A hollow cathode lamp is controlled during normal running, by a pulsing circuit, and the lamp is in series with a diode which protects the pulsing circuit when the lamp is initially ionised by a large voltage applied across the anode and cathode of the lamp. The pulsing circuit drives an operational amplifier which in turn drives the control terminal of a transistor in series with the lamp. The operational amplifier presents a high impedance to the pulsing circuit, and therefore a number of similar lamps can be controlled, each through its own amplifier, without interaction.
