While many power
supplies can be set to limit their output current to a defined level, to
protect the circuit they are powering, no such protection is available
if you are powering a circuit from a battery. If a fault develops, the
circuit can blow before you have a chance to disconnect it. Of course,
you can fit a fuse in series with the supply line to the circuit under
test but it will blow if a fault develops. Or perhaps it won’t blow
sufficiently quickly to protect the circuit. And repeatedly having to
replace fuses becomes a nuisance as well.
The alternative is to use an electronic fuse. This circuit uses a
relay to make and break the circuit. The current drain of the circuit
under test is monitored by a 1O 2W resistor which is placed in series
with the supply line. The voltage across this 1O resistor is monitored
by op amp IC1a which has an adjustable gain of between 11 and 16, as set
by trimpot VR1. The resultant DC voltage from pin 1 of IC1a is fed to
pin 5 of IC1b which is configured as a comparator. Trimpot VR2 provides
an adjustable voltage reference to pin 6 of IC1b and this is compared
with the amplified signal from IC1a.
If IC1b’s threshold is exceeded, its pin 7 goes high and this is fed
to Schmitt trigger inverter IC2a which then “sets” the RS flipflop
comprising gates IC2c & IC2d. Pin 11 of IC2d then goes high to turn
on transistor Q2 and LED1 while pin 4 of IC2b also goes high to turn on
Q1 and the relay which then disconnects the load. The circuit stays in
this state until the RS flipflop is reset by pushing switch S1.
Capacitor Cx, across the feedback resistance of IC1a, is used to
simulate a slow-blow or fast-blow fuse and can be selected by trial and
error. Changing the gain of IC1a or the value of the sensing resistor
changes the fuse rating of the circuit.