4 ms·
> Smartphones already do this. They essentially measure the resistance of the cable by measuring the voltage and correlating it with the current drawn, and regu
by amluto 1y ago
> Smartphones already do this. They essentially measure the resistance of the cable by measuring the voltage and correlating it with the current drawn, and regulate the current accordingly.
Not sufficient. It’s easy to have a cable where the long conductors can safely carry a lot more current than the terminations. Or one could have a 1 foot thin cable connected in series with a 10 foot thick cable.
For that matter, a short low-ampacity cable would look just like a long high-ampacity cable.
- lxgr 1y agoOn top of that, a slightly imprecise voltage source could also easily look like a very low resistance cable.
- userbinator 1y agoIt's differential impedance that they measure, not the absolute voltage.
- lxgr 1y agoWhat would they measure that for?
- userbinator 1y agoTo determine how much current they can draw.
- lxgr 1y agoThey can only measure combined voltage sag of the source and resistive losses in the cable though that way, no? Doesn’t help with the concern of high-resistance spots somewhere in a low-current-rated cable.
- NetMageSCW 1y agoThey aren’t trying to deal with poorly manufactured out of spec cables.
- ajb 1y agoBut resistance in series is additive - the resistance of the combination is dominated by the resistance of the worse cable: 0Ω + 1Ω = 1Ω Not sure what problem you're thinking of.
- lxgr 1y agoMost of the resistance occurring in a small part of the entire cable, potentially causing a fire, seems possible, no?
- NetMageSCW 1y agoNo. You could also suppose a dead short in the cable that causes a fire, or just a cable with a length of detcord in the middle.
- Tharre 1y agoIt's not resistance per se that you care about, it's how much the cable is heating up which is mostly dependent on resistance per meter. Imagine 2 cables with the same resistance, one 0.5m long and the other 2m. At a set amount of amps, the 0.5m long cable would need to safely dissipate 4 times as much heat than the 2m cable over the same distance. And you don't know how long the cable actually is (and they make 10cm usb cables after all) so you can't make any decision based on resistance that doesn't risk some cables going up in flames.
- userbinator 1y agoFortunately copper and aluminium, the two most common metals for cables, have resistance that increases with temperature. Ultimately what I'm saying is that the endpoints have or can measure all the information they need to adapt, and this is even more accurate than requiring a cable to self-identify which is worthless if it's lying (and that can happen unintentionally if it's damaged or the connector is worn.)
- amluto 1y agoThe copper and aluminum will not become resistive enough to make a difference at temperatures low enough to prevent the rest of the cable from becoming crispy.