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Power supply electronics overview

AC rectifier circuits

Voltage regulator circuits

Zener voltage regulator circuit

7805, 7812, and 78** series regulators

Specifications (specs)

Over-voltage Protection Circuits

- simple thyristor or SCR circuits to provide over-voltage protection for use in a power supply.

Although modern power supplies are now very reliable, there is always a small but real chance that they can fail. Although they can fail in many ways, one particularly worrying possibility is that the series regulator element, i.e. transistor or FET may fail in such a way that it goes short circuit. If this happens a very large voltage (overvoltage) could appear on the circuitry that is being powered causing catastrophic damage to the whole equipment. By adding a little extra protection circuitry in the form of over-voltage protection, it is possible to protect against this unlikely but possibly worrying possibility.

Overvoltage protection basics

In most power supplies there is a series regulating element, such as a transistor or a FET. On one side there is normally a relatively high voltage, and on the other is the final output voltage.

In order to ensure that the required voltage on the output of the series regulator, control circuitry monitors the output voltage and applies a suitable signal to the series regulator device so that the required voltage is obtained.

This scenario is fine while the series regulator device is operating normally, but if it fails the consequences may be disastrous. There are two main ways in which a series device may fail:

1. Open circuit



2. Short circuit

If the series device fails such that it becomes an open circuit, then there is not a major problem. There will simply be no output voltage. While this is a fault condition, no further damage is caused.

The problem occurs if the series device fails so that it becomes a short circuit. If this occurs, then the full voltage appearing from the previous stages will appear at the output. In the case of a linear regulator, the voltage may be double the required output voltage. For example a 5 V line for microprocessors and logic may turn have 10 Volts of easily more. This is more than sufficient to destroy the expensive ICs that are connected to the line. It would effectively write-off the equipment as repair of the circuit boards would almost certainly not be economical.

In order to protect against this occurrence, a circuit is required that very quickly detects that the voltage is rising too high and then shuts down the power supply while not allowing the voltage to rise. While this may seem to be a tall order a simple overvoltage protection circuit can be made relatively easily based around a thyristor circuit./p>

Circuit

Many good power supplies will incorporate some form of over-voltage protection, but for those that do not it is possible to add a simple external overvoltage protection circuit. It can comprise of just four components: a thyristor, a zener diode, a resistor and a capacitor.

Thyristor crowbar circuit

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.

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.

However it is necessary to ensure that the power supply has some form of current limiting. Often a fuse is ideal because the SCR will be able to clamp the voltage for long enough for it to blow.

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 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.

Limitations

Although this power supply overvoltage protection circuit is widely used, it does have some limitations. Most of these are associated with the zener diode. The zener diode is not adjustable, and these diodes come with at best a 5% tolerance. In addition to this 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. 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.

It is also necessary to ensure that there is some means of limiting the current should the overvoltage crowbar circuit fire. If not then further damage may be caused to the power supply itself. Often a fuse may be employed in the circuit. In some circuits a fuse is introduced prior to the series regulator transistor, and the SCR anode connected to the junction node where the output of the fuse is connected to the input of the series regulator. This ensures that the fuse will blow swiftly.

Despite its drawbacks this is still a very useful circuit which can be used in a variety of areas.