Electronic dimming circuit

Electronic dimming circuit can be used to adjust the brightness of the bulb to meet different lighting requirements. The figure below is an electronic dimming circuit.

This circuit is a thyristor voltage regulator circuit composed of a controlled rectifier circuit and a trigger circuit. After the 220V mains electricity is rectified by a bridge composed of diodes VD1~VD4, the voltage applied to the A and G ends of the thyristor VS is a sinusoidal pulsating voltage. This voltage is then stepped down by the current limiting resistor R1 and supplied to the trigger circuit as a DC power supply and synchronization voltage. The unijunction transistor VT, resistor R4, potentiometer RP and capacitor C form a relaxation oscillator. In each half cycle, when the charging voltage on capacitor C reaches the peak voltage of the unijunction transistor, the unijunction transistor changes from cut-off to on, and capacitor C discharges through R2. In this way, a pulse is output on R2 and sent to the thyristor control electrode G, triggering the thyristor to turn on, so that current flows through the light bulb HL. After the thyristor is turned on, the forward voltage drop between AG is very small (about 1V), so the trigger circuit stops working. When the power supply voltage passes through zero, the thyristor is turned off, and when the next half cycle begins, capacitor C is recharged and the above process is repeated.

Adjusting the potentiometer RP can change the length of the thyristor conduction time in each half cycle, thereby controlling the power supplied to the bulb HL, that is, adjusting the brightness of the bulb. When the resistance of the potentiometer RP is increased, the capacitor C needs to charge for a longer time to make the thyristor VS conduct, and the conduction angle of the thyristor is smaller, and the bulb is dim; conversely, when the resistance of the potentiometer RP is adjusted to a smaller value, the bulb is bright. Under normal circumstances, adjusting the potentiometer RP can make the voltage at both ends of the bulb change within the range of 0 to 200V, thereby effectively controlling the brightness of the bulb.