One-way thyristor stepless dimmer

The 220V alternating current passes through the bulb H and is bridge-rectified by VD1-VD4, and a full-wave pulsating voltage is output and added between the anode and cathode of the thyristor vs
to provide the forward anode voltage required for the thyristor to turn on. This pulsating voltage is stepped down by R1 and
sliced ​​by VD5 to obtain a trapezoidal voltage of 14 to 17V. This voltage is added to the trigger circuit composed of a single-junction transistor V'r2. When
the trapezoidal wave voltage drops to mild each time, it starts When rising, the power supply charges capacitor C through resistors RP and R6, causing
the voltage across capacitor C to continue to rise. When it rises to the turn-on voltage of Y12, VT2 is turned on, and C discharges to resistor R5 through VT2
. The terminal receives a forward tip pulse on the base of the transistor VT1. VT1 quickly turns on, and a
forward trigger voltage is injected into the gate of the thyristor VS, forcing the thyristor VS to turn on. During the turn-on period of vs, the voltage drop of vs is very small, so the trapezoidal
wave voltage drops almost to zero. When the alternating current crosses zero, the thyristor VS turns off, the trapezoidal wave voltage starts to rise from zero again,
the capacitor C starts charging again, and the circuit repeats the above process. Adjusting the potentiometer RP can change the charging time of the capacitor C.
When the resistance value of RP is small, in one cycle of the trapezoidal wave (that is, half cycle of AC), the voltage across the capacitor C
reaches , so the conduction angle of the thyristor VS is larger. If the average current flowing through the bulb H is large, the brightness of the bulb
will be larger; if the resistance of RP is adjusted larger, due to the larger charging time constant, in one cycle of the trapezoidal wave, the voltage
across will reach VT2 later. With the conduction voltage , the conduction angle of the thyristor
VS is small, and the average current flowing through the bulb is small, so the brightness of the light is darker. Therefore, the brightness of the bulb H can be changed by adjusting the potentiometer RP.