120V AC Lamp Dimmer

The full wave phase control circuit below was found in a RCA
power circuits book from 1969. The load is placed in series with the AC
line and the four diodes provide a full wave rectified voltage to the
anode of a SCR. Two small signal transistors
are connected in a switch configuration so that when the voltage on the
2.2uF capacitor reaches about 8 volts, the transistors will switch on
and discharge the capacitor through the SCR gate causing it to begin conducting.

Circuit diagram:

The time delay from the beginning of each half cycle to the point where the SCR
switches on is controlled by the 50K resistor which adjusts the time
required for the 2uF capacitor to charge to 8 volts. As the resistance
is reduced, the time is reduced and the SCR will conduct earlier during each half cycle which applies a greater average voltage across the load.

With the resistance set to minimum the SCR
will trigger when the voltage rises to about 40 volts or 15 degrees
into the cycle. To compensate for component tolerances, the 15K resistor
can be adjusted slightly so that the output voltage is near zero when
the 50K pot is set to maximum. Increasing the 15K resistor will reduce
the setting of the 50K pot for minimum output and visa versa. Be careful
not to touch the circuit while it is connected to the AC line.

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