# How to provide reverse voltage protection?

### How to protect Circuit against applying reverse voltage? OR How to provide reverse voltage protection?

Consider the below example circuit. If we apply proper voltage (connecting properly to the supply/battery) to the electrical circuit, then it will work as it is.
If we connect the terminals in opposite manner, then it will spoil the circuit.

The circuit should have the ability to protect against applying reverse voltage.

In this post, let us see various methods of protecting electrical circuit / electrical equipment against applying reverse voltage.

#### Method#1 Reverse Voltage Protection by PN Junction Diode

• By inserting a diode in between supply and equipment, we can protect the circuit against applying reverse voltage.
• It act as a one way switch.
• ie, when the battery connected properly it is in forward biased and thus it will close the complete circuit.
• If the battery connected in reverse manner, the diode is in reverse biased. It will block the supply. Thus protect the electrical circuit.
• The power dissipation in this method is huge compare to other methods. Click here to know about How to Calculate Power Dissipation of Diode?

Example Calculation:

• Consider Load Current = 1A, If we use 1N4007 diode, then the power loss will be
P = Vf x If = 0.9 x 1 = 0.9 watts

• Vf for corresponding 1A current is obtained from the Vf vs If curve given in the 1N007 diode datasheet
• Nearly 1watt of power loss for reverse voltage protection of 1A load is an inefficient way to do it.

#### Method#2 Reverse Voltage Protection by Schottky Diode

Example Calculation:

• Consider Load Current = 1A, If we use 1N5817 schottky diode, then the power loss will be
P = Vf x If = 0.45 x 1 = 0.45 watts

• Vf for corresponding 1A current is obtained from the Vf vs If curve given in the
• Observe that the power loss is just half of that using normal PN junction diode.

#### Method#3 Reverse Voltage Protection by P- Channel MOSFET

• The most effective method of reverse voltage protection is using P- Channel Mosfet in the circuit.
• When the battery is connected properly and the portable equipment is powered, the P-Channel MOSFET’s gate voltage is taken low and its channel shorts out the body diode
• When the battery is connected incorrectly, the P-Channel MOSFET’s gate voltage is high, preventing it from turning on.
• In this case, the Power loss is I2 x RDS(ON).
• Comparing to other methods the power loss is very less in this method.

Example Calculation:

• Consider Load Current = 1A, If we use FQP47P06 MOSFET, then the power loss will be
P = I2 x RDS(ON)= 1 x 0.00026 =0.26 milliwatts
• The   RDS(ON) value is given in the datasheet
• Observe that the power loss is reduced drastically.

We can use N- Channel Mosfet in the return path. But that is not usually used by the designers due to various complications.

The improvised method of MOSFET based reverse voltage protection is discussed in detail in separate post... Please click here to know about the Reverse Voltage Protection using Low VDS MOSFET