Over Voltage and Over Current Protection

Introduction:

             Sometimes due to faults in Power System, Voltage in our homes increases beyond its rated value (220 or 230 V in India) which may damage the sensitive devices like mobile or laptop chargers, motherboards etc.

To save these sensitive devices from Over Voltage I designed and made a very simple and easy circuit. The circuit diagram is shown below

 Circuit diagram:                

                         

Components:

1. 9-0-9 Transformer(1)

2. 1N4007 diodes (4)

3. 470 micro Farad 25 volt Capacitor(1)

4. 10 k variable resistance(1)

5. 7812 Voltage regulator(1)

6. 1 k resistance(2)

7. 10 k resistance(2)

8. lm358 comparator

9. Green and Red LED(1,1)

10. Zener diode from 5 to 10 volt(1)

11. BC 547 transistor(1)

12 12 v 7 amp relay(1)

Operation:

            It is based on LM358 comparator which compares two voltages, one is reference voltage and second is input voltage. The input AC voltage is first stepped down using a 9-0-9 transformer of 500 mA rating. Then it is converted to DC using a bridge rectifier. A 470 micro farad , 25 V capacitor must be used to remove the ripples from the DC supply.  Then it is fed to LM358 through a 10 k variable resistor .It is used to set the desired value beyond which the circuit will trip. 7812 IC is used to regulate the voltage at 12 V.  Zenor diode is used to set the reference value which operates in breakdown region and current is limited using a 10k resistance. When input voltage rises from the desired value output of LM358 becomes high which triggers the base of BC547 transistor. 12 V 7 A Relay is connected in the collector circuit of BC 547 which operates after the triggering. The main supply is connected via NC(normally closed) terminal of relay. When relay operates it breaks the main circuit to protect the devices which are connected to the main supply.  

Green LED is used to indicate the main supply and Red LED is used to indicate over voltage. These are connected through 1 k resistance to limit the current.

            If short circuit occurs in the protected supply then fuse connected in series with main will be blown and it will save the components from over current. 

            In this circuit relay will not work for all the time therefore power consumption is low. But if any component of the circuit fails then the circuit will not work properly. This can be overcome by making the small change in the circuit. If we interchange the Input voltage and reference voltage then output of LM 358 will be high for normal conditions and relay will be on. We have to connect the main supply through NO(normally open) terminal of the relay so that if input voltage increases beyond the limit or any component fails then the relay will trip and it will break the circuit. Red LED which indicates the over voltage, must be connected between the Vcc and output of the LM 358.  This method is more reliable but in this method power consumption will be more than the previous method.