SCR AC Voltage Regulator
SCR AC Voltage Regulator
The SCR (Silicon Controlled Rectifier) is a modern semiconductor device offering advantages such as compact size, light weight, high efficiency, long lifespan, rapid response, and ease of use; consequently, SCR-based units are widely used in AC voltage regulators today. This article introduces an SCR AC voltage regulator featuring a simple circuit, easy assembly, and convenient control. It is suitable for regulating household appliances, such as dimming lights, adjusting electric fan speeds, or controlling the temperature of electric irons. With an output power of 100W, it is compatible with most standard household appliances.
1: Circuit Principle (Circuit diagram shown below)
The SCR AC voltage regulator consists of two main sections: a controlled rectification circuit and a trigger circuit. As shown in the diagram, diodes D1–D4 form a bridge rectifier circuit, while a unijunction transistor (UJT), T1, acts as a relaxation oscillator serving as the synchronous trigger circuit for the SCR. When connected to the mains, the 220V AC supply passes through the load resistor (RL) and is rectified by diodes D1–D4, creating a pulsating DC voltage across the SCR's anode (A) and cathode (K) terminals; this voltage is stepped down by resistor R1 to serve as the DC power supply for the trigger circuit. During the positive half-cycle of the AC input, the rectified voltage charges capacitor C through resistor R4 and potentiometer W1. When the charging voltage (Uc) reaches the peak voltage (Up) of transistor T1, T1 switches from a cut-off state to a conducting state. Capacitor C then discharges rapidly through T1's emitter-base 1 (e-b1) junction and resistor R2, generating a sharp pulse across R2. This pulse acts as a control signal applied to the gate of the SCR, triggering it into conduction. Once the SCR conducts, the voltage drop across it becomes very low (typically less than 1V), causing the relaxation oscillator to cease operation. As the AC waveform passes through the zero-crossing point, the SCR automatically turns off. During the negative half-cycle, capacitor C begins charging again... This cycle repeats continuously, allowing for the regulation of power delivered to the load (RL).
2: Component Selection
For the voltage regulator, a WH114-1 type carbon film potentiometer with a resistance of 470 kΩ is selected; this type can be soldered directly onto the circuit board. Regarding resistors, a 1 W metal film resistor is used for R1, while 1/8 W carbon film resistors are used for the others. D1 through D4 are silicon rectifier diodes with a reverse breakdown voltage exceeding 300 V and a maximum rectified current exceeding 0.3 A (e.g., 2CZ21B, 2CZ83E, 2DP3B). The SCR selected is a silicon-controlled rectifier with forward and reverse voltage ratings exceeding 300 V and a rated average current exceeding 1 A, such as the domestic 3CT series.
The SCR (Silicon Controlled Rectifier) is a modern semiconductor device offering advantages such as compact size, light weight, high efficiency, long lifespan, rapid response, and ease of use; consequently, SCR-based units are widely used in AC voltage regulators today. This article introduces an SCR AC voltage regulator featuring a simple circuit, easy assembly, and convenient control. It is suitable for regulating household appliances, such as dimming lights, adjusting electric fan speeds, or controlling the temperature of electric irons. With an output power of 100W, it is compatible with most standard household appliances.
1: Circuit Principle (Circuit diagram shown below)
The SCR AC voltage regulator consists of two main sections: a controlled rectification circuit and a trigger circuit. As shown in the diagram, diodes D1–D4 form a bridge rectifier circuit, while a unijunction transistor (UJT), T1, acts as a relaxation oscillator serving as the synchronous trigger circuit for the SCR. When connected to the mains, the 220V AC supply passes through the load resistor (RL) and is rectified by diodes D1–D4, creating a pulsating DC voltage across the SCR's anode (A) and cathode (K) terminals; this voltage is stepped down by resistor R1 to serve as the DC power supply for the trigger circuit. During the positive half-cycle of the AC input, the rectified voltage charges capacitor C through resistor R4 and potentiometer W1. When the charging voltage (Uc) reaches the peak voltage (Up) of transistor T1, T1 switches from a cut-off state to a conducting state. Capacitor C then discharges rapidly through T1's emitter-base 1 (e-b1) junction and resistor R2, generating a sharp pulse across R2. This pulse acts as a control signal applied to the gate of the SCR, triggering it into conduction. Once the SCR conducts, the voltage drop across it becomes very low (typically less than 1V), causing the relaxation oscillator to cease operation. As the AC waveform passes through the zero-crossing point, the SCR automatically turns off. During the negative half-cycle, capacitor C begins charging again... This cycle repeats continuously, allowing for the regulation of power delivered to the load (RL).
2: Component Selection
For the voltage regulator, a WH114-1 type carbon film potentiometer with a resistance of 470 kΩ is selected; this type can be soldered directly onto the circuit board. Regarding resistors, a 1 W metal film resistor is used for R1, while 1/8 W carbon film resistors are used for the others. D1 through D4 are silicon rectifier diodes with a reverse breakdown voltage exceeding 300 V and a maximum rectified current exceeding 0.3 A (e.g., 2CZ21B, 2CZ83E, 2DP3B). The SCR selected is a silicon-controlled rectifier with forward and reverse voltage ratings exceeding 300 V and a rated average current exceeding 1 A, such as the domestic 3CT series.
Aug 13,2026