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Let's say I have

  1. A very old Micro-USB cable. It's USB 2.0 and rated for a max current of 5V, 0.5A (2.5W).
  2. A modern, power-thirsty, quick-charging tablet that charges over micro-USB at a max of 100W.
  3. A modern, QC, power-supply that outputs a max of 100W

I plug my power supply into the 220V wall outlet. I plug my cable into the power supply. I plug my tablet into the cable.

What prevents the USB cable from catching fire?

Michael Altfield
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    Your question seems very hypothetical and afterall you probably don't have such a cable anymore, but even if you did, the power dissipated in such a cable would NOT catch on fire. If anything, there may be a large enough voltage drop that may render the charging ineffective, and you may just find that charging does not occur at maximum current possible ... – citizen May 12 '22 at 14:00
  • I'd say the "old" cables are quite under-rated, and have a good safety margin. – Eugene Sh. May 12 '22 at 14:07
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    You say it is a USB 1.0 designed cable. USB 1.0 was for 5V at 10mA. It didn’t get to 100mA until USB 1.1, and 500mA wasn’t until USB 2.0. – Puffafish May 12 '22 at 14:22
  • @Puffafish thanks. I updated the question for USB 1.0, – Michael Altfield May 12 '22 at 14:40
  • Sorry for moving the goal posts, but after some research I see that Micro-USB was released in 2007. USB 1.1 was released in 1998. USB 2.0 was released in 2000. USB 3.0 was released in 2008. Therefore, I'm changing it back to USB 2.0 at 500 mA. – Michael Altfield May 12 '22 at 14:54
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    No such thing as 100W over micro USB, so no problem there. I think with QC or old USB charger spec you could get up to ~ 2.5A through an old micro USB, which should generally be safe. Even 28 gauge wire, 1 m long gives you less than the 4.5V minimum at 2A, so you'd in practice be limited to safe values just by the wire resistance. – user1850479 May 12 '22 at 14:57
  • @user1850479 Micro-USB PD does exist https://en.wikipedia.org/wiki/USB_hardware#USB_Power_Delivery – Michael Altfield May 12 '22 at 14:58
  • USB-PD supports a lot of connectors, but not all of them support 100W. IIRC the maximum for micro on a standard cable was 1.5A, increasing to 3A on specially marked cables. I think its actually the older pre-PD standards that will put the most current into a micro USB connector. – user1850479 May 12 '22 at 15:15
  • the wire doesn't care about watts, just amps, and those methods of delivering higher watts use higher voltage to push more power, still keeping the amps under 2 or 3. – dandavis May 12 '22 at 20:37

2 Answers2

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First of all, an old cable has VBUS, GND , D+ and D-. This is only four wires. The full 100W power draw is only possible when using USB 3.0 compatible cables, which have 8 wires in them (two extra data pairs). I have not read the full USB spec in detail for a few years, but I know that the negotiation of power delivery can get quite convoluted, but for it to work both sides of the cable make sure everything is correct, including the cable type (I’m guessing that this is done with some negotiation using the extra conductors).

Another thing to note is that the high transfer of power can cause other issues with data transfer (there are notes in the wiki page you’ve linked to this effect). So you need a high quality cable to keep the noise level down far enough for both ends to be happy to keep the power high. Old cables are unlikely to have been made at the quality level required (though it is possible the stars aligned, and you had the best cable ever made in the 2000s).

Couple those two facts with safety margins: an old USB cable was designed for 100mA at 5V, but the will have had a huge safety margin (multiple orders of magnitude). Even back in the old days of dial up internet there were safety requirements on things like cables: self-extinguishing materials for insulation etc. So even if they got hot, they might start melting which would be unpleasant, but not quite burst in to flames event.

As soon as anything starts going wrong with the cable, the USB handshaking will go wrong, and the power supply will very quickly shut the supply of power down. Everything fails safe is the idea.

Pushing high current through small wires causes voltage drop. This will be more than expected with a poor cable, this will be detected by the device being powered, which will then alert the power supply, which will scale everything down again to safe levels.

Puffafish
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    My understanding is that the power negotiation will only apply if there's >4 wires. In this situation (An old Micro-USB cable) there are only 4 wires. So while I'm sure that modern USB-C cables can safely negotiate the power requirements, the "curve ball" part of this question is that Micro-USB doesn't have that power handshake afaik. – Michael Altfield May 12 '22 at 14:49
  • @MichaelAltfield Wikipedia says: "Micro plugs have a resistor or capacitor attached to the ID pin indicating the cable capability" – user253751 May 12 '22 at 16:31
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    @user253751 I can confirm this, we designed a power system (circa 2007) that had a USB power output. Phones etc. would only pull 100mA until we put the resistors across the data pins, then they would pull 500mA. – Aaron May 12 '22 at 17:00
  • @Aaron that's a different one, that indicates the charger's capability – user253751 May 12 '22 at 22:57
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As a generic answer, the USB specs, and QC specs.

The USB specs require that a PD source detects the cable type from the plug.

If the plug does not identify as PD capable plug then PD will not be used.

Specific to your MicroUSB question, PD capable cable assembly connects the MicroUSB ID pin with a specific resistance to ground, which the non-PD cable assemblies don't have.

QC specs can be speculated, but they should have some kind of voltage and current monitoring to know how much voltage drop the cable has and the charged device can limit the current to a level that is safe.

Justme
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  • I think the "voltage drop" and "current monitoring" here is key. I tested this with a cable and a very old USB Extension Cable. I found was that the current was higher when charging over the cable without the extension cable. When charging *through* the Extension Cable, the current was less (actually varied +/- 0.2A). In both cases, voltage was steady at 5V. So, I guess the device detected voltage drop and adjusted the current accordingly to keep the voltage at 5V. And without the resistors or additional wires for the PD handshake, I guess the device will safely limit voltage to exactly 5V. – Michael Altfield May 13 '22 at 10:18