powering Raspberry Pi from bench supply with USB - Page 1
Forum section: electronics / beginners

author: metebalci
author_rank: Frequent Contributor
timestamp: January 23, 2022, 08:09:52 pm
content: I know I can power RPi from GPIO and using it like that, but I wonder if I can with USB. So I get a USB cable and connected +5V and GND to bench supply, and to comply with USB power spec. I shorted the D+ and D-. However, RPi is still not properly booting, I dont have a screen attached so not sure if it writes anything but I see some activity in LEDs then it seems like it is rebooting. What am I missing ?

author: Ian.M
author_rank: Super Contributor
timestamp: January 23, 2022, 09:20:47 pm
content: A screen?Before the Pi 4, the Pi's power in port was very dumb, IIRC simply Vbus and Ground with no data pins connected. The Pi 4 has some resistors to signal the charger its expecting 5V power (which wasincorrectly implementedon early board revisions).Many cheap USB cables have too much resistance to reliably power even a Pi Zero! Measure the 5V at the GPIO header, to check you are actually delivering the correct voltage to the board, and scope it to check it isn't dipping. *DON'T* be tempted to crank up the voltage past 5.25V in an attempt to compensate for a crappy cable. Instead, cut it down to 3" long and splice it onto much thicker wires back to your PSU.

author: metebalci
author_rank: Frequent Contributor
timestamp: January 23, 2022, 11:13:21 pm
content: Quote from: Ian.M on January 23, 2022, 09:20:47 pmI checked with the scope and sometimes it seems it actually falls down, not sure to what extent it is OK. But it seems to be working now. I dont know what was the problem. I guess it might be because of the cables.

author: Ian.M
author_rank: Super Contributor
timestamp: January 23, 2022, 11:51:31 pm
content: I believe anything below approx 4.63V will trip the undervoltage detector. USB connector contact resistance can be significant - I have one Pi with a SATA HDD (in a USB<=>SATA 'pod') that needs its power in USB jack and plug cleaned and reseated at regular intervals to prevent the Pi detecting undervoltage, and the HDD restarting.

author: rpiloverbd
author_rank: Regular Contributor
timestamp: January 24, 2022, 12:18:54 pm
content: Personally I always recommend using an official RPI power supply to power up the RPI. You know, RPI needs 5v,2A. Is your power supply really giving this output?

author: Ian.M
author_rank: Super Contributor
timestamp: January 24, 2022, 12:33:46 pm
content: 5V 2A is grossly inadequate even for a Pi 2B if you have four fully loaded USB ports. OTOH if all that's on there is a single unit load wireless HID mouse/keyboard dongle, the same Pi 2B could be happy on a 500mA PSU (that's 500 Western mA as Chinese mA are usually more wimpy, like 'Sears horsepower'). Seehttps://www.pidramble.com/wiki/benchmarks/power-consumption

author: metebalci
author_rank: Frequent Contributor
timestamp: January 24, 2022, 01:48:23 pm
content: Quote from: rpiloverbd on January 24, 2022, 12:18:54 pmJust in terms of basic specs that should not be a problem I think for my power supply (RS NGE) (or is there sth special about USB power?). I also normally use USB adapters.

author: metebalci
author_rank: Frequent Contributor
timestamp: January 24, 2022, 01:54:24 pm
content: Quote from: Ian.M on January 24, 2022, 12:33:46 pmMy use case for the purpose of this question is only RPi with SPI/GPIO connections, no USB devices, even no keyboard etc. but wifi. It is consuming around 0.5A like this.

author: mazurov
author_rank: Frequent Contributor
timestamp: January 24, 2022, 06:18:27 pm
content: Quote from: metebalci on January 23, 2022, 08:09:52 pmThis looks wrong to me. I has been powering USB devices from a bench supply for many years and don't remember having any issues. The only connections are plus to vbus and minus to gnd, this worked for all devices so far including pies up to v.3.

author: metebalci
author_rank: Frequent Contributor
timestamp: January 24, 2022, 07:36:42 pm
content: Quote from: mazurov on January 24, 2022, 06:18:27 pmIt is from USB charging specs. Less than 200ohms between D+ and D- means it is a dedicated charging port, not capable of enumeration, so just like a power supply. I guess if a device gets power by USB (but no data comm use) or maybe for old devices it may not matter, but it matters for some devices at least.

author: Renate
author_rank: Super Contributor
timestamp: January 26, 2022, 07:03:47 pm
content: Quote from: metebalci on January 24, 2022, 07:36:42 pmWhat do you mean?Everythingis capable of enumeration! "That'soneUSB power supply!"I used to run a shorting wire from the four USB sockets to the regular 5V.Then I could feed the power in on a USB A plug instead of the wimpy micro USB.

author: metebalci
author_rank: Frequent Contributor
timestamp: January 26, 2022, 11:54:39 pm
content: Not an expert, but I take it to mean no other/some more/normal USB signalling. No handshake, negotiation etc.

author: Renate
author_rank: Super Contributor
timestamp: January 27, 2022, 12:34:59 am
content: Oh, yeah, I get USB enumeration and low-level USB.I guess that you're not familiar with Count von Count from Sesame Street.He likes to count things.