120 vac lamp dimmer

Posted on Feb 6, 2014

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

120 vac lamp dimmer
Click here to download the full size of the above Circuit.

8 volts, the transistors will switch on and discharge the capacitor through the SCR gate causing it to begin conducting. 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 tollerances, 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.

Leave Comment

characters left:

Related Circuits

  • New Circuits



    Popular Circuits

    Clamp-on-current probe compensator
    Crystal checker
    12V to +/- 20V DC Converter Circuit
    RF-Radio Frequency 6
    IR Bug
    voltage to current convertor
    LM12 audio amplifier circuit diagram electronic project
    LM2876 40W audio power amplifier circuit design
    Adjustable voltage with adjustable current limit circuit
    Practical Data Acquisition using a Windows based Power Meter