120 VAC Lamp Dimmer Circuit

A lamp dimmer circuit can be conceived in so many ways but the challenge is cost effectiveness and reliability. Thyristors (or SCRs) and TRIACs are the major components associated with the lamp dimmer circuit.


Direct on line (120 VAC) dimmers can be configured either by two TRIACs connected anti parallel or it can be configured by 4 number of SCRs in the same topology for a full wave phase control. But this is a costly methodology.


The full wave phase control topology, as shown in the circuit was found in a RCA (Radio Corporation of America) power circuit book,1969. In fact, so far as power circuit is concerned, this is one of the cheapest effective typologies.


A full wave rectifier bridge is being employed in series with the AC line along with a SCR, the rectified AC also had its zero crossing.


The frequency of which is double that of the supply line and hence commutation of SCR at its natural zero is possible. Apparently it may appear that the SCR is working on a DC line but that is not the truth.

Two small transistors are connected as shown in the circuit. A 2.2 micro Farrad capacitor is employed in such a way so that when the voltage on the capacitor reaches about 8V the transistor will switch on.

As soon as the transistor is switched on, they discharge the capacitor through the SCR gate causing it to conduct.


As natural zero crossing phase is embedded in the topology, the conduction of the SCR is not a problem.


The 50K variable resistance is employed to adjust the time required for the capacitor to charge up to 8V. As the resistance is reduced, the time reduces and the SCR shall conduct at an early time at each half cycle. 

This applies more voltage across the load. Hence the 50K resistor can be used as a steering of the Dimmer circuit.

A standard SCR having 400V rating or above and respective current rating according to the load shall be a handy choice for this circuit. The whole circuit can be accommodated inside a match box size cabinet. A multi turn radio potentiometer of 50K value can be mounted on top of the box.



With the resistance of the potentiometer set to minimum the SCR shall only trigger when the supply voltage rises above 40V that is equivalent to 15o of the full cycle.


The 15K resistor plays an important role as it determines the setting of the capacitor voltage of 8V mostly.


If we increase the value of the resistance from 15K to higher value, that shall reduce the setting of the potentiometer (50K) for minimum opposition. It is a caution for the hobbyist not to touch any electronic component while the power is given, as the components are on direct supply mains.