Replace normally-open and normally-closed switch 15A

Hi.

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.

met_fredrik:
Hi.

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 ?

Thank you for your answer.

Yes, I would like to make a circuit that from the microcontrollers view looks the same if your choose normally open or normally closed. So that 0v from the microcontroller would turn the first switch on and the second off. And then at 3v it would turn the first off and the second on.

This looks like a bit of a challenge.

met_fredrik: Yes, I would like to make a circuit that from the microcontrollers view looks the same if your choose normally open or normally closed. So that 0v from the microcontroller would turn the first switch on and the second off. And then at 3v it would turn the first off and the second on.

The standard approach would be to use two microcontroller pins, one to drive the normally-on-mosfet and one to drive the normally-off mosfet. If you really want to do it with just one pin, you can use a logic inverter (e.g. one of the 6 inverters in a 74HC04) to generate the negated single.

What will the output "switches" be connected to? Normally I would recommend a relay for this kind of application, so that you have isolation between the switched circuit and the microcontroller.

If it's an engine STOP system you are looking at, would I be correct in assuming it either operates a fuel solenoid or some form of "engine management" device.

If so you need it "bomb proof" capable of being installed by idiots who invariably endeavour to wire things up wrong.

That being the case surely a simple relay is all that's required - or am I just too long in the tooth to appreciate that simple devices can be replaced by much more complicated ones, at greater expense and more liable to failure due to user abuse.