I was using a thermoelectric cooler for a camping trip and discovered the next morning that my car battery was dead from the load. I never expected a 3.5-A load to drain the battery in eight hours. So I went about making a circuit that could be contained in a standard automotive plug (MPD APP-001). The plug houses a fuse and an LED that's also used in the circuit shown in the figure.
The main problem was getting enough drive on the MOSFET to achieve the lowest RDS(ON). High-side drive did not seem practical, so I inverted the logic and went with a pnp switching transistor (Q1). Q1 is driven with zener diode D2 that sets the On/Off voltages. R1 limits the current through the transistor. The existing parts (R2 and D1) form an ON indicator, showing when the load has current. Q2 is the output switch. By supplying at least 12-V dc to the gate, the IRF540 is fully driven and should reach its RDS(ON) of 44 mΩ.
At 7 A of load, the FET would have about 2 W of heat generation. By setting the value of D2, the operating point may be tailored for desired operation. A 12-V zener will permit operation for some time after the engine is stopped. But if you forget to unplug the load, turn it off to save the main battery.
