Power supplies are one of the most important components in electronic systems. They are normally reliable, but if they fail then they can cause significant damage to the circuitry they supply. Luckily, the thyristor or SCR can offer a very easy but effective method of providing a crowbar circuit to protect against this eventuality.

This share would explain why is important to prevent power supplies failure, how does the thyristor or SCR overvoltage crowbar circuit protect the power supplies, and the limitations of crowbar circuit. If you have been or are being suffered from power failure, you can better learn how to solve the problems with a crowbar circuit. Let's keep reading!

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Content

● Why is Important to Protect Analogue Power Supply?

● How Does a Thyristor / SCR Overvoltage Crowbar Circuit Protects Power Supplies?

● What are the Limitations of a Crowbar Circuit?

● FAQ

● Conclusion

Why is Important to Protect Analogue Power Supply?

One failure mode is for many analogue regulated supplies is that the series pass transistor can fail with a short circuit appearing between the collector and emitter. If this happens the full unregulated voltage can appear at the output, and this would place an intolerably high voltage on the whole system causing many ICs and other components to fail.

By looking at the voltages involved it is very easy to see why the inclusion of overvoltage protection is so important. A typical analog power supply may provide 5 volts stabilised to logic circuitry.

To provide sufficient input voltage to give adequate stabilisation, ripple rejection and the like, the input to the power supply regulator may be in the region of 10 to 15 volts. Even 10 volts would be sufficient to destroy many chips used today, particularly the more expensive and complicated ones. Accordingly preventing this is of great importance.

The Introduction to Power Supplies in Analog Circuits

How Does a Thyristor / SCR Overvoltage Crowbar Circuit Protects Power Supplies?

The thyristor crowbar circuit shown is very simple, only using a few components. It can be used within many power supplies, and could even be retro-fitted in situations where no over-voltage protection for power supplies.

It uses just four components: a silicon controlled rectifier or SCR, a zener diode, a resistor and a capacitor. And they work together like this:

STEP#1

The SCR over voltage crowbar or protection circuit is connected between the output of the power supply and ground. The Zener diode voltage is chosen to be slightly above that of the output rail. Typically a 5 volt rail may run with a 6.2 volt Zener diode. When the Zener diode voltage is reached, current will flow through the Zener and trigger the silicon controlled rectifier or thyristor. This will then provide a short circuit to ground, thereby protecting the circuitry that is being supplied form any damage and also blowing the fuse that will then remove the voltage from the series regulator.

STEP#2

As a silicon controlled rectifier, SCR, or thyristor is able to carry a relatively high current - even quite average devices can conduct five amps and short current peaks of may be 50 and more amps, cheap devices can provide a very good level of protection for small cost. Also voltage across the SCR will be low, typically only a volt when it has fired and as a result the heat sinking is not a problem.

STEP#3

The small resistor, often around 100 ohms from the gate of the thyristor or SCR to ground is required so that the Zener can supply a reasonable current when it turns on. It also clamps the gate voltage at ground potential until the Zener turns on. The capacitor C1 is present to ensure that short spikes to not trigger the circuit. Some optimisation may be required in choosing the correct value although 0.1 microfarads is a good starting point.

STEP#4

If the power supply is to be used with radio transmitters, the filtering on the input to the gate may need to be a little more sophisticated, otherwise RF from the transmitter may get onto the gate and cause false triggering. The capacitor C1 will need to be present, but a small amount of inductance may also help. A ferrite bead may even be sufficient. Experimentation to ensure that the time delay for the thyristor to trigger is not too long against removing the RF. Filtering on the power line to / from the transmitter can also help.

Thyristor overvoltage protection circuit

What are the Limitations of a Crowbar Circuit?

Although this power supply overvoltage protection circuit is widely used, it does have some limitations.

● Crowbar Firing Voltage

A zener diode is used for firing thyristor crowbar circuit. It is necessary to choose a Zener diode with the right voltage. Zener diodes are not adjustable, and they come with at best a 5% tolerance. The firing voltage must be sufficiently far above the nominal power supply output voltage to ensure that any spikes that may appear on the line do not fire the circuit.

● Susceptibility to RF

If the power supply is to be used to power a transmitter filtering on the line to / from the transmitter is required along with some careful design of the spike protection on the gate.

● Circuit Threshold

When taking into account all the tolerances and margins the guaranteed voltage at which the circuit may fire can be 20 - 40% above the nominal dependent upon the voltage of the power supply. The lower the voltage the greater the margins needed. Often on a 5 volt supply there can be difficulty designing it so that the overvoltage crowbar fires below 7 volts where damage may be caused to circuits being protected.

Frequently Asked Questions

1. Q: What is a Crowbar Protection in SCR?

A: Crowbar protection is a fail-safe protection mechanism which shorts circuits the output of a power supply under failure conditions such as overvoltage. Crowbar protection can also refer to a circuit which has its sole purpose to cause a fuse to blow by subjecting it to high current.

2. Q: How does Crowbar Protection Circuit Over Voltage Protection Work?

A: A Crowbar circuit for overvoltage protection monitors the input voltage and when it exceeds the limit it creates a short circuit across the power lines and blows up the fuse. Once the fuse is blown the power supply will be disconnected from the load and thus preventing it from high voltage.

3. Q: What is the Purpose of a Crowbar Circuit?

A: A crowbar circuit is an electrical circuit used for preventing an overvoltage or surge condition of a power supply unit from damaging the circuits attached to the power supply.

4. Q: How does an SCR Crowbar Circuit Protect Sensitive and Delicate Loads?

A: It protects the load by shorting the output terminals of the power supply when a overvoltage is detected. When the output terminals of the power supply are shorted, the huge current flow will blow the fuse and thus disconnecting the power supply from the rest of the circuit.

Conclusion

In this share, we learn why is important to protect the power supplies, how the thyristor / SCR protects the power supplies from failure and the limitations of the crowbar circuit. This simple thyristor crowbar circuit can be very effective and protect an expensive item of equipment from the possible failure of the series regulator element. What do you think of the thyristor / SCR crowbar circuit? Leave your comments below and we'll reply you as soon as possibile. If you think this share is helpful for you, please feel free to share it!

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