output voltage under normal operating load is significantly lower than the peak voltage of the transformer, averaging around 0.9 times the RMS voltage. A bleeder resistor is normally used to ensure a minimum load keeps this voltage regulated. However, if the bleeder resistor burns out and fails open while no external load is connected, the continuous current stops. The choke-input filter effectively transforms into a capacitor-input filter because the choke ceases to regulate the charging cycle. Without current flowing, the filter capacitor will charge up to the absolute peak voltage of the AC waveform (RMS multiplied by 1.414). Because of this specific risk, capacitors must have a voltage rating that can safely handle the full peak transformer voltage. Otherwise, a simple component failure like a blown bleeder resistor would cause the capacitors to over-voltage and potentially explode.