AC electronic voltage regulator using TL431

TL431 is an adjustable precision voltage regulator with low noise and continuously adjustable output voltage (25~36V). The output dynamic resistance is small and the current is large. Therefore, it has been widely used in electronic circuits.

Applying TL431 to an AC voltage regulator can simplify the circuit structure and improve circuit performance. The AC electronic voltage regulator designed in this article automatically adjusts the output voltage to 220V+-7% when the mains voltage changes within the range of 158~270V. When the mains voltage is lower than 158V or higher than 270V, it can automatically cut off the power supply. Therefore, it has under-voltage and over-voltage protection functions.

The figure below is the circuit diagram of an AC electronic voltage regulator. In the figure, R1 to R5 are 10k ohms, RP1~RP5 are 10k ohm potentiometers, A1~A5 are TL431, V1~V4 are IN4001, C is 4700uF/50V, and K1~K5 are JQX-14F (DC12V). After the mains power is filtered by the autotransformer T, V1~V4 bridge rectifier and capacitor C, it is used for voltage sampling by the detection circuit. The +12V power supply is obtained by 7812 stabilizing the voltage filtered by capacitor C (note: omitted in the figure). Taps 1 and 3 of the transformer are used as fixed mains input, and the voltage at both ends of the capacitor C also changes within the range of 158~270V. When the potentiometer changes, when the adjustable terminal voltage of RP1~RP4 exceeds 2.5V, A1~A4 will be turned on in sequence, causing relays K1~K4 to be closed in turn, thus changing the tap of the output end and the voltage regulator from 5->2 to achieve the purpose of automatic voltage regulation.

When the mains input is from 158~180V, A1 is turned on, and taps 1 and 5 output 220V±7%; when the mains input is from 180~205V, A1.A2 is turned on, and tap 1.4 outputs 220V±7%; When the mains input is from 205 to 235V, A1, A2 and A3 are turned on, and the 1.3 tap outputs 220V+-7% directly; when the mains input is from 235 to 270V, A1, A2, A3 and A4. On, the 1.2 tap outputs 220Vt7%; when the mains input is lower than 150V, A1 does not conduct, K1 releases the closed contact, cuts off the output power supply circuit, and plays the role of under-voltage protection; when the mains input is higher than 270V , A6 conducts, K5 pulls in, cuts off the output power supply circuit, and plays the role of overvoltage protection.

Since the voltage regulator introduced in this article uses TL431, it is easy to adjust the voltage regulating range and overvoltage and undervoltage protection values ​​of the voltage regulator by changing the position of the adjustable ends of the potentiometers RP1~RP5.