What’s all this low side switching stuff, anyhow?
There are two basic ways to control the electricity that flows through a circuit. ‘High side switching’ (sometimes called ‘positive switching’) puts a switch (or a relay or some other sort of controller) between the source of power (the battery in our cars) and the device that’s being controlled (‘the load,’ we call it). The circuit is completed by connecting the other side of the load to ground and thus back to the negative terminal of the battery. In ‘low side switching’ (‘negative switching’ or ‘ground switching’) the battery connects directly to the load (through a fuse of course, I hope), and the switch is located between the other side of the load and ground.
They both work fine, though low side switching is against the National Electrical Code in higher voltage situations such as home AC wiring, as it can put electricity in places where those unfamiliar with the wiring might not expect to find it.
Why then do we see it showing up so often in cars? The major reasons, quite intertwined, are complexity and economics. It starts with the nearly universal practice of connecting everything back to negative by way of the car’s body and frame—the ‘ground’ (‘earth’ in Brit-speaking countries) to borrow a term from the earliest days of electricity, when the real dirt of the earth was used as part of communications circuits. The wetter it was the better it was! This saved many miles of wire. Use the car’s own metal as ground (it conducts much better than dirt), and already you’ve eliminated half the wires that would otherwise be needed to wire up the car. But now this introduces some interesting situations, complete with additional impact in both complexity and economics, often leading to the choice between high or low side switching.
The classic illustration of low side switching is the switches that turn on the inside lights when the doors are open. Take one apart and you’ll often find just one wire connected to it. Before the days of dimmers and other fancy controllers, that wire would go straight up to the low side of the dome light bulb, as would a wire from each of the other door switches. Or perhaps some if not all of them would be strung out together, with one wire then going up to the light. The other side of each switch went to ground, usually directly through its mounting in the door frame without even using any wire. The high side of the dome light went straight to the battery. So when any one of the switches closed, the circuit to ground was completed and the light came on. We got the dome light to work at the cost of as many wires as there are doors, running between the light and the switches. If we chose to employ high side switching it would have taken twice as many—wires from the battery to the switches, and another set of wires up to the light. Pennies count in the design of mass produced items.
In the other direction, you would usually find high side switching employed in controlling loads that are located some distance from their switch, again for reasons of economics. Tail lights, backup lights, and directionals are common examples. One wire from the switch out to the load; return via ground.
Now for the big caution: aftermarket devices!
The directions that come with these frequently make the implicit assumption that your car uses high side switching most everywhere. You will often see instructions such as “Tap into the high beam wire.” That won’t work for us, as our Foresters (except the 2009-2013 SH series) use low side switching on the headlights. The so called ‘high beam wire’ comes off of the (electrically) low side of the high beam filament. It’s switched to ground to turn on the high beams, and when they’re off it still connects back through the bulb to the battery, offering an intriguing ‘sneak path’ or ‘back feed’ that electrons have been known to take advantage of under certain conditions.
So, any time you see simplistic directions like “Tap into the x wire” on an aftermarket device, analyze the circuits first—the circuit you’re tapping into and the circuit that the aftermarket device appears to require. Check things out with a meter. Assure yourself that the ‘x wire’ behaves the way the aftermarket people figured it would (probably high side switched) before wiring things up that way. If you need to use a low side switched situation to control something else (such as driving lights), you can usually succeed by connecting a relay directly across the existing, switched device, so that the relay will close under whatever the conditions might be that turn on that device. No need to worry about which terminal is ground or where the source of power might be; if the existing wiring can turn on that device, it will also turn on your relay. We call this setup ‘floating’.
Don't succumb to the urge to double up on the use of whatever the low side switch might be, in order to control two separate things powered by two separate circuits. This can lead to all sorts of puzzling interactions. Invest a dollar in that relay or, under some low-current circumstances, use a couple of diodes to provide the required isolation.
Here are a few other threads that discuss specific aspects of this situation.
The “What's all this ... stuff, anyhow?” title of this post is used in homage to the late Bob Pease, MIT '61, a respected designer of analog integrated circuits and writer of many “What's all this ... stuff, anyhow?” articles for EDN Magazine. An engineer!