An alternating current generator for use as a shot exploder suitable for firing electric detonators coupled through toroid transformers e.g. `Magnadet` detonators. The detonating system is energized by means of a signal that is automatically generated at the resonant frequency, thus avoiding the usual time-consuming and dangerous practice involving in-situ determination of inductance and selection of capacitors. A power oscillator having a transistor is used to supply the energizing signal. This transistor is switched by a current feedback signal. In the preferred form a transformer is located in series with the output, the secondary of the transformer driving the transistor.