A lamp drive circuit includes an electronic switch in series with the lamp and a source of DC voltage, a control input of the switch being pulse-width-modulated by a control circuit which includes a temperature-sensing circuit for reducing the pulse-width-modulation duty cycle when lamp temperature exceeds a predetermined temperature. The temperature-sensing circuit may include a thermal switch in series with one of two parallel-connected resistors in a timing circuit. Duty cycle may also be automatically adjusted in response to changes in the source voltage.