Abstract: In a d.c. chopper controller having an "ON" controlled rectifier for interrupting a load current and a discharge circuit connected in parallel with the "ON" controlled rectifier, said circuit incorporating a commutating capacitor, an "OFF" controlled rectifier which is arranged to apply voltage formed on the capacitor across the "ON" controlled rectifier in a direction to reverse the current flow therethrough and a charging circuit for the commutating capacitor including a third controlled rectifier connected to a reference source, the improvement comprising operating the controller by means of a control circuit such that a charge for the commutating capacitor is provided at a selectable instant during the period that the "ON" controlled rectifier is conducting. This improvement enables secure commutation irrespective of the interruption rate of the "ON" controlled rectifier or the magnitude of the load current.