this post was submitted on 23 Aug 2025
12 points (100.0% liked)

Ask Electronics

3839 readers
20 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
12
USB PD for older device (piefed.social)
submitted 4 weeks ago by SorryImLate@piefed.social to c/askelectronics@discuss.tchncs.de
11 comments fedilink hide all child comments
 

I have an older electrical device which I would like to switch to charging with a USB C cable and a standard USB power supply. The device's power supply unit supplies 14V and 400 mA, so 5.6 W. The device documentation certifies it for 12V with 2 Ah.

I found a useful answer on stackexchange about the Source Power Rule for USB 3.1. This implies to me that my device is not compatible with 3.1 because at a power draw below 15W, PD 3.1 defaults to 5V.

According to Wikipedia, a USB4 DFP is required to supply at least 7.5W Type-C current, which is above the power requirements for my device.

Based on this, it seems to me that the power supply of my device is too non-standard to be supplied by a standard USB power supply. Have I understood this correctly?

Thank you for your help.

you are viewing a single comment's thread
view the rest of the comments
[โ€“] czardestructo@lemmy.world 2 points 3 weeks ago (1 children)

That is not correct, you won't get a random lower voltage. You are correct that thw PD supply needs to support the voltage but PD always starts with 5V for backwards comparability then digitally its negotiates the PD voltage it wants, if it asks for 12v and its not available it with just stay at 5V. It won't spit out the closest value unless the device asks for it. Its a very strict and wonderful standard.

[โ€“] cmnybo@discuss.tchncs.de 1 points 3 weeks ago

It must be the PD trigger chip that does that then. With every PD trigger module I've used, I get a lower voltage if the requested one is not supported. It doesn't stay at 5V unless that's all the power supply supports.