Overvoltage crowbar circuit
an over voltage crowbar protection circuit using a silicon
controlled rectifier or SCR
Power supplies are normally reliable, but if they fail then
they can cause significant damage to the circuitry they supply on some
occasions. The SCR overvoltage crowbar protection circuit described provides a
very simple but effective method of protecting against the certain types of
power supply failure.
In most analogue power supply arrangements a control voltage
is fed into a series regulating device such as a transistor. This controls the
current and hence the output voltage. Typically the input voltage to this may be
well in excess of the output voltage. If the series regulator transistor in the
power supply fails and goes short circuit, then the full input voltage will
appear on the circuitry that is being supplied and significant damage may
result. To overcome this SCR over voltage crowbar circuits are widely used.
These overvoltage protection circuits are easy to design, simple to construct
and may prevent significant levels of damage in the unlikely event of a power
supply failure.
By looking at the voltages involved it is very easy to see
why the inclusion of overvoltage protection is so important. A typical 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.
Circuit
Most good bench power supplies include a form of overvoltage protection, but for
those power supplies or for other applications where over voltage protection is
required, a simple over voltage crowbar circuit can be built. It uses just four
components: a silicon controlled rectifier or SCR, a zener diode, a resistor and
a capacitor.
SCR overvoltage 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 recifier, 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.
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