the nor gate 45 activates the turn off transistors q 7 q 8 pulling current out of the base of the power transistors q 1 q 2 the second set of driver transistors q 9 q 10 q 11 q 12 are analagously biased and connected to be triggered by an output from a nor gate 47 of the control circuitry of fig3 and to drive the bases of the second pair of power switching transistors q 3 q 4 the manner of triggering outputs from the nor gates 45 47 will now be described with reference to fig3 and fig4 as before noted the alternating current trapezoidal waveform produced in the primary of the current transformer t 2 and illustrated in fig4 a is fed back via the secondary of the transformer t 2 to the control circuitry of fig3 this signal is full wave rectified by diodes cr 7 cr 8 cr 9 cr 10 of the control circuitry the resulting rectified trapezoidal current waveform is converted to a voltage waveform fig4 e by a resistor r 7 which may be connected to a parallel capacitor c 7 for noise suppression the trapezoidal waveform developed across the resistor r 7 is then applied via a resistive divider network r 8 r 9 to the positive input 49 of the comparator 25 whose negative input 53 is supplied by a reference voltage v ref the trapezoidal signal thus presented to the positive input 49 of the comparator 25 is representative of the current level passing through the primary of the power transformer t 1 and hence of the output current additionally its duration is representative of the duration of.