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Power supply circuit design on a PDM

Practical Harness Construction - Club Level

Discussion and questions related to the course Practical Harness Construction - Club Level

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Hey folks,

I've got a quick question about translating the power supply design portion of the course from a relay/fuse basis over to a PDM. Rather than getting into my specific example, I'll just ask about the theory presented in the course.

In the course the circuit design is broken down like this:

Main Power Relay:


5A Oil Metering Pump

Enable Power Relay:

15A Ignition Supply

10A Injector Supply

5A O2 Heater

5A Aux SOL [BCS, idle control, secondary butterfly]

Fuel Pump Relay:

20A Fuel Pump

Put simply, I'm assuming that I'd want to use a PDM circuit per fuse rather than per relay. Not the worst, particularly given that many items can fit on the more plentiful 8A circuits rather than the limited 20A circuits.

But in a more complex system with additional accessories to power, what will be the most logical way to distribute circuits? I'll focus in on the computer/electronics portion of the equation. Let's say a vehicle has:


Wideband controller

CAN expander

Dash display

Shift light module

GPS/accelerometer module

Let's also assume for the sake of this discussion that all of those items are on the same CAN bus.

In a case such as this would it make sense to put *all* devices on the same CAN bus on the same power circuit? Or should they be fully broken down by device? Or should the ECU be isolated with the other devices lumped into a secondary circuit?

I assume it's a case-by-case decision, I just want to try and understand any underlying logic that might lead to one decision versus another. Other than the financially-motivated desire to gang circuits so as to not need an additional PDM.


I put everything I can on its own PDM circuit, particularly anything that is critical, and would kill the car if it was to lose power. I'll often bundle up accessory items like loggers and dash displays onto one output, as if one of the items on the circuit fails, and kills power to the others, the car will still function.

Have a think about when you'd like the different elements to be powered too, you've got more options now with a PDM, rather than just 'Main Power' and 'Enable Power'.