The drive circuit for a discharge lamp, e.g. a halogen lamp, provides a variable pulse width to vary the light output of the lamp. A failsafe control computes the actual voltage applied to the lamp taking into account the input voltage to the control, the clocking frequency and the pulse width modulation. If the computed voltage is outside a set range the lamp is switched off. The control system includes a microprocessor which varies the clocking rate and the pulse width modulation. If the computed voltage is outside the permitted range the control system can vary the clocking rate and pulse width to bring the voltage back into the range.
