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Topic: Having trouble with MOSFET (Read 874 times) previous topic - next topic

ticklechicken

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.

majenko

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.

LarryD

Do you have 0 volts on the Arduino going to 0 volts on you controlled circuit?
The way you have it in your schematic isn't the same as how you have it wired up!

Chagrin

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.

MarkT

You will need a flyback diode across the relay winding to prevent inductive spikes damaging the MOSFET.
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ticklechicken

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.

majenko

Quote

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?

ticklechicken


Quote

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.

smeezekitty



Quote

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.
Avoid throwing electronics out as you or someone else might need them for parts or use.
Solid state rectifiers are the only REAL rectifiers.
Resistors for LEDS!

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