Solid State Relay Wiring Details


After receiving feedback from Jean on the MSExtra forum in this thread (I’m jb007), it appears I had made an assumption about the need for +5V on the input of the SSR. You don’t, it can be connected to +12V the same as the positive output terminal is.

Secondly, I’ve connected the fused +12v from the main relay as the fuel pump PWM output will only work with the FP pin, full stop. I believe like Jean does that this posses a safety risk as it overrides the original intent of this FP pin, namely to control a (fuel pump) relay that will shut off the injectors and ignition coils if there is a fault or the engine stops.

I’m considering abandoning the PWM of the fuel pump until this problem is hopefully resolved by the software guys James or Matt. If I do go back to a normal return system, I’ll still monitor the pressure, as I previously suspected I was loosing pressure, and it also was fluctuating. Time will tell. 

I’ve had some correspondence from members on the MegaSquirt MSExtra forum concerning how to drive the SSR’s from the MegaSquirt MS3 or MS3X output pins.

Anyway after I did some bench testing with a motor, bench power supply and my MS3 connected to a JimStim, I discovered that:

  • the output polarity of the FP pin can be changed (inverted) and the drive to the input of the SSR can be set to a high side input e.g. FP to the positive pin and negative to the earth.
  • when it comes to using one of the generic PWM outputs, things go tits up because there is no way (at the moment) to invert the output polarity. The result is if you use a high side input (see above for description) then the output is turned on when you want it turned off.

Bottom line is I will be using a low side input for both my PWM fuel pump (hopefully it will work!) and using the generic PWM output A.

My fuel pump has its negative terminal connected internally inside the tank to earth, so I can’t connect the SSR as a high side output. So I will connect as per the first diagram.

I will be using a FR607 diode to snub the back EMF (inductive spike when voltage is removed from it’s inductive windings).

I guess that’s all I can write about, next is to put it into practice.

Driving SSR on the low side (this is how I do it)
Driving SSR on the low side (this is how I do it)
Driving on the high side.


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