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This is my first project.  I'm trying to make a remote start for my portable generator.  On the generator, I have found the two wires that control the electric starting, and they measure 12V across them.  When I connect them with my ammeter, 2.4 amps flows and the generator starts.  According to the generator's schematic, these wires control a relay and not the actual starter motor.

So with this in mind, I tried to use a IRF520 MOSFET to control the starting.  This may be the wrong MOSFET, I don't know.  I picked it up at Radio Shack to experiment with.  I don't know how to choose the correct MOSFET from the data sheets.  Maybe I need some help here?

To test whether I had it connected correctly, I tested the MOSFET with this setup on my breadboard:
ext 12V positive --- 220ohm --- diode --- drain --- source --- ground
Everything worked when I connected the gate to an Arduino output and switched between LOW and HIGH.  I'm not sure why this won't work when connected to the generator wiring.  From what I can tell, all I'm doing is controlling a relay.  Why won't this work?  I connect the two generator wires to the drain and source, but nothing happens when I actuate the gate.  I've switched the drain/source polarity to be sure, but I still get nothing.
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Firstly, do you know how the relay is wired up?

Are you controlling the circuit between the power + and the relay, or between the relay and - ?

If the former (which is most likely), then you really need a P channel MOSFET, as that is much better at high side switching.
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The last thing you did is where you should start looking.
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Do you have 0 volts on the Arduino going to 0 volts on you controlled circuit?
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Are you controlling the circuit between the power + and the relay, or between the relay and - ?

Small engines always switch between Bat + and the starter relay's coil. I think all cars do as well. The coil will be around 5 ohms.
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You will need a flyback diode across the relay winding to prevent inductive spikes damaging the MOSFET.
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Thanks for the help everyone.

Firstly, do you know how the relay is wired up?

Are you controlling the circuit between the power + and the relay, or between the relay and - ?

If the former (which is most likely), then you really need a P channel MOSFET, as that is much better at high side switching.
Here is the schematic from the generator.  I highlighted the two wires I'm tapping into for the remote start (red and red/white).  From what I can tell, I'm controlling between the power and the relay, as Chagrin stated.  I did test this with a relay and got everything to work.  I wanted to use a MOSFET due to space constraints and I thought it was a simple swap.

So I need a P channel instead of the N channel that I have?  I'll look into that.  If anyone has any good links for tutorials, I appreciate it.


Do you have 0 volts on the Arduino going to 0 volts on you controlled circuit?
Yes, they are all grounded together.  This is easy since I'm powering the Arduino off the generator battery.

You will need a flyback diode across the relay winding to prevent inductive spikes damaging the MOSFET.
I thought that was only needed if the MOSFET was controlling something with a winding like a motor.  Since I'm controlling a relay, I thought the diode wasn't needed.
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I thought that was only needed if the MOSFET was controlling something with a winding like a motor.  Since I'm controlling a relay, I thought the diode wasn't needed.
What do you think makes a relay work?  Magic Cheese?
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I thought that was only needed if the MOSFET was controlling something with a winding like a motor.  Since I'm controlling a relay, I thought the diode wasn't needed.
What do you think makes a relay work?  Magic Cheese?
Somehow in my mind, I interpreted "something with a winding" as "something with a moving armature".  I didn't count the winding on the solenoid portion of the relay.  But thinking about it now, I see it.
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I thought that was only needed if the MOSFET was controlling something with a winding like a motor.  Since I'm controlling a relay, I thought the diode wasn't needed.
What do you think makes a relay work?  Magic Cheese?
Somehow in my mind, I interpreted "something with a winding" as "something with a moving armature".  I didn't count the winding on the solenoid portion of the relay.  But thinking about it now, I see it.
LOL Usually I find relays to be much more inductive than motors.
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