Moazzam Hussain

Electronic Circuit and Devices Lab Over-/ Under Voltage Protection of Appliances EE-17B Prepared by: Moazzam Hussain - OBJECTIVE: The purpose of this project was to create a circuit that protects the electronic appliances from over and under voltages which can greatly damage the appliances. APPARATUS: • • • • • • • • • • • • Transformer Diodes Zener 6V Zener 6.8V Capacitor Resistor LM 7812 LM 741 BC 547 Potentiometer Relay LEDs CIRCUIT DIAGRAM: Figure: Circuit Diagram of over-/ under voltage protection of appliances I used LM741 instead of LM324. I used 6V relay instead of 12V relay. INTRODUCTION: PURPOSE OF TRANSFORMER: Figure: Transformer A transformer is a device which can step up or step down voltage levels. A step down transformer of 12 V was used. The voltage coming to our homes is 230𝑉𝑅𝑀𝑆 . So this ratio was set that if normal voltage of 230VRMS comes as input, the transformer should give 12 VRMS. PURPOSE OF BRIDGE RECTIFIER: Figure: Bridge Rectifier Transformer only stepped down the voltages but it remains AC, so in order to convert it into DC we need good rectifiers that would convert sine waves to specific positive peak. Rectifiers are made by using four diodes 1N4007connected in a loop. When input AC is supplied the diodes behave as forward bias and two behave as reverse biased and blocks the electric current. the output, we are getting is in the form of pulsating DC. Since, at a time, only 2 diodes remain in the ON state. Due to which, we get voltage drop of 0.7+0.7 = 1.4 V (because of internal resistance in diode there is 0.7 V drop. In normal conditions of 230V as input, the output is 12*√2V – 1.4 = 15.57V. PURPOSE OF FILTER CAPACITOR: A capacitor filter is used when we need to convert a pulsating DC into pure or to remove distortion from signal. The rectifier circuit converts the incoming AC to DC but unluckily it does not make it a pure DC. So, the rectified DC is not very clean and has ripples. It is the job of the filter to filter out these ripples and to make the voltage compatible for regulation. Zener diode is used is used to supply the fix voltage. Zener diodes: are like simple diodes. But when supply voltage exceeds the breakdown voltage/ Zener voltage, diode starts to conduct in both directions i.e. now, it conducts in reversed biased direction too. PURPOSE OF LM7812: Figure: LM7812 LM7812 is a voltage regulator IC. It has 3 pins. Pin 1 is input pin whereas pin 3 is an output pin. Pin 2 is ground pin of IC 7812. The 7812 Voltage Regulator IC does not need any component to balance or satisfy its output voltage. The 7812 IC has built-in protection from high current. Attached to it is a hot-sink with common ground. The heat sink protects our regulator IC from overheating and short circuits. PURPOSE OF LM741: Figure: pin Diagram of LM741 LM 741 is an op amp IC. It is very useful as it protects the circuit from short circuiting. It consumes very less power and it has large CMMR ratio and differential voltages. Four stages of op amp are shown in figure: PURPOSE OF RELAY: Relays are switches that aim to close and open the circuit electronically as well as electromechanically. It controls the opening and closing of the circuit contacts of the electronic circuit. WORKING OF CIRCUIT: The purpose was to design a circuit that gives over and under voltage protection to the electrical appliance. The limits of over and under voltages are 180 and 240 VRMS. So, the circuit comprises of a transformer the steps down the AC voltage that is coming. The primary to secondary inductance ratio was set to 336:1, so that when an optimum voltage of 230 𝑉𝑅𝑀𝑆 is provided, the output after stepping down will be 12𝑉𝑅𝑀𝑆 . That stepped down voltage is fed to the bridge so that is can be rectified and converted into DC. The capacitors further smoothen the voltage to DC. After the conversion, the voltage is fed to the regulator IC so that, the IC regulates the voltage 12 Vas output. The unregulated supply of voltage is sent to resistors the series setup of resistors R8, R7 and potentiometer RV3. The same supply is also fed to Zener of 6.8V through R6. These 2 series setups will set the upper reference voltage of comparator U3. The voltage of potentiometer RV3 is adjusted in such a way that for optimum voltages between 180 and 240 𝑉𝑅𝑀𝑆 , the voltage at the non-inverting terminal should be less than 6.8V. So, this way whenever the input will be greater than 240 𝑉𝑅𝑀𝑆 , the voltage at non-inverting amplifier will be greater than 6.8 V hence the op amp will give high output. This will trigger the transistor switch hence the relay will get active and break the connection of the electronic appliance with the supply. For the under-supply voltage check a second comparator is used. The unregulated supply of voltage is sent to resistors the series setup of resistors R2, R3 and potentiometer RV1. The same supply is also fed to Zener of 6V through R1. The voltage of potentiometer RV3 is adjusted in such a way that for optimum voltages between 180 and 240 𝑉𝑅𝑀𝑆 , the voltage at the inverting terminal should be greater than 6V. So, whenever a low voltage is applied as input, the voltage at the inverting terminal becomes less than 6V, hence the amplifier gives a high output that will trigger the transistor switch hence the relay will get active and break the connection of the electronic appliance with the supply. Under normal conditions, both the op amps will give low output. Hence the transistor will remain in off state, so the relay will not be active and the connection will not be broken. Simulations: Under voltage condition: Normal voltage condition: Over voltage condition: Limitations: When the supply voltages are between 237-240 Volts, the relay keeps on activating and deactivating. Same is the case when voltages are 180Volts. The relay doesn’t stay in one position, it keeps on switching. Conclusion: It was concluded that I successfully designed a circuit that protects the appliances from over and under voltages.