PDM30 and 12v power line as input

[kristap]

This is for a new race car build and I'm looking for advice or recommendations on how I might use a 12v power wire as an input signal. I would like to power the ecu with the pdm and treat that 12v line, from a kill switch originally intended to power the ecu, as an input signal. The kill switch is a Cartek isolator.

When 12v is present, the pdm would power the car as normal. When that 12v is cut at the kill switch, the pdm would react to the loss of "signal" and activate a master shutdown function. This complies with my race class rules.

Diagram illustrates the plan I would like pursue.

In testing, the voltage drop registered is minimal (~2v) when that 12v line is opened at the kill switch. This should be fine under normal circumstances but I would like more of a buffer if feasible and efficient, no relays.

Is this a case for a pull-down resistor? If so, what size of resistor and where should it sit? Should it be connected inline between the 12v line and pdm input pin? Should it connect the 12v line to the pdm 0v pin?

[adrian]

All of the PDM input pins have 10k pull-ups to battery voltage, so you are right, you will need a pull-down resistor if you are feeding it a 12V signal.

A 2.2k resistor is about the best, it will pull the voltage down to around 2.5V (with a 13V supply) which gives you plenty of voltage swing between on and off but also doesn't draw too much current.

Wire the resistor from the input pin to 0V.

[the_bluester]

I take it you have the Cartek GT isolator? The XR (Which I plan to use on my car) has an output that is grounded when the battery is isolated which is perfect for a kill input to the PDM.

[kristap]

I take it you have the Cartek GT isolator? The XR (Which I plan to use on my car) has an output that is grounded when the battery is isolated which is perfect for a kill input to the PDM.

I do have the GT and was not aware that the newer XR offered this functionality. Figures, there is a newer and more expensive option once I've committed to a path

All of the PDM input pins have 10k pull-ups to battery voltage, so you are right, you will need a pull-down resistor if you are feeding it a 12V signal.

A 2.2k resistor is about the best, it will pull the voltage down to around 2.5V (with a 13V supply) which gives you plenty of voltage swing between on and off but also doesn't draw too much current.

Wire the resistor from the input pin to 0V.

Thank you for the simple explanation; it is appreciated. Would using chassis ground instead of 0v be a viable alternative?

[the_bluester]

I take it you have the Cartek GT isolator? The XR (Which I plan to use on my car) has an output that is grounded when the battery is isolated which is perfect for a kill input to the PDM.

I do have the GT and was not aware that the newer XR offered this functionality. Figures, there is a newer and more expensive option once I've committed to a path

I have done that a few times!

Another option if you are prepared to use a relay would be to wire a normally open relay in the circuit. Use the 12V active from the Cartek to energise the relay, with the N-O contact wired to the PDM input pin and the common to the PDM switch 0V. That way when the isolator is on, the relay will be on and the PDM input pin at 0V (Pulling the pin low as is preferred by MoTec) When you turn off the isolator (Or if the relay fails) the internal pullup in the PDM will have the pin go high and trigger the shutdown using appropriate switch logic in your PDM setup.

[adrian]

Would using chassis ground instead of 0v be a viable alternative?

It will work wiring it to chassis ground and because there is a fair voltage swing it shouldn't give you any issues.

Technically speaking you should always wire switch inputs to the 0V not chassis ground as this is the reference point for the PDM's voltage measurement. If you happen to have an electrically noisy car, or a bad earth somewhere this can cause problems if you wire it to chassis ground rather than 0V.