PCB design consideration - input power relay

I have an instance where I’d like to enable the PCB’s input power relay with my ignition’s 12v accessory. Currently, it seems most PCBs only have the option to ground out the relay coil with a switch or jumper. How about changing that to allow an option for powering the relay coil?

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You can mount a separate relay on the vehicle, operated by its ignition and use the contacts to ground the board relay.

I use a double throw switch on the PCB case, so normally the main relay is switching on with the ignition (via a relay) or press the switch and the main relay is always on regardless of ignition.

Yeah, I can work around it but I’m suggesting it for new/upgraded designs.

I like to turn my power on after I start my tractor so it doesn’t receive the low voltages during starting. I was wondering if a capacitor could be added to compensate for that, as I’d like to power with the key as well

Depending on what you’re all powering from that line, you might need a fairly large cap. If you’re boards are all USB into the laptop then your laptop battery should provide backup. Also, most of our boards run at 3.3 or 5v so I figure out should hold enough voltage for that.


On my tractors (and combine), I connect the input relays, with the coil
between positive accessory signal and the command signal for crank.
They are both available behind the ignition/start keyswitch.
So when I crank the engine all the peripherial devices (headlight, wippers, fan )
are switched off and when stopping the engine.
On the combine, I’ve put a 600 A relay for all this stuff and it works fine.

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