A switching circuit (Q11, Q12, D11, D12, D13, L11, L12) is repetitively configured to charge an energy transfer capacitance (C11) from an electrical supply (V11) and then inject a discrete pulse of energy into a resonated load circuit by discharging the capacitance. The load circuit is formed by a resonating capacitance (C12) and an inductive load device (T11, R11), eg motor, or induction heating or power transfer device. Energy circulates in the load circuit at or near to its natural resonant frequency. There is no injection of energy into the load circuit while energy for charging the transfer capacitance is being delivered from the supply to the switching circuit. During injection, the two capacitances and the inductive load device may be connected together in parallel or in series, or the transfer capacitance may be connected in series with the inductive load device but not the resonating capacitor.