Zener Diode

12 V AC Dimmer Schematic Circuit Diagram

The circuit presented here is a variation of a traditional lamp AC dimmer design. If you visualize a diac connected between points A and B, you’ll recognize its basic structure. However, a significant deviation from a standard diac circuit lies in its inability to function at 12 V. Typically, diacs have trigger voltages ranging from 30 to 40 V, rendering them unsuitable for operation at 12 V. Consequently, the AC dimmer circuit fails to operate under these conditions due to the limitations of the diac.

The section of the circuit between points A and B simulates the behavior of a diac with an approximate trigger voltage of 5.5 V. Within this network formed by R1, P1, and C1, a phase shift relative to the supply voltage is generated. This “diac equivalent” circuit produces a phase-shifted trigger pulse to activate the triac during both the positive and negative half-cycles of the sinusoidal AC voltage. Here’s how it operates:

Positive Half-Cycle:

• During the positive half of the sine wave, C1 starts charging as the voltage begins to rise. The rate of charging is determined by the time constant created by C1, R1, and P1. T1 initially remains non-conductive, waiting until the voltage across D2 reaches 4.7 V, causing the Zener diode to conduct. Once this happens, current flows, activating both T1 and T3. This results in the generation of a pulse at point B.

Negative Half-Cycle:

• Similarly, during the negative half of the sine wave, D1, T2, and T4 play equivalent roles in generating a pulse at point B.

The trigger angle, which determines when in the AC cycle the triac is activated, can be adjusted using P1 within a range spanning approximately 15 to 90 degrees. C2 provides a degree of noise decoupling. Depending on the load, a heat sink might be necessary for the triac. Various transistors can be used; the ones specified here are just examples. If the circuit doesn’t dim adequately, you can alter the value of P1 to 25 kΩ, enabling an increase in the trigger angle to 135 degrees.

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