Fuse protection for electronics circuit seems to be an outmoded idea, for there are circuits (in computers etc) that consume high currents initially at lower stable value. In case high rated fuse are employed in such equipment to accommodate the initial surge current, it defeats the very purpose of protection. Slow-blow fuses often turn out to be too slow while quick-action fuse may blow out at the mere hint of a surge.
An electronics fuse circuit of the kind described here provides an ideal solution in such cases. Electronic fuse circuit offers another advantage as replacing of a fuse in this case is done at the mere pressing of a button.
The circuit has a ‘slow-blow’ feature. When a gadget draws more than the prescribed current, the circuit monitors it for some time, and if the situation persists it ‘blow up’ the fuse.
The circuit of electronic fuse uses a JFET type TL081 (IC1) op-amp. A small reference voltage is set up across the IC’s inverting input, and in case the simple voltage fed across resistor R1 (in series with the load) exceeds it, the output is taken high and the relay energized to disconnect the load.
In case it is desired to latch up the relay, atwin-contact relay may be used. The other set of contacts in that case can be wired in parallel to the emitter /collector of transistor T1.
The assembled circuit of electronic fuse can be mounted on the upper stack of the transformer, whereas the relay can be placed at the point where the switch would have been.
Resistors (all ¼-watt, ± 5% Carbon unless stated otherwise)
R1 = 0.5 Ω, 10W (Wirewound)
R2 = 100 Ω
R3 = 10 KΩ
R4 = 1 KΩ
VR1 = 22 KΩ
C1 = 220 µF/40V (electrolytic capacitor)
IC1 = TL081
D1, D2 = 1N4007
ZD1 = 12V, 2W Zener diode
T1 = BC547
RL1 = 12V, 100 mA Relay
X1 = 230V AC Primary To-0-15V, 200mA SEC. Transformer
SW1 = Push-To-Off Switch