Power supplies that are intended to be used with TTL logic
circuitry must guard against over-voltage, which can destroy
TTL chips very rapidly. The duration of over-voltage that can
destroy TTL chips is much too brief to trigger any
conventional fuse, so that only other semiconductor circuits
can play any useful part in protecting a circuit against the
type of failure of a stabilizer that leads to excessive
voltage. As it happens, this is the most common type of
stabilizer failure, so that the protection is necessary for
any TTL circuit of any significance. Many modern digital
circuits make extensive use of MOS devices, which are less
susceptible to damage from over-voltage, but it is unusual to
find a large digital circuit, which does not contain at least
one or more TTL devices.
In the circuit shown below, if the voltage at the output
terminals rises above 6.2V, zener conducts charging capacitor
C4. This voltage will fire the silicon-controlled rectifier
(SCR), which quickly shorts-or puts a ‘crowbar’ – across the
supply rails blowing the fuse.
The regulator IC is the familiar fixed voltage type such as
LM7805. The 7805 is a three-pin regulator which requires a
minimum voltage input of 7.5V to sustain stabilization, which
an absolute maximum input voltage of 35V. The maximum load
current is 1A and the regulation against input changes is
typically 3-7mV for a variation of input between 7.1V and 25V.
The regulation against load changes is of the order of 10mV
for a change between 5mV and 1.5A load current. The noise
voltage in the band from 10Hz to 100KHz is 40-50uV, and the
ripple rejection is around 70db. Maximum junction temperature
is 215 degrees C, and the thermal resistance from junction to
case is 5 degrees C/W. This stabilizer is used extensively for
power supplies in digital equipments.
In the circuit, the capacitors that are shown connected each
side of the IC are very important for suppressing oscillations
and must not be omitted. In particular, the 330nF capacitor at
the input must be wired across the shortest possible path at
the pins of the IC.
I chose a TIC 106A SCR because it can handle an anode current
up to 4A at 100V, but has a very sensitive gate requiring only
about 200uA to fire it. The SCR is only ‘on’ for a very short
time and thus does not need a heat sink. If you use another
type, it is not likely to display the same characteristics.
Most SCR’s with a heavy anode current rating have insensitive
gates. SCR’s with a lower anode current rating will need a
The rating of fuse depends on the power supply you are using
the circuit with. Assuming a maximum power supply output
current of about 200 to 300 mA, you can fit a 500mA fuse.
The voltage at which the SCR switches on is largely dependent
on the value of the zener. You can experiment with different
values of zeners to protect other types of circuits. The 4000
series CMOS ICs have supply voltage limits of 15V (AE suffix)
or 20V (BE suffix) and crowbar protection can be used for
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