A circuit for driving gas discharge lamps (102, 104, 106) from a nominal-level voltage supply includes: a voltage boost IC (144); an oscillator (196, 198, 178, 180) producing a high-frequency output voltage applied to the lamps via a transformer (212); and a voltage clamp (215A, 215B) coupling the transformer to the oscillator input (174, 176). The IC (144) regulates the power drawn by the circuit to a constant level if the supply voltage is greater than 95 % of its nominal value. If the supply voltage falls to less than 95 % of its nominal value, regulation is lost and the circuit draws less power in proportion to the fall in the supply voltage. If the supply voltage falls to less than 90 % of its nominal value the clamp operates to reduce the power drawn by the circuit at a rate greater than that of the fall in the supply voltage. The circuit thus enables the power drawn by the circuit to be reduced by reducing the supply voltage to less than 95 % of its nominal value.