variable reference voltage source 118 such as a potentiometer connected across low voltage supply rails to vary the power output of the dimmer the operator varies the potentiometer slider the higher the reference voltage the smaller the width of the pulses output by the comparator 117 as a smaller proportion of each sawtooth generated by the generator 116 rises above the reference voltage from the potentiometer 118 where the dimmer is to operate in a rack as shown in fig4 the variable reference voltage may be supplied from the control electronics 505 or could comprise a digital to analogue converter receiving a digital control word from the control electronics 505 pwm controllers comprising single integrated circuits providing components 116 and 117 are known in order to improve the stability against changing loads a negative feedback system may be employed measuring the output voltage and adjusting the pwm ratio to achieve the desired output preferably however the pulse generator 115 comprises a control unit 150 such as a microprocessor with a program storage read only memory rom and may also comprise an analogue to digital converter 151 providing a signal input to the control unit 150 and a potentiometer 152 variable by the user to provide an analogue control signal the value a of the analogue control signal is read as shown in fig1 by the control unit 150 via the adc 151 and an output line of the control unit 150 is switched high for a.