Beginner question. I’m reading articles and my head is spinning.
Situation… I’m getting a raspberry Pi 5 and it can either draw 3A and limit the USB to 600mA, or draw 5A full power. It will be powered by a USB power bank which I’m sure is 3A max per type A port.
I want to ensure that the negotiation doesn’t end up with the Pi trying to draw 5A, and I want to ensure it only attempts to pull 3A max.
The only way I can envisage doing this is by using a cable with the correct resistor configuration to tell the Pi to only pull 3A. But I’d like to be sure that the cable will do this. I’m looking at USB testing equipment and they are expensive (knocking on for £80) and are telling me things I don’t want to know (and am not sure how to interpret as they’re testing for video and other signals that are now in USB C)
I believe that it’s down to resistor configuration and to that extent I’ve ordered some USB A and C breakout boards, so I can plug a cable in and test for the presence of a resistor, but I’m going a bit ga-ga as to what I’m testing for. There are discussions about various resistance values on various lines, C1 to C2, C1 to ground… er… I’m not sure.
I just need a way of testing a cable to see if it will restrict the load that a device tries to pull from a charger.
Welcome to the forum.
You should be asking this over at the official support forum, any answer you get here is very likely to be inaccurate at this early date. At the official support forum you will get answers from the actual engineering team that designed and supports the RasPi 5.
The RasPi 5 never limits its own current, all it does is allow more 5V current on the external USB ports when USB PD responds that it can provide 5V @ 5A.
Most USB PD supplies can’t provide 5V @ 5A, so the most common situation is that people will want the full power for USB connected devices but can’t get it unless they buy the new RasPi 5 power supply.
So the easiest way to make sure the RasPi 5 never delivers more than 5A the USB ports is to simply not connect anything to the USB ports except a keyboard and mouse.
Another method is to power it from the GPIO header instead of the USB C port. When powered via the header RasPI 5 does not allow the higher current for external USB devices.
Yet another option that I believe will work is to set an option in the config.txt startup file:
I’m not positive this setting will work because nobody has asked about forcing the low limit on the offical forum yet. I do know that by setting it to 1, the USB negotiation will be skipped and it will allow extra current on the USB ports even if it will crash the Pi.
Thanks very much for the response. I’ve asked on the forum.
However, I’m aware that I’ve got a collection of cables over the years and at the moment my review method is to think, “that’s too thin, chuck it,” … and I’d love to be able to get a more “confident” way of knowing what my cables are capable of, and what their behaviours are, particularly with power capability and control so as I don’t cause a melt/fire.
Melt and fire are only caused by cables that are too small.
A cable with a higher current capacity will NEVER cause melt/fire, period.
So you can always use a cable with larger gauge wires than are needed and it won’t overheat, the wire will always be cooler.
To ensure the cable you buy does not overheat you just need to buy a cable rated for 5A and no matter what you do in the future it will stay cool on a RasPi 5.
From a practical standpoint, in 6 years of closely following the RasPi community I have never seen a credible report of a USB cable melting or catching fire when used for RasPi power regardless of how small the wire gauge is.
What does happen is smaller gauge cables cause the RasPi to crash long before the wire gets hot. This is because the voltage drops too low for the RasPi to stay working long before the cable gets hot.
Really the absolute best option is to use the official Raspberry Pi 5 power supply. It is perfectly engineered to power a RasPi 5 and is lower cost than all the alternatives I’ve seen.
Also if you are intending to run it on USB battery power bank, be aware it will not run for very long even on the largest size.
There is circuitry/chips in the 5A cables. Won’t this tell the Pi (or other device) that it’s OK to draw 5A regardless of what the transformer is capable of supplying?
No, that won’t happen.
The cable doesn’t talk to the RasPi or the power supply, they talk to each other through a cable that allows them to talk. (not all cables allow them to talk, when that happens the 5A capability is turned OFF by the RaPi)
So a RasPi 5 will only disable the USB port limit is the power supply itself says it can provide 5V @ 5A.
Thanks. I think I’m more confident about the Pi5 now. I’m still left in a quandary about how I evaluate my cable pile, or do I just throw the lot out and start again?
You’re very welcome.
I just happen to be preparing to build a power supply for a RasPi this holiday morning so RasPi power topics are on my mind right now. BTW - To all the veterans out there, Thank you for your service!.
The easy way is to test your cables is with a RasPi.
The only bad thing that can happen is the RasPi crashes and corrupts the uSD card. But that can be fixed by simply writing a new copy of the OS image to the card.
All RasPi’s have a built in power check to let you know of a problem before it crashes.
So go ahead and try all your cables and watch for the little lightning bolt icon to show on the monitor. If it shows then the cable is not suitable and might lead to crashing if you run the RasPi harder.