Powering C125 CAN Modules, GPS, etc

[Slagathor]

I understand sensors should always be grounded/referenced to the Sensor 0V (C125 Pin 9), and not the battery/chassis ground, because the 5V/8V sensor power (C125 Pins 8,7) is regulated relative to the Sensor 0V.

But what about CAN "modules" and other powered devices such as RS232 GPS?

For example, the Motec L10 GPS wiring diagram says that it should be powered by the 5V sensor lead, but grounded to the battery ground. That seems like you are mixing circuit loops. Can the battery and sensor grounds develop a voltage difference? If so, then using the sensor 5V and battery ground is mixing circuit loops, which seems like bad practice.

A similar question arises with CAN modules such as tire temperature monitoring systems. The Izze system required 5V or 8V power. If you use Sensor 5V or 8V should you then use Sensor Ground? It is not really a sensor circuit... It is an independent device that lives on the CAN network. My first instinct was to add a voltage regulator off the battery lead, and power the CAN module with that, and reference to battery ground, rather than using a sensor voltage and ground....

[David Ferguson]

The suggestion for using 0V, is for sensors that provide "ratiometric" output -- ie, their output could vary relative to the input voltage -- these are connected to the Analog Voltage inputs of your dash.. In the case of GPS module and IZZE CAN modules, the data provided is not relative to that 5V supply, so ground is fine.

However, your bigger problem is that the 5V supply on the color dashes is only 250mA, yes the GPS only draws 40mA, but you should check the specs on the IZZE products. You may decide to just have a separate power supply for external devices such as those. There are inexpensive power supplies designed to supply USB 500mA to 1A or more, and those have worked well for some of my customers. At least one customer reporting better results with his GPS-L10

[Stephen Dean]

I have the IZZE sensors in my car as well, I have a similar setup to what Dave suggested to power them, in that I got a wired in USB 2 dash socket, took the USB connector off of it and used it as a regulated 5V supply. The Dash 5V supplies are marginal for the IZZE units.