10-04-2017, 07:27 PM
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This circuit protects refrigerators as well as other electrical appliances from over and under voltage. By the name itself we can say that if the input voltage is more or less than the required voltage then the electrical appliance is turned off nothing but it gets disconnected from its respective power supply.
This circuit uses an operational amplifier IC LM324 which is used as a comparator. Generally, the IC LM324 consists of four operational amplifiers, of which only two operational amplifiers are used. In this IC, we have an inverting terminal and a non-inverting terminal. The voltages at the inverting terminal and the non-inverting terminal are 6.0v and 6.8v respectively.
When the input AC voltage exceeds 240v, the voltage at the non-inverting terminal increases. So now if the voltage at the non-inverting terminal increases more than 6.8v then the output of the operational amplifier is pulled high. So the electrical appliance is turned off by means of a relay connected to the output pin of the op-amp. Thus the electrical appliance is now protected against over voltage. Now let us consider under voltage condition. When the line voltage is below 180v, the voltage at the inverting terminal is less than the voltage at the non-inverting terminal. Thus the output of the operational amplifier now goes high and the AC supply gets disconnected and the electrical appliance also turns off. Thus, the appliance is now protected against under voltage.
Thus the circuit protects any electrical appliance from over and under voltages.
BLOCK DIAGRAM
INPUT
OPAMP1
REGULATED POWER SUPPLY
TRANSISTOR DRIVER CIRCUIT
OPAMP2
RELAY
ELECTRICAL APPLIANCES
POWER SUPPLY:
The input to the circuit is applied from the regulated power supply. The a.c. input i.e., 230V from the mains supply is step down by the transformer to 12V and is fed to a rectifier. The output obtained from the rectifier is a pulsating d.c voltage. So in order to get a pure d.c voltage, the output voltage from the rectifier is fed to a filter to remove any a.c components present even after rectification. Now, this voltage is given to a voltage regulator to obtain a pure constant dc voltage.
230V AC
50Hz
D.C Output
Regulator
Filter
Bridge Rectifier
Step down transformer
Fig: Power supply
Transformer:
Usually, DC voltages are required to operate various electronic equipment and these voltages are 5V, 9V or 12V. But these voltages cannot be obtained directly. Thus the a.c input available at the mains supply i.e., 230V is to be brought down to the required voltage level. This is done by a transformer. Thus, a step down transformer is employed to decrease the voltage to a required level.
Rectifier:
The output from the transformer is fed to the rectifier. It converts A.C. into pulsating D.C. The rectifier may be a half wave or a full wave rectifier. In this project, a bridge rectifier is used because of its merits like good stability and full wave rectification.
Filter:
Capacitive filter is used in this project. It removes the ripples from the output of rectifier and smoothens the D.C. Output received from this filter is constant until the mains voltage and load is maintained constant. However, if either of the two is varied, D.C. voltage received at this point changes. Therefore a regulator is applied at the output stage.
Voltage regulator:
As the name itself implies, it regulates the input applied to it. A voltage regulator is an electrical regulator designed to automatically maintain a constant voltage level. In this project, power supply of 5V and 12V are required. In order to obtain these voltage levels, 7805 and 7812 voltage regulators are to be used. The first number 78 represents positive supply and the numbers 05, 12 represent the required output voltage levels.