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How to Make an Electronic Fuse Circuit
Most modern voltage regulators have an integral current limiter. These are, unfortunately, normally provided with a thermal `fuse', which in the first instance does not control the current but the dissipation. Since this type of fuse is very slow, in many applications a standard glass fuse is added to protect a number of components against over-current.
Also, many small power supplies and mains adaptors have no short-circuit protection. During the design and testing of new equipment, this may have catastrophic consequences. In all these cases, the fast electronic fuse described here may be found of great value.
The fuse is connected between the power supply output and the load. After it has ‘blown’, and the current has returned to a normal value, it can be reset with S1. Transistor T1 and resistor R1 form a 6 mA current source that, together with zener diode D1, provides a constant volt age of 5.6 V with respect to the +ve supply line. This gives the gate of T3 a sufficiently high negative voltage to keep this p-channel MOSFET conducting (and thus providing the link between power supply and load).
When the load current rises to about 1A the potential drop across T3, D2, and part of P1 reaches a level at which T2 switches on, so that the gate-source junction of T3 is short-circuited. Diode D3 then lights to indicate that the fuse has ‘blown’. In practice, the LED will already light dimly when the critical current is approached. The circuit is reset by pressing S1. With values as shown in the diagram, the maximum load current is around 300 mA. The drop across T3 is only 100 mV.
Higher load currents may be accommodated by taking a higher rated p-channel MOSFET for T3 or connecting two or more FETs Type IRF9530 o in parallel. A heatsink is not needed. Since the maximum dissipation does not exceed 40 mW. The LED should be a low-current type, as a current of only 6 mA is available.
The circuit is calibrated by pro viding a current into K1 at the level at which the fuse should operate and then adjusting P1 until the fuse ‘blows' As already stated, other types of p-channel MOSFETs may be used for T3. Note. However, that the lower the on-resistance of this transistor, the less precise the value of the load current at which the fuse will act.