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Switching ground vs. hot

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BMW schematics always show electrical switches on the hot 12V wire, while I’ve seen recommendations to switch the ground wire. Electrically, there seems to be no difference, e.g., lights turn on. I would like to get opinions on when/whether to switch the ground wire or the hot wire. I guess this would also apply to low-current switches like relays’ terminal 85 vs. 86. 

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It doesn't matter, electrically.

 

Switching ground leaves a hot wire exposed at the location (headlight, for example)

whereas switching hot does not.

Building codes require that the hot be switched, so that in the 'off' position, there 

is no potential in the box between hot and ground.

 

Electronically, it's often easier and safer for the semiconductors  to switch ground, which is 

why EE210 has you doing it that way.

 

I usually switch hot, rather than lift ground, because I find it more intuitive.

Also, less likely to short out hot to chassis.  Or if it does, the fuse only blows

when I turn on the accessory, which makes troubleshooting a lot quicker.

 

t

 

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High-side switching is the preferred mode for the reasons Toby mentioned.  While it is also true that low side switching with FET's is less expensive, the benefits of high-side are worth it.  In a crash, you lower the risk of a hot wire touching the chassis.

 

If you google "high-side switching in automotive electronics", you'll find some really nice explanations.  Manufacturers like Infineon have excellent app notes on the subject.

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While what Toby And Healey said is the normal way for most circuits, the exception being for circuits with multiple triggers where they tend to switch the ground side to reduce the number of wires needed, for example on a 2002, dome light circuit where the door switches make or break the circuit same thing for the seat belt warning systems and so on. It's never simple.

Edited by Son of Marty

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12 minutes ago, Son of Marty said:

the exception being for circuits with multiple triggers

Yeah this is where I was going to to too.  I make most automotive relay circuits low side switched, so that the circuit won't work at all unless the ignition is on (ignition then powers high side of the coil), and then the switch/trigger mechanism controls the low side of the relay coil.

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