(57) @ A dwell control for an ignition system of the general type including a closed loop control which operates to vary the instant after each spark at which energy storage is commenced for the next spark. At low speeds the closed loop control is overridden and the circuit output (Q44) is operated at constant duty ratio. Switch over between the two modes of operation is effected under the control of a bistable circuit (Q₁₈, Q₁₉) which is set to one state in which a variable timer (C₁, Q₂, Q₃, 04) incorporated in the closed loop control determines the instant of energy storage commencement, whenever the time duration for which the energy stored exceeds a threshold in less than a predetermined duration. The bistable is set to its other state, to provide fixed duty cycle operation, whenever the timer period exceeds a limiting value, or when a pulse arrives at the circuit input (Q₁) from a transducer (10) before the expiry of the timer period. This control scheme for the bistable prevents undesirable hunting between the two operating modes.