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Switching a relay.

EFI Wiring Fundamentals

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Discussion and questions related to the course Motorsport Wiring Fundamentals

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Hi guys,

When switching a relay, where is the placing of the switch best located? IE: Do you switch the feed side (12V supply, 86) of the relay or the grounding side (85)? Or, does it not matter? Cheers.

hello Rob on Efi cars it is far better to be switching the ground side via the ecu. the 12volt side still needs to be 12 volt ignition switch switching the 12 volt on and off

Regards Ross

Thanks Ross.

More questions related to switching a relay -

I'm building a harness and will be PWMing a solid state relay for a meth injection solenoid. I plan on using a low-side trigger and want to be sure I protect the circuit.

How do I calculate any current draw that will be seen with the low-side trigger/PWM circuit? I have search the forums and found a "dead link" Zac had posted awhile back, but no luck otherwise.

I'm assuming my low-side trigger wire size will be determined by the outcome above, correct? Or is 22awg okay? I see that's what was used for trigger wires in the HPA RX7 Racecar documentation.


G'day Jason. Can you throw up the URL of the post with the dead link? I'll see if I can fix it.

The triggering current of the solid state relay is really, really small, you'd be able to run 30awg wire and not have a problem, so the size of the terminals on the connectors you're using will more likely drive the wire size selection. 22awg is a good starting point :-).

Hi Zac,

Okay, thanks for the information. Does the low triggering current apply to both a SSR and traditional (non-SSR) relay?

Here is the broken link, found in this thread in one of your posts.

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