this post was submitted on 17 Mar 2026
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Lenovo USB-C PSU for laptops powers a Rasberry Pi, but cannot simply charge bicycle lights. WTF? (infosec.pub)
submitted 6 days ago by evenwicht@lemmy.sdf.org to c/askelectronics@discuss.tchncs.de
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I bought a 2nd-hand Lenovo USB-C PSU (ADLX65YLC3D) which indicates a range of voltages (20v, 15v, 9v, 5v) on the label. Tried to charge a few different bicycle lights but the charging indicators did not light up on any of them. I almost tossed it because the 2nd-hand market I bought from is definately dodgy. But then I tried to power a Rasberry Pi and it seems to work on that. So wtf? An a/c adapter either works or it doesn’t. What would cause this: works on some devices but not others? The Rasberry Pi needs 5v just as the bicycle lights. That is the default voltage for USB-c.

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[–] jet@hackertalks.com 27 points 6 days ago* (last edited 6 days ago) (1 children)

Welcome to the wonderful world of power delivery negotiations.

Basically your bike lights are too dumb to tell the power brick what they need. Use a cheap charger that will just send out the default without negotiation

Here is a 40MiB zipfile if you want the nitty gritty details: https://www.usb.org/document-library/usb-power-delivery

You are ending up in the PE_SRC_Disabled state on the source power delivery state machine.

[–] rizzothesmall@sh.itjust.works 5 points 5 days ago

Don't understand why it wouldn't provide like 5v 2a by default until a PD negotiation happens.

I have a USB C Dell dock which can whack out 180w but won't power anything that doesn't give it a proprietary Dell signal, making the USB C-ness of it fairly worthless.