35 v r3 r3 r4 u00d7 peak ac line voltage when switch sw is closed indicated by position b this voltage is made available to the collector of q1 and further charging of c1 is prevented by the low resistance path between switch sw and capacitor c1 through resistor r9 q1 q2 and their associated base rc 39 s provide for turning on q1 and therefore the scr if sw is closed for several tenths of a millisecond immediately following each positive going zero crossing of the ac line transistors q1 and q2 are each protected by a diode d2 and d3 respectively the rc circuit which drives the base of q1 is an order of magnitude faster than the rc circuit which drives the base of q2 when q2 turns on it clamps the base of q1 turning off q1 q1 is therefore only on for the short time provided by the difference between the r8c2 time constant and the r7c3 time constant the result is that if switch sw is closed and c1 is charged q1 will drive a pulse into the gate of the scr and turn it on immediately following each positive zero crossing of the ac line capacitor.