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What are the pros and cons of Low-Side versus High-Side relay driving concepts in an automotive environment?

The questions popped up when I was looking for an 8 channel relay driver IC for a automotive ECU project. It seems like there are more Low-Side driver IC's available, but what's actually the pros and cons from the ECU designer point of view?

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  • \$\begingroup\$ P-devices have higher resistance, and are referenced to the supply, not the ground. Next? \$\endgroup\$ – Ignacio Vazquez-Abrams Nov 7 '13 at 17:40
  • \$\begingroup\$ Is it really a bad question? \$\endgroup\$ – sergej Nov 7 '13 at 18:52
  • \$\begingroup\$ It's... one with a very short answer. The difference between high-side driving and low-side driving is the differences between P-devices and N-devices. And the differences aren't pros or cons per se, they're merely points that you need to factor into your design. \$\endgroup\$ – Ignacio Vazquez-Abrams Nov 7 '13 at 19:00
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    \$\begingroup\$ There's more to it though. Low-side switching means your device will float at a higher voltage compared to chassis ground, which may be a shock or damage hazard. And there's a whole other slew of problems you can get with AC hot/neutral switching, which is a related problem. \$\endgroup\$ – user36129 Nov 7 '13 at 19:16
  • \$\begingroup\$ @user36129. Thanks, floating might indeed be an issue, I haven't considered it yet. \$\endgroup\$ – sergej Nov 8 '13 at 9:55
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I'll interpret your question as being about driving the relays, not about what a relay drives.

Low side switches are generally easier for electronics since the driving circuits are generally ground-referenced and the low side is the ground. That means the gate or base signal can often be directly driven by the logic that decides when the relay should be on or not.

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  • \$\begingroup\$ Thanks for your answer. Right, my question is about driving the relays. I have added some background information to my question to make it clearer. \$\endgroup\$ – sergej Nov 8 '13 at 9:50
  • \$\begingroup\$ Whoever downvoted this: Please explain what you think is incorrect, misleading, badly written, etc. I looked this over again and still think it is right. \$\endgroup\$ – Olin Lathrop Jan 28 '14 at 19:31
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Olin's answer put another way would be that to turn off a high side driver you typically need to use a voltage nearly equal to the high side of the relay coil. This voltage, say 12V is not compatible with the logic devices which are typically used to control the relay.

With a low side driver, the necessary "turn-off" voltage is around 0V which is compatible.

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