Introduction: Crowbar Circuit

Hello guys,

A crowbar circuit is a method of protecting a circuit against high voltages (overvoltage) in the event of a power supply malfunction or power surge. This is especially useful in a device using TTL components as these are very sensitive to overvoltage. However, there are many other devices which can be damaged by overvoltage.

If the input voltage to the circuit reaches a certain threshold, a Zener diode breaks down and causes either a TRIAC or SCR to short power and ground... as if you threw a crowbar across the terminals. This forces a lot of current through the device but immediately lowers the voltage. An inline fuse will then electrically disconnect the load from the supply. In the case of an SCR, when the Zener diode breaks down, a voltage appears on the gate terminal of the SCR. If this is above the SCR's gate activation voltage, the device turns on.


1. 3A FUSE


3. LM431 IC

4. BT137S

5. RESISTOR SMD - 200, 3.5K, 2.5K, 220


The circuit diagram of a crowbar circuit is very simple and easy to build and implement making it a cost-effective and quick solution. The complete crowbar circuit diagram is shown above.

An adjustable Zener diode LM431 and a TRIAC as opposed to an SCR. The diode breaks down whenever the voltage at the reference input reaches 2.5 V. This means it can be set to pretty much any level with a simple voltage divider. R1 and R2 were chosen such that the limit voltage is just about 6 V.

That's due to the fact that TRIAC and the SCR don't trigger in the same way. The LM431's cathode current when off is about 1 uA. This means there is a very small voltage drop across R4, essentially keeping MT1 and the gate of the TRIAC at the same voltage. When the trigger voltage is reached and the Zener breaks down, current begins to flow through R4, causing a larger drop across it.

This puts the TRIAC into 3rd quadrant operation, since both MT2 and the gate are at lower potentials than MT1. Essentially, a small amount of current flows from MT1 to the gate which causes a large amount of current to flow from MT1 to MT2. If this is more than a few milliamps, the TRIAC "latches" (latch current) and stays conducting until that current is less than a quantity known as the holding current.

When the TRIAC conducts, a 3A automotive fuse will blow, protecting the circuit. There's also a handy, dandy LED to let you know if the fuse has blown or not.

Step 3: DESIGN

The above circuit is converted into a PCB. I have shared you the layout created using EAGLE CAD tool.


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