the opto receiver when the three way switch is set at the medium setting thus the transistor q3 will not always be on instead the on time and off time of the transistor q3 will be determined by the capacitor charge and discharge times compared to a symmetric duty cycle the net reduction in light output that results from the change in duty cycle from the active stage operating symmetrically the transistor q3 always off is approximately 20 of the total light output thus in the minimum light output stage the total reduction in light intensity from the maximum output is 80 i e the total light output is approximately 20 of the maximum light output in the third switch position both the lines 122 and 124 are energized the voltage across the rail voltages will again be twice the input voltage because the voltages across the rails is determined by the input line voltage and by the voltage drop across the diode d1 in the positive half cycle and across the diode d3 in the negative half cycle thus the voltage across the rail provides maximum power to the lamp likewise the dimmer control circuit 110 also will provide maximum power to the lamp when the lines 122 and 124 are both energized the photo coupler or optocoupler transmitter tu1x is shorted out of the circuit since the opto coupler transmitter is off the opto coupler receiver tu1r will also remain off and thus the capacitor c13 will not be charged the transistor q3 will be kept off because the base emitter voltage will be less than the turn on.