Unidirectional thyristor dimming light circuit triggered by unijunction transistor (1)

In order to improve the stability of the voltage at both ends of the bulb, the trigger circuit shown in Figure 26 uses a crystallized transistor
.
    The 220V alternating current passes through the bulb E and is rectified by VDl - VD4 bridge type. The output barrier wave pulsating voltage is added between
the anode and cathode of the thyristor VT to provide the _[F-directed anode current required to turn on the VT. This pulsating voltage
crosses through R, step-down, and vS clipping to obtain a trapezoidal wave current of about 12V, and is added to the trigger circuit J composed of a single-junction transistor vu. When the trapezoidal voltage drops to zero each time and starts to rise, the power supply charges the capacitor C through RP and R, so that the voltage at both ends of ( and ) continues to rise. When it rises to the turn-on voltage of vu, vu turns on, and C It discharges to R. through vu, the discharge current is in R, and the pulse voltage generated at both ends is added to the f1 pole of the thyristor V1I - causing VT to open. During the opening period of VT, the voltage drop of VT is very small, and the trapezoidal wave voltage almost drops to Zero. When the alternating current crosses zero, VT is turned off, the trapezoidal transition voltage starts to rise from zero, the capacitor C cross restarts charging, and the circuit repeats the process described in E. The side node RP can be changed (, the charging time is slow, thereby changing The time when the voltage j across C rises to the peak point voltage of the single-crystal transistor vU (that is , the time when the relaxation oscillator outputs pulses) plays the role of changing the conduction angle of the thyristor VT to achieve stepless dimming . Purpose.