Hacker News new | ask | show | jobs
by myself248 926 days 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...

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.
1 comments
Kirby64 925 days ago
You're correct in all of that in regards to mechanical relays, but none of this matters for solid state devices. I'm not aware of any actual mechanical relays in Tesla's. They do have contactors for the HVDC connections, but that's about as close as you get to relays. Hell, they don't even have fuses; they use self reset-able 'soft' fuses.

You have to worry about shutting down current quickly (i.e., inductor flyback), but that's a pretty trivial problem to solve.

link