Crowbar Circuit

What is a Crowbar Circuit?

This description is for my benefit, but others may also find it useful. A crowbar circuit prevents damage to the circuit in the event of an overvoltage of the power supply. It protects the load by shorting the output terminals of the power supply when an overvoltage is detected. When the output terminals of the power supply are shorted, the huge current flow will blow the fuse, thus disconnecting the power supply from the rest of the circuit. The job of the crowbar is to detect the overvoltage and blow the fuse or in some cases a circuit breaker is tripped, as in the Thorn 3000 power supply (see diagram at foot of page).

Crowbar circuits are designed using a Thyristor Silicon Controlled Rectifier (SCR).

Simple Crowbar Circuit using a Thyristor

The following image shows a Thyristor used as a crowbar. All the components required to build this circuit are below.

• Thyristor (Q1)
• Zener Diode (ZD1)
• Schottky Diode (SD1)
• Filter Capacitor (C1)
• Snubber Capacitor (C2)
• Pull-down Resistor (R1)
• Fuse (F1)

Using simple values, the output of the power supply is 8V. The threshold voltage of the Zener Diode is chosen to be 9.1V. All the other components and their values are shown in the following image.

How The Crowbar Functions

The Zener Diode is the component that detects the over voltage. Usually, the threshold voltage of the Zener Diode is selected just over the output voltage of the power supply 1V more than the output voltage, in this example ZD1 value is 9.1V

When an overvoltage occurs and if the voltage reaches the threshold voltage of the Zener Diode, it then starts conducting. If the voltage still increases, the voltage drop across the resistor R1 and the Gate terminal of the Thyristor (Q1) will increase.

Initially, when the Zener Diode in not conducting, the resistor R1 acts as a pull-down resistor for the gate terminal of the Thyristor to keep it low. But when the Zener Diode starts conducting and the voltage across the resistor R1 increases, the gate voltage also increases.

When the voltage at the gate terminal is more than its threshold voltage (usually between 0.6V and 1V), the Thyristor starts to conduct and essentially provides a short circuit between the power rails. As a result of this short circuit, the fuse blows.

An important point to remember here is that the current rating of the Thyristor should be more than that of the Fuse. Also, the overall trigger voltage is the sum of the Zener Diode threshold voltage and the Thyristor threshold voltage.

There are few other components in the circuit, the capacitor C1 is a filter capacitor used to reduce noise and small voltage spikes, which avoids unnecessary triggering of the circuit.

Capacitor C2 is a Snubber capacitor, it prevents accidental triggering of the Thyristor during power up of the circuit. Finally, the Schottky Diode acts as a reverse protection diode to prevent the main circuit from triggering the Crowbar Circuit.

Real life implementation of a crowbar in the Thorn 3000 power supply