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A little background: I have a 12 VDC "cigarette" socket in my car which is a non-switched circuit (so, always on). I'm in need of a switched socket, whereby power is cut when the car is not running. Rewiring the socket to an existing switched power source in the car is not really an option.

So I'm considering introducing an N-channel MOSFET (like this), where the gate would be driven by a USB line's +5v. There are two standard USB 2.0 ports right next to the 12VDC socket, which are both switched, so the idea is the USB ports go live when the car is started, which will drive the MOSFET to complete the 12VDC circuit. Car is shut off, and the MOSFET switches off. The peripheral hooked into 12VDC socket would only draw about 2A.

USB +5V -> Gate, with pulldown resistor to GND Socket GND lead -> Drain Batt GND -> Source

So,

1) Does this plan sound reasonable?

2) As I understand it, with the V(GS) at 0V (the car is off) the Drain-Source connection is essentially an open circuit. Is this accurate, or would the car battery still see a small (even negligible drain) if the peripheral is left connected while the car is off?

3) Any concerns, or better ways to achieve my result? Would a relay be more appropriate than a MOSFET?

Any advice is greatly appreciated.

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For this situation (not knowing what the load will be) it's better to use a P Channel FET as a "high side" power switch.

The PFET should be pulled up to 12V to turn it off, with a resistor, and the 5VDC USB signal can be used with a simple NPN transistor to pull the PFET gate to 0V and thus turn it on.

schematic

simulate this circuit – Schematic created using CircuitLab

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I'd use a relay where the ignition key either ON or in the ACCESSORIES position would energize the relay, and its normally open contacts would be wired in series with the line going from the lighter socket's fuse to the socket.

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