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Thyristor Crowbar: overvoltage protection circuit
yristor or SCR can deliver an easy means of providing over-voltage
tion for power supplies using a crowbar circuit.
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r Circuit Design Includes:
circuit design primer
Circuit operation
Triggering / firing circuit design
Overvoltage crowbar
Triac
pplies are normally reliable, but if they fail then they can cause significant damage to the circuitry they
though unlikely, it can and does happen.
fear is that the power supply circuit could fail in such a way that a much greater voltage could be placed
supply rail for the electronics circuitry within the partcular item. This much larger than normal voltage could
ant destruction to many of the devices within the circuit.
Anode
Gate
Cathode
31 MARCH 2023
Thyristor or SCR circuit symbol
Fact of the day: It was on 31
Robert Bunsen was born. He w
stor or SCR can offer a very easy but effective method of providing a crowbar circuit to protect against this
y, and providing additional security and peace of mind for anyone using the equipment.
ogue power supply failure modes
chemist who invented the Buns
in many chemical laboratories.
Quote: Strength does not come
capacity. It comes from an in
Mahatma Gandhi
re mode is for many analogue regulated supplies is that the series pass transistor can fail with a short circuit
g between the collector and emitter.
The way things are: The rem
reach a goal increases as
ppens the full unregulated voltage can appear at the output, and as many analogue series regulators require
approaches
voltage of at least 1.5 to two times the output voltage.
ppened it would place an intolerably high voltage on the whole system causing many ICs and other
nts to fail.
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g at the voltages involved it is very easy to see why the inclusion of overvoltage protection is so important. A
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pply may provide 5 volts stabilised to logic circuitry. To provide sufficient input voltage to give adequate
online
on, ripple rejection and the like, the input to the power supply regulator may be in the region of 10 to 15
providers that will give you an
training
courses
fro
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volts would be sufficient to destroy many chips used today, particularly the more expensive and complicated
cordingly preventing this is of great importance.
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stor / SCR overvoltage crowbar circuit
stor crowbar circuit shown is very simple, only using a few components. It can be used within many power
and could even be retro-fitted in situations where no over-voltage protection may be incorporated.
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st four components: a silicon controlled rectifier or SCR, a zener diode, a resistor and a capacitor.
TR1
ZD1
SCR1
Regulator
control
R1
C1
Protected
output
Smoothed
DC input
Fuse
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Thyristor overvoltage protection circuit
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over voltage crowbar or protection circuit is connected between the output of the power supply and ground.
r diode voltage is chosen to be slightly above that of the output rail.
Electronics Notes offers a host o
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a 5 volt rail may run with a 6.2 volt Zener diode. When the Zener diode voltage is reached, current will flow
association with Amazon). Ch
he Zener and trigger the silicon controlled rectifier or thyristor.
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hen provide a short circuit to ground, thereby protecting the circuitry that is being supplied form any damage
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blowing the fuse that will then remove the voltage from the series regulator.
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on controlled rectifier, SCR, or thyristor is able to carry a relatively high current - even quite average devices
uct five amps and short current peaks of may be 50 and more amps, cheap devices can provide a very good
rotection for small cost.
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age across the SCR will be low, typically only a volt when it has fired and as a result the heat sinking is not a
EVENTS
l resistor, often around 100 ohms from the gate of the thyristor or SCR to ground is required so that the
n supply a reasonable current when it turns on.
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tor also clamps the gate voltage at ground potential until the Zener turns on. The capacitor C1 is present to
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at short spikes to not trigger the circuit.
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More events
imisation may be required in choosing the correct value although 0.1 microfarads is a good starting point.
wer supply is to be used with radio transmitters, the filtering on the input to the gate may need to be a little
phisticated, otherwise RF from the transmitter may get onto the gate and cause false triggering. The
C1 will need to be present, but a small amount of inductance may also help. A ferrite bead may even be
ntation to ensure that the time delay for the thyristor to trigger is not too long against removing the RF.
on the power line to / from the transmitter can also help.
wbar circuit limitations
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this power supply overvoltage protection circuit is widely used, it does have some limitations.
bar firing voltage:
The firing voltage of the tthyristor crowbar circuit is set by the Zener diode. It is
sary to chose a Zener diode with the right voltage . Zener diodes are not adjustable, and they come with at
5% tolerance. The firing voltage must be sufficiently far above the nominal power supply output voltage to
e that any spikes that may appear on the line do not fire the circuit.
eptibility to RF:
If the power supply is to be used to power a transmitter filtering on the line to / from the
mitter is required along with some careful design of the spike protection on the gate.
it threshold : When taking into account all the tolerances and margins the guaranteed voltage at which the
may fire can be 20 - 40% above the nominal dependent upon the voltage of the power supply. The lower
ltage the greater the margins needed. Often on a 5 volt supply there can be difficulty designing it so that the
PXI & PXIe Timeline: How PXI s
oltage crowbar fires below 7 volts where damage may be caused to circuits being protected.
ple thyristor crowbar circuit can be very effective. It is simple even if a little crude, but can protect an
e item of equipment from the possible failure of the series regulator element.
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