Design of JS20 Unijunction Transistor Time Relay Circuit

　　Circuit principle: The circuit consists of five parts: delay link, amplitude detector, output circuit, power supply and indicator light. The voltage stabilizing part of the power supply is composed of resistor R and voltage regulator tube vs, which supplies power to the delay link, the thyristor VTH and the relay in the output circuit. KA is directly powered by the half-wave rectifier circuit. When the power is turned on, the capacitor G is charged through RP1 and R4 after being rectified by diode VD1, filtered by capacitor cl, and stabilized by RJ and VS. The voltage on capacitor G gradually increases according to the exponential law. When this voltage is greater than the peak voltage of the single junction transistor VT, the single junction transistor is turned on, G discharges through the eb1 pole and the wind of VT, and outputs a pulse voltage on the wind, triggering the gate tube VTH to turn on and make the relay KA close. At the same time, the contact of KA short-circuits C3, causing it to discharge rapidly, and the voltage between the ebl of the single junction transistor drops rapidly. When it drops to the valley voltage of Vr., Vr is turned off. At the same time, the LED indicator lights up. When the power is cut off, the relay KA is released, the circuit returns to its original state, and waits for the next action. By adjusting RP, the delay time can be adjusted.