I have been thinking it'd be nice to have a video on the differences of high side and low side switching, and what "ground" is in different contexts.
I know that road cars currently ground the negative to chassis and mostly switch the positive (except forties e.g. coils), and I know that old cars switched the ground, ostensibly because they didn't want therthe electrons to leak out (another physics concept which tbh still slightly confuses me--feels like electrons should be interchangeable).
When telegraph systems were first set up, they operated with "earth return" circuits, so one side of the battery supplying the system was returned to (real Earth ---the dirt).
It was found that if the negative side was earthed, the wire connecting to the "earth stake" would corrode badly.
With the opposite connection, this corrosion did not happen, so "positive earth" became the standard in that service.
When cars were first made, some manufacturers , mainly in the UK,followed this standard, & others did not.
Over time, it was found that this corrosion was not a problem with cars, &
eventually, after many years, the industry standardised on the " negative earth" connection.
Switching on the ground side of devices in cars was done for cost reasons.
It's cheaper for most stuff which is close to the battery, to just direct connect one side of the device, & run a wire to the switch on the dashboard.
OK, you will say, how about the accessories & other stuff that comes off the ignition switch?
It's still cheaper for most as you only need half as many wires in the form going to them.
From memory, lights were normally "supply side"switched.
I'm working on a race car that switches "groundsheet", and all the circuitry works just fine with rectifier negative and battery negative on one side of the primary switch, and the chassis ground and all other circuits on the other side. As a matter of fact I think the switched ground is a bit safer since accidentally shorting the main switch battery terminal to chassis will at worst turn the electronics on, vs maybe melting a wrench. So why did positive side switching cars replace the alternative?
It is probably because a lot of devices in modern cars are not just simple electrical or electromechanical things.
Switching the supply side is standard with the neg ground circuitry used with
Electronics, so maybe that just seemed to be the way to do it.
Does there exist any black box combination of diodes, resistors, inductors, capacitors etc that behaves differently if the main DC power to the system is switched on the high side vs on the low side?
Passive devices like that, no, but there may be some active circuitry where it makes a difference (can't really think of any offhand,though).
It's obvious that a mains-powered device needs to be high-side switched and have a permanently earthed chassis if it's made of metal, since the human body will short the chassis to earth if the device doesn't, but is it really necessary to connect DC "ground" with earth ground, other than maybe for EMI?
It isn't actually necessary, nor always used, but is useful, & has become a standard over many years.
If all the devices you are using have the same DC ground, you may avoid some interference & noise problems.
Of course, cars don't have any real connection to the real Earth.( no connection to the real dirt).
Plus, although I do know the difference between Earth ground, chassis ground, and common/negative/"ground" and their symbols, clearing up best practices for e.g. schematics symbols for everyone would be great since they all seem to get tossed around.
Good luck with that, although there are standard symbols, but they are not always used.
I also just recently found out the functional difference between PNP and NPN transistors and I've been making Arduino circuits and watching EEVblog videos for months. I figured everything was just high-side switched, though I still don't know if/why NPN's are "better". I've been missing some pretty critical information in hindsight.
When transistors were first made, they appeared in both NPN & PNP types.
These first ones were made using Germanium, & it was found easier to achieve consistent good performance with PNP devices.
When the early Silicon transistors were made, it was found that it he opposite applied, with NPN being the easiest to achieve consistent results.
Silicon transistors are superior to Germanium in all but a few special applications, so they took over in most jobs.
The difficulties with good performance with PNP Silicon devices was overcome, but NPN had already become the default type of device for most circuits.