6 ms·

Hi, automotive electrical is my job. There's quite a bit of very thick wiring in a car, not just the starter wire, but boring stuff like audio amplifiers, rear

Hi, automotive electrical is my job. There's quite a bit of very thick wiring in a car, not just the starter wire, but boring stuff like audio amplifiers, rear window defrosters, power seat motors. Those things don't draw a ton of power, like maybe just a few hundred watts, but at 12 volts even modest powers require extraordinarily thick wires, especially when you account for bundle derating. This requires large terminals, which requires larger connectors, and there's the complexity, because MOST of the wiring in the car is just signals, or low-power stuff, which can run over thin wires and small terminals. (Minimum size is limited by mechanical durability rather than electrical conductivity.) Making a "hybrid" connector that has a couple large cavities for large terminals, and a bunch of small cavities for small terminals, is a pain. Having separate connectors for heavy power and for signals introduces more assembly work and negatively impacts testability. The wires have different stiffness and bend behaviors, they exert different amounts of force on weather seals, they have to be terminated on different machines at different points in the assembly process. By allowing power wires to be nearly as thin as signal wires, you can use simpler connectors with unified terminals. Manufacturing gets simpler, harnesses get lighter, assembly gets faster and easier. Weight is also a huge deal, every ounce counts. There's upwards of 100 lbs of wiring harness in most cars, more in larger or premium models with a lot of accessories. If half of that weight is signals and won't change with voltage, but the other half is heavy power circuits that'll get 4x thinner at 48v, it's significant weight savings. Furthermore, switching heavy current means massive relays or FETs and the heatsinks thereon. If you can reduce the current, those components get lighter too. Audio amplifiers get lighter, speakers get lighter (stupid heavy-wound 2-ohm speakers to get reasonable volume out of low voltage drive? Nah, use standard 8-ohm now that you have real voltage at the amplifier!), all sorts of things get lighter. That's all in addition to the electric power steering already mentioned by others. EPS can easily move 1kw for short periods, and has stupidly huge wiring to do that at 12v. It's still chunky at 48v, but a lot less so, and can use more common terminals and connectors. Replacing a hand-assembled bolted connection with a machine-crimped and clicked-together connector improves reliability or reduces testing process overhead. It's really significant, and it's embarrassing that the industry fell flat on its face in the late 90s last time they tried. Here's hoping this takes off.

panick21_ 3y ago
Awesome to hear from an expert. Im looking forward to some teardowns to see how the set this all up.
mcguire 3y ago
The rumor I heard was that the higher voltage resulted in lower switch lifetimes. Any truth to that?
- cogman10 3y ago
  Yes, but not meaningfully. The higher the voltage you get, the more arching there is when a relay trips (also depends on if there's any sort of inductive load, think the sparks you see when you unplug a vacuum without turning it off). But when you think about the impact that has on switches and relays, realize that in your own home you have 120V controlled by switches. Very cheap switches last decades (though admittedly not switched as often as something like a blinker).
  - myself248 3y ago
    Ahh, no. AC is fundamentally different from DC when it comes to arcing behavior, because it has zero-crossings. If a switch arcs while switching AC, the arc goes out 1/120th of a second later. An arc would have to be pretty enormous to have enough thermal mass to remain ionized long enough for the next half-wave to re-energize it and sustain it. (HV AC transmission and distribution tends to have SF6-filled switches for this reason.) But around the house, your AC switches are really simple because they're not moving anywhere near that much power. And statistically, some fraction of switch openings happen with near-zero instantaneous current anyway. DC, by comparison, is brutal to switch. It doesn't have zero crossings, so the arc has to be blown out by the design of the switch. That means nice wide contact openings, and on really large ones, magnetic blowouts to divert the arc into chutes that cool it. If you look at a switch datasheet.... pulling up a randomly-selected one from Digi-Key now.... https://mm.digikey.com/Volume0/opasdata/d220001/medias/docus/4897/toggle-switches-serie-600H-600NH.pdf https://mm.digikey.com/Volume0/opasdata/d220001/medias/docus... Look at the cycle ratings. It has a bunch of different ratings depending on the contact form (some that're forced apart, some that're sprung apart), but in all cases, the DC rating is equal or much lower current than the AC rating. And the DC ratings only go to 24V, this switch IS NOT RATED for use at 48VDC at all, despite happily going to 250V when switching AC. So, if you're comparing apples to apples, if you had 48VAC for instance, that would be easier to switch than 120VAC. (At constant current, that is. If you want to move the same power, you need more current at the lower voltage, and it gets harder again.) But DC is oranges. Yes, switching 48VDC is harder than switching 12VDC, but only at constant current. And it may require _different_ switches than 12VDC. Given that you only need a quarter as much current to move the same power, it's still a net win, but it's not at all comparable to switching AC.

panick21_ 3y ago

Awesome to hear from an expert. Im looking forward to some teardowns to see how the set this all up.

mcguire 3y ago

The rumor I heard was that the higher voltage resulted in lower switch lifetimes. Any truth to that?

cogman10 3y ago
Yes, but not meaningfully. The higher the voltage you get, the more arching there is when a relay trips (also depends on if there's any sort of inductive load, think the sparks you see when you unplug a vacuum without turning it off). But when you think about the impact that has on switches and relays, realize that in your own home you have 120V controlled by switches. Very cheap switches last decades (though admittedly not switched as often as something like a blinker).
- myself248 3y ago
  Ahh, no. AC is fundamentally different from DC when it comes to arcing behavior, because it has zero-crossings. If a switch arcs while switching AC, the arc goes out 1/120th of a second later. An arc would have to be pretty enormous to have enough thermal mass to remain ionized long enough for the next half-wave to re-energize it and sustain it. (HV AC transmission and distribution tends to have SF6-filled switches for this reason.) But around the house, your AC switches are really simple because they're not moving anywhere near that much power. And statistically, some fraction of switch openings happen with near-zero instantaneous current anyway. DC, by comparison, is brutal to switch. It doesn't have zero crossings, so the arc has to be blown out by the design of the switch. That means nice wide contact openings, and on really large ones, magnetic blowouts to divert the arc into chutes that cool it. If you look at a switch datasheet.... pulling up a randomly-selected one from Digi-Key now.... https://mm.digikey.com/Volume0/opasdata/d220001/medias/docus/4897/toggle-switches-serie-600H-600NH.pdf https://mm.digikey.com/Volume0/opasdata/d220001/medias/docus... Look at the cycle ratings. It has a bunch of different ratings depending on the contact form (some that're forced apart, some that're sprung apart), but in all cases, the DC rating is equal or much lower current than the AC rating. And the DC ratings only go to 24V, this switch IS NOT RATED for use at 48VDC at all, despite happily going to 250V when switching AC. So, if you're comparing apples to apples, if you had 48VAC for instance, that would be easier to switch than 120VAC. (At constant current, that is. If you want to move the same power, you need more current at the lower voltage, and it gets harder again.) But DC is oranges. Yes, switching 48VDC is harder than switching 12VDC, but only at constant current. And it may require _different_ switches than 12VDC. Given that you only need a quarter as much current to move the same power, it's still a net win, but it's not at all comparable to switching AC.