this post was submitted on 17 Mar 2026
7 points (88.9% liked)

Ask Electronics

4075 readers
1 users here now

For questions about component-level electronic circuits, tools and equipment.

Rules

1: Be nice.

2: Be on-topic (eg: Electronic, not electrical).

3: No commercial stuff, buying, selling or valuations.

4: Be safe.

founded 2 years ago
MODERATORS
DavideAndrea@discuss.tchncs.de
linker3000@discuss.tchncs.de
7
Lenovo USB-C PSU for laptops powers a Rasberry Pi, but cannot simply charge bicycle lights. WTF? (external-content.duckduckgo.com)
submitted 4 days ago by evenwicht@lemmy.sdf.org to c/askelectronics@discuss.tchncs.de
14 comments fedilink hide all child comments
 

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.

top 14 comments
sorted by: hot top controversial new old
[–] jet@hackertalks.com 20 points 4 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

[–] rizzothesmall@sh.itjust.works 5 points 4 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.

[–] ThePantser@sh.itjust.works 9 points 4 days ago (1 children)

The lights most likely do not have the extra circuitry to talk to the charger to negotiate voltages. Since it's a charger that can change voltage as you stated then the device must be able to say "hey give me 5v". You will need to use a dumber 5v only charger for those devices.

[–] evenwicht@lemmy.sdf.org 2 points 4 days ago (1 children)

What would be the meaning of a default voltage then? My understanding of USB PD is that 5v is a default, which I took to mean it would deliver 5v in the absence of a handshake.

[–] jet@hackertalks.com 6 points 4 days ago (1 children)

Yeah, but some power bricks want to be safe and wont give any power without power delivery negotiation. It's not unreasonable, and it is safe, it wont burn anything out.

[–] evenwicht@lemmy.sdf.org 3 points 4 days ago (1 children)

Is the spec ambiguous on that then? Is a 5v default and a PSU without default both compliant?

[–] jet@hackertalks.com 2 points 4 days ago (1 children)

The spec is very clear, the source does not need to provide any amperage, just voltage. PE_SRC_Disabled (see my other comment in this thread)

[–] evenwicht@lemmy.sdf.org 0 points 4 days ago (1 children)

voltage = current × resistance, IIRC my high school physics correctly. If current is zero, then voltage must also be zero, no? I don’t understand how voltage can be positive if amperage is zero.

[–] jet@hackertalks.com 4 points 4 days ago

Your right, but it only needs a tiny amount to signal 5V.

The power brick engineers can choose to fail safe (just 5V only minimal amperage), or fail dangerous (5W power delivery) - for this lenovo power brick they decided to fail safe.

[–] Notsosuperfloh@discuss.tchncs.de 6 points 4 days ago (1 children)

Most usb- c pd adapters wont put ouz any voltage on their own. the end device must tell the adapter what voltage they want via the cc pins. on the raspberry pi this is done via resistors between cc1 snd cc2 to get 5v. those pins are most likely unpopulated on your bycicle lights, so they get no power.

[–] brillotti@lemmy.world 3 points 4 days ago

This is correct. I made my own USB C to 30 pin adaptors for old iPods and this is precisely how I had to build them.

[–] Romkslrqusz@lemmy.zip 3 points 4 days ago

In addition to what others have said, I would hazard a guess that these bike lights shipped with a charger that uses a 5V brick and a Type A to Type C cable.

With A to C, no negotiation happens - 5V just gets sent.

[–] FartsWithAnAccent@fedia.io 1 points 4 days ago (1 children)

What lights are you using? I've been able to charge everything from Sofirn to Cateye without issue off my laptops.

[–] evenwicht@lemmy.sdf.org 1 points 2 days ago

They are actually headlights. The kind that strap to your head, which I happen to use for cycling. I suppose they were intended for joggers. I don’t know the makes but it’s two different manufactures, likely some cheap chinese stuff. One is an LED strip across the forehead with a side beam, 7 or so different functions with different colors and intensities. The other has selectable red, white, or yellow colors, blinking or solid.