Safe 12V car adapter described here can be used to limit a +12 volt car battery current, available from the in-dash cigar lighter power port, to below 2.6 Amperes for use with portable electronic gadgets and travel chargers on long car journeys. This circuit will protect the car electric system against possible short circuits across the cigar power port.
The 12 volts car adapter circuit is connected to the car +12V electric system via the cigar lighter plug J1. The +12V arrives on the board via a reverse voltage protection diode D1. Capacitor C1 decouples the input to the circuit. The current limited 12V output can be taken from the cigar lighter socket J2.Red LED (D2) is a simple power status indicator. After construction,boxed up the unit using a suitable ABS enclosure.
Working of this electronic fuse is very simple. Usually,the mosfet switch T1 is driven via resitor R2 and the 12V from the car battery is available at the output jack J2. The current flow through sense resistor R1 produces a voltage drop,which is at a certain level will force transistor T2 to switch on. This in turn switches off T1 somewhat and the output supply current to the connected electrical load is reduced to prevent costly disasters.
Note: Use a good quality heatsink for T1. Only for cars with negative ground only!