Crowbar Circuit Calculator

A crowbar circuit is an overvoltage protection mechanism that short-circuits a power supply the moment voltage exceeds a safe threshold, protecting downstream components from damage. Enter your supply voltage, trigger voltage, and maximum current, then select your zener diode voltage, SCR gate current, and circuit type to get the required gate resistor value. Secondary outputs include zener current, current limiting resistor, zener power dissipation, and a recommended fuse rating.

V

Normal operating voltage of the power supply

V

Overvoltage threshold that triggers the crowbar

A

Maximum current the circuit needs to handle

Zener diode breakdown voltage

mA

SCR gate trigger current requirement

Results

Gate Resistor

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Zener Current

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Current Limiting Resistor

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Zener Power Dissipation

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Recommended Fuse Rating

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Frequently Asked Questions

What is a crowbar circuit and how does it work?

A crowbar circuit is an overvoltage protection device that shorts the power supply when voltage exceeds a predetermined threshold. It uses an SCR (Silicon Controlled Rectifier) triggered by a zener diode to create a short circuit, blowing a fuse and protecting downstream components.

What's the difference between basic SCR and TL431-based crowbar circuits?

Basic SCR crowbar circuits use a zener diode for voltage reference and are simple but less precise. TL431-based circuits offer better accuracy and stability with adjustable trigger voltages, making them suitable for precision applications.

How do I select the right zener diode voltage?

Choose a zener voltage that's about 20-30% higher than your normal supply voltage but lower than the maximum safe voltage for your circuit. For a 12V supply, a 15V zener provides good protection margin.

Why is a fuse necessary in crowbar circuits?

The fuse is essential because the crowbar circuit creates a deliberate short circuit to protect against overvoltage. Without a fuse, the short circuit would continue indefinitely, potentially damaging the power supply or creating a fire hazard.

What happens if the SCR gate current is too low?

If the gate current is insufficient, the SCR may not trigger reliably or at all, leaving your circuit unprotected during overvoltage conditions. Always ensure adequate gate current based on your SCR specifications.

Can I use a triac instead of an SCR in crowbar circuits?

Yes, triacs can be used in crowbar circuits and work with both AC and DC supplies. However, SCRs are more commonly used for DC crowbar circuits due to their simpler triggering characteristics and lower cost.

How do I calculate the current limiting resistor value?

The current limiting resistor prevents excessive current through the zener diode. Calculate it as R = (Trigger Voltage - Zener Voltage) / Desired Zener Current. Typical zener currents are 5-20mA for reliable operation.