4 ms·

To be fair, I've cut open pound store USB-B cables before only to discover that they contained literally 2 strands of copper (Or at least I hope it was copper).

To be fair, I've cut open pound store USB-B cables before only to discover that they contained literally 2 strands of copper (Or at least I hope it was copper). The idea that people are trying to sink 2 amps down that type of cable is a bit frightening. I guess the point I'm trying to make is, specs are great, but the reality is that there are infinitely many types of cable and the only thing which definitively defines the capabilities is physics.

KirinDave 7y ago
Even bad USB cables are gonna have total resistances below 150mΩ, and that's including contact resistance on both ends. Unless you're shorting the cable inside the contacts (you do short pins to make USB deliver power, but upstream ports gotta spec for shorts anyways), even bad cables are unlikely to be a severe risk. The nice part of DC is you get to use the entire cross section of the wire. So 2A at 5V isn't gonna start too many fires even on thin wire. Now hopefully we don't see too many folks start using USB-PD 3 with crappy but compliant cables, as my comfort with DC drops as we go up into 20V 5A territory.
- fortran77 7y ago
  This isn’t quite right. High current at low voltages can start fires, too.
  - KirinDave 7y ago
    That's not really what anyone is talking about here. We're talking about if a USB cable itself could be an issue. For that to happen the cable would have to be shorted in a way that still coerced a USB upstream port to still deliver 2A. Even long and bad cables are going to have incredibly small resistances compared to any piece of electronics they're attached to that can make the USB port think that's okay. Which means that the majority of the energy dissipated in a USB short will likely be in the downstream electronics. You could probably use something besides copper and still have it be the minority of the resistance in a circuit that could get 2A. If a USB device gets more current than it's supposed to get, it difficult not to blame the upstream power port for providing so much under the conditions of a short or the downstream port for not being designed correctly. Furthermore, when people see USB cables with "only 2 strands of wire" these are power cables where the data pins are shorted with a resistor, which locks a USB port to a more modest 500mA. That's definitely fine.
  - Filligree 7y ago
    In fact, ohmic (i.e. reistance-based) heating is entirely independent of voltage; it depends only on amperage. Higher voltage is only dangerous inasmuch as it may increase the amperage.
    - KirinDave 7y ago
      Which is why I pointed out the resistance of the cable relative to the other components. It's not where you're going to get resistive heating unless it's a full short and that's the dominant resistor. You may know, but USB-PD has a pretty smart design here that makes it harder for such a condition to occur. I refers to the 5/20V fear only in terms of how safe I feel about handling such voltages.

KirinDave 7y ago

Even bad USB cables are gonna have total resistances below 150mΩ, and that's including contact resistance on both ends. Unless you're shorting the cable inside the contacts (you do short pins to make USB deliver power, but upstream ports gotta spec for shorts anyways), even bad cables are unlikely to be a severe risk. The nice part of DC is you get to use the entire cross section of the wire. So 2A at 5V isn't gonna start too many fires even on thin wire. Now hopefully we don't see too many folks start using USB-PD 3 with crappy but compliant cables, as my comfort with DC drops as we go up into 20V 5A territory.

fortran77 7y ago
This isn’t quite right. High current at low voltages can start fires, too.
- KirinDave 7y ago
  That's not really what anyone is talking about here. We're talking about if a USB cable itself could be an issue. For that to happen the cable would have to be shorted in a way that still coerced a USB upstream port to still deliver 2A. Even long and bad cables are going to have incredibly small resistances compared to any piece of electronics they're attached to that can make the USB port think that's okay. Which means that the majority of the energy dissipated in a USB short will likely be in the downstream electronics. You could probably use something besides copper and still have it be the minority of the resistance in a circuit that could get 2A. If a USB device gets more current than it's supposed to get, it difficult not to blame the upstream power port for providing so much under the conditions of a short or the downstream port for not being designed correctly. Furthermore, when people see USB cables with "only 2 strands of wire" these are power cables where the data pins are shorted with a resistor, which locks a USB port to a more modest 500mA. That's definitely fine.
- Filligree 7y ago
  In fact, ohmic (i.e. reistance-based) heating is entirely independent of voltage; it depends only on amperage. Higher voltage is only dangerous inasmuch as it may increase the amperage.
  - KirinDave 7y ago
    Which is why I pointed out the resistance of the cable relative to the other components. It's not where you're going to get resistive heating unless it's a full short and that's the dominant resistor. You may know, but USB-PD has a pretty smart design here that makes it harder for such a condition to occur. I refers to the 5/20V fear only in terms of how safe I feel about handling such voltages.