How Crowbar Protection Circuit (Over Voltage Protection) Works?

Over Voltage Protection - How Crowbar Circuit Works? 

The Crowbar protection circuit is a fail-safe protection circuit. It protects the load against overvoltages.
It is placed across the power supply output terminals. 

The crowbar circuit mechanism contains, crowbar device and sensing circuit( monitoring circuit).
The two commonly used components for the crowbar device are Silicon Controlled Rectifier (SCR) and the MOSFET.
A crowbar circuit is usually placed across the power supply’s output terminals, to protect the load against any overvoltage

Crowbar Circuit Operation:

To understand the working principle of crowbar circuit, refer the following circuit.
SCR_Over_Voltage_Protection_Circuit

Normal Condition:

  • Assume that the supply voltage is VDC = 6V.
  • The 10K resistor and the Potentiometer (POT) forms the voltage divider circuit.  
  • Adjust the pot to 10K, so that the non-invertering input of opamp is 3V.
  • The zener voltage of the the zener diode is VZ = 3V which is applied to the invertering input of the opamp.
  • As the both the inputs are equal (3V) the output of the opamp is zero. (Remember that here the opamp act as a comparator.)
  • In other words the gate voltage of the SCR is zero.
  • So the SCR is in OFF state.

Faulty Condition:

  • When overvoltage (Surge voltage) occurs, across the voltage divider circuit, the over voltage will appear which leads to high voltage at non-inverting input terminal of opamp.
  • The voltage at inverting input of opamp is same as the zener voltage VZ=3V.
  • Now the comparator output is high consequently voltage appeared at SCR gate terminal.
  • Thus the crowbar SCR turns ON and shorts the circuit. Eventually the fuse will blow/trip the circuit breaker.

Advantages of Crowbar Protection Circuit:

A crowbar protection is:

  • Easy and cheap to construct
  • Prevent serious damage to sensitive and expensive electronic equipment
  • Has a low holding voltage, thus allows high fault currents to flow without dissipating much heat
  • It draws attention to the equipment or fault condition when it deactivates the protective devices by tripping the circuit breaker or blowing the fuse

Read More:

How to provide reverse voltage protection?  
How Snubber Circuit protects SCR?  
Fuse Terminology 
Power Supply related Interview Questions with Answers -Set10

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