I am making an engine stop system for boats that should be able to replace both normally-open and normally-closed systems.
The finished system will consist of both switches so it will be up to the user to choose when mounting.
The system should be able to switch 12V@15A
For the normally-open part I figured I could just use a n-channel logic level MOSFET. But for the normally-closed I got kinda stuck. To use a p-channel I would need a negative voltage to switch it, if I have understood it correctly.
So, anyone got some tips for how this could be solved?
EDIT: Should be switchable by no more than 3V.
nope, to switch on a p-channel, the gate voltage must be Xvolts below the source voltage , X depends on the mosfet .
3V is a low value, you’ll have to search for a mosfet that is fully ON (meaning the lowest RDS value) at Vgs = 3V , not easy to find I’m afraid.
Even logic level mosfet will allow you to “full” switch-on with a Vgs of 5V. (-5V for a p-channel) example : http://www.onsemi.com/PowerSolutions/product.do?id=NTD25P03L
look at the electrical characteristics, and at the graphs, they give very useful information.
BTW, I supposed that by “normally” open or “normally” closed, you mean that the Gate is at Ov ?