I'm trying to make a remote start circuit for a motorcycle. I have wiring diagrams, it seems as though there are two connections to close to activate ignition, and one connection to close for the starter motor switch to actually start the engine.
I've opted to use N-channel MOSFETs as switches (60v 30A). My question is, can I use these MOSFETs if the load is on either side (or both sides) of the MOSFET?
The load needs to be on the drain or you'll blow the MOSFET (ie use the first circuit) - you want the on-voltage across the MOSFET to be as small as possible and the source-follower circuit (second example) cannot do this and will put loads of power straight into the MOSFET die and likely melt it.
The third example makes no sense - why do you have two loads in series?
If you want the load switched on the high-side then you need a p-channel MOSFET and a level-shifting circuit to drive its gate.
If using an n-channel as in circuit 1 you will want a logic-level MOSFET (or you'll need a level-shifting circuit too).
When selecting a MOSFET ignore the current rating, you need to get the Rds(on) resistance and calculate the power dissipation (I-squared-R). Try and keep the power dissipation low (or at least select an appropriate heatsink). All the current rating is is the maximum-power-with-infinite-heatsink rating - normally you don't go anywhere near this current since you'll be losing volts across the device.
If you have to switch high-side then a p-channel MOSFET and level-shifter (or MOSFET driver) is going to be required. Switching low-side is prefered (although a lot of things on a vehicle return to ground via the chassis of course...)
A slightly dodgy approach is possible too: Have the Arduino +5V connect to battery positive and its GND powered via a -5V regulator from battery negative/ground. Then logic-level p-channel MOSFETs can be driven direct. The problem with this setup is that any contact from the vehicle ground to the Arduino and its circuitry will appear as -7V relative to Arduino ground and would fry it.
Relays are a better choice for this application. Connect the relay contacts across the switches you are trying to bypass and it is done.
If you really want to use FETs: If you are connecting battery voltage to a circuit (IGN and Starter Relay), it looks like figure 2. If you are connecting ground to a circuit (top link in ignition switch), it looks like figure 1 (Normal).
I think the middle arrow is point to the wrong switch. If I am reading it correctly, the top link in the ignition switch box connects ground to the Ignition Control Unit which you need. The middle link powers the lights, which you don't need until you start riding.