USB charger output voltage

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colinuu

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#281442 17-Feb-2021 22:57

Maybe not the right forum for this, but ...

I have just repaired a Asus T100 tablet for a friend. The point of interest for me is the charger port, which uses a micro-usb connector (like lots of stuff these days). The markings on the device give the voltage rating for this port as 9V, which is also repeated in the user manual found on line.

Now, for all these years I have believed that usb voltage is 5V. I don't have the original charger that came with the tablet so can't verify the 9V. But it seems to suggest that not all chargers feeding a usb cable are limited to the standard 5V. It sounds very dangerous to me, that a user could accidentally plug a 9V supply into a device that is expecting only 5V.

From the Asus manual:

Any thoughts?

toejam316

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#2658766 17-Feb-2021 22:59

It's done by negotiation by the USB-PD standard, or one of the alternative non-standard standards made by Qualcomm or the like for fast charging.

Anything I say is the ramblings of an ill informed, opinionated so-and-so, and not representative of any of my past, present or future employers, and is also probably best disregarded.

colinuu

246 posts

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#2658773 17-Feb-2021 23:20

Thanks, I wasn't aware of that one.

So the charger starts up with a default 5V, then moves to 9V if the device requests it?

Oblivian

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#2658774 17-Feb-2021 23:26

^^ USB-C sensing.

BUT, there is still risks if you have say a phone, and a 19V laptop adaptor. And the phone doesn't quite do the job that high...

It seems to be largely taken up to prevent stupid. Or people that like to think there is no difference and use the same charger for basically anything and just switch the lead (if not like Ithing adaptors and a fixed cable)

Makers will always say, be sure to use the provided/designed power adaptors. Yet another out for if it goes bad. As people see USB socket, assume it's the same.

There was 5V USB with 500mA/1A. Some 5/2A. (Huawei 'super charge' 5V/2 4.5/5 and 5/4.5A). 9V/2 And laptops can be 18/19/24V and various. Idea is they step down based on what the device says the profile it can handle.

sbiddle

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#2658782 18-Feb-2021 07:20

Regular USB-A chargers are 5V, with 9 V and 12V available when negotiating Qualcomm QC2.0 or QC3.0 (or other fast charging standards) charging rates on a supported device and charger.

On a USB-C PD charger with PD2.0 it can negotiate 5V, 9V, 12V, 15V, and 20V at varying rates right up to 20V@5A (100W). Extensions to the PD spec with PD3.0 will allow granular voltage as well but this isn't yet common.

The reason why USB charging is so confusing is that is that many modern devices plugged into a "high power USB port" such as the many common generic USB 5V 2A chargers and things such as wall sockets will simply slow charge at around 500mA because they're unable to negotiate a proper charging standard so will drop all the way back.

When you plug a QC or PD device into a a charger it negotiates the appropriate charging rate, which can also vary during the charging process.

timmmay

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#2658793 18-Feb-2021 08:45

My very old Asus Android 4 tablet had standard USB, charged anything, but stepped the voltage up when it was used with that tablet for faster charging.

colinuu

246 posts

Master Geek

#2659279 18-Feb-2021 22:12

Thanks for the explanations everyone.