The project “Auto Cut Circuit for Over and Under Voltage” is designed to protect electrical and electronics device from over and under voltage which ensure long lifespan of these device. An “Auto Cut Circuit for Over and Under Voltage” have been presented here serving multiple purpose security; under voltage protection, over voltage protection, auto reset, auto cut off and regulation.
Circuit Description of Auto Cut Circuit for Over and Under Voltage
The circuit diagram of “Auto Cut Circuit for Over and Under Voltage” is shown in figure 1. The entire circuit of auto cut circuit is built around two timer 555 IC, configured in comparator mode. Although the circuit looks complex, it comprises of building block you have been familiar with.
As seen in left part of the circuit, input mains are connected to primary of step down transformer which lower the input voltage to 12V AC. A bridge rectifier made from four general purpose rectifier diode converts 12V AC to pulsating 12V DC which is further smoothen using capacitors as shown in circuit diagram. The ripple free 12V DC is obtained at this point which is further given to relay and input of 9V series voltage regulator IC. The output of series voltage regulator is 9V DC.
Two timer IC (IC2 and IC3) are configured in comparator mode which compare the mains voltage and produce output as per our desire. The output of 1st comparator (IC2) is given to the reset pin (pin 4) of 2nd comparator (IC3) through the collector of transistor T1. The voltage at input pin of IC3 (pin 2) is 1/3 of Vcc i.e. 3 volts. Till the input voltage (voltage at pin 2) is less than 1/3 of Vcc the output of IC3 (pin 3) is high. As long as the output at pin 3 is high the LED connected to pin 7 become off due to high impedance. Similarly, when the input pin voltage (voltage at pin 2) is greater than 1/3 of Vcc the output become low as a result the LED connected to pin 7 start to glow.
The circuit of “Auto Cut Circuit for Over and Under Voltage” can only operate in range of 160V AC to 270V AC, other than this range of voltage i.e. under 160V AC or over 270V AC the relay connected to the output of IC3 is deenergized as a result load will be off. Transistor T1 is use to drive the reset pin of IC3, similarly transistor T2 is use to drive the relay.
- At first the circuit is fabricated on a PCB board.
- Adjust variable resistor VR1 in such a way that LED1 start to glow when the mains voltage is lower than 160V AC.
- Adjust variable resistor VR2 in such a way that LED2 start to glow when the mains voltage becomes higher than 270V AC.
PARTS LIST OF AUTO CUT CIRCUIT FOR OVER AND UNDER VOLTAGE
|Resistors (all ¼-watt, ± 5% Carbon)|
|R1, R5 = 220 Ω
R2, R3, R6, R7 = 1 KΩ
R4 = 100 KΩ
VR1, VR2 = 100 KΩ (Preset)
|C1 = 0.1 µF
C2, C3 = 1000 µF/25V
C4, C5 = 0.01 µF
|IC1 = LM7809
IC2, IC2 = NE555
T1 = BC547
T2 = SL100
D1 – D4 = 1N4007
D5 = 1N4001
LED1 = Yellow Color LED
LED2 = Red Color LED
|X1 = 230V AC Primary to 12V, 250mA Secondary Transformer
RL1 = 12V, 200Ω Relay