mosfets 9 10 on and off in opposition the voltage at node a is switched alternately and substantially between the voltage of the ht rail and the voltage of the 0v rail the lamp arc is sustained by current flowing via the primary winding of current transformer 20 and node c to the capacitors 21 and 22 which allow the current to return to the ht and 0v rails the arc impedance is sufficiently low to divert most of the current flowing in inductor 17 away from capacitor 18 and via the lamp 19 the current limiting circuitry operates through current transformer 20 and voltage comparator 5 in just the same way as in the first mode of operation described above since the oscillator frequency in the second mode of operation is substantially lower than in the first mode of operation the inductor 17 will pass much higher currents as its impedance at low frequency is much lower than its impedance at high frequency therefore the operation of the current limiting circuit described above is an essential feature in this second mode of operation indeed due to the operation of the current limiting feature it may be seen that the lamp current waveform illustrated schematically in fig3 approximates a square wave with a small amount of high frequency ripple due to the operation of the.