[throw84944994]

> negotiates 5V5A

Even the voltage is not matching spec (Pi power supply has 5.1 volts, not 5.0 volts!). That is because historically Pi had shitty cables, with high resistance and voltage drops. 5V5A is not even in spec, limit for 5 volts is 3 amps!

> fire hazard than 20V5A

That would be 100 watts! Many people just grab any usbc cable, and solder it directly to GPIO power pins. But good luck with that!

Initial batches of Pi4 did not even had a resistor, to request 3.0 amps!

[toast0]

5.1 volts is 2% off 5.0 volts. I don't have a copy of the USB specs, but a voltage 2% higher than nominal is almost certainly within specifications.

[shadowpho]

Pretty sure it’s 10% and many psu do give out 5.25 or 5.4v out for this exact reason

[klempner]

> That would be 100 watts!

The point is that power dissipation in a cable is a function of the current going through it. The cable will get exactly as hot carrying 5 amps with a voltage of 5 volts as it will carrying 20 or 48.

(now, that is more *wasteful* -- you lose the same amount of power to heat carrying 25W at 5V5A as you do at 100W 20V5A, but that's 4x the relative waste in power)

> Many people just grab any usbc cable, and solder it directly to GPIO power pins.

You're not going to get *any* 5 amp mode out of a standard PD power supply unless the cable indicates it is 5 amp capable, which isn't going to happen unless that "any usbc cable" has the right emarker on it.

> limit for 5 volts is 3 amps.

There is no such limit.

What there is is two things: 1. There are a standard set of PDOs a standard "X watt" PD power supply is supposed to provide. 5V3A 9V3A 15V3A 20V5A, (then 28, 36, and 48 volts for EPR) with the highest one limited to the power limit of the supply. These only go up to 3 amps until you get to 20 volts. 2. Devices are supposed to support those standard PDOs.

Anything other than those standard PDOs is optional (at least before 3.2 which starts introducing AVS as a requirement at 27W+). 12V support is common, as for that matter is PPS support. 5A support below 20V in fixed PDOs is 100% allowed but is super rare.

(5A lower voltage PPS is a different story, but unfortunately the RPi5 doesn't know how to negotiate 5V PPS. That is a shame because it would 100x its power supply compatibility because most chargers targeting higher end Samsung phones support it.)

A power supply is 100% allowed to support 5V5A. It just isn't required to. It would have been 100% legitimate for the RPi5 to have a buck circuit to handle a standard 27W 9V3A power supply and then turn that buck off if the power supply and cable support 5V5A.

> Initial batches of Pi4 did not even had a resistor, to request 3.0 amps!

To be precise, it had *a* resistor (connected to the shorted together CC pins) when it was supposed to have one separate resistor for each pin, and that broke cables with emarkers.