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Electronics => Projects, Designs, and Technical Stuff => Topic started by: AussieBruce on May 28, 2022, 02:03:21 am

Title: Powering target device using a USB charger
Post by: AussieBruce on May 28, 2022, 02:03:21 am

Hi,  I’ve googled this intensively, and nothing seems to hit the nail on the head….

I’m working with a protoboard with a USB connector for power and connectivity. The nice thing about it is that it comes with enough information to achieve programmatic access to the FIFOs in the USB interface chip, both ways. I’m designing an onboard application that will have the device running unattended, with just power being provided through the USB connector. Occasionally it will be connected to a host machine for uploading of data, and of course that connection will power the device.

My question relates to powering in unattended mode. When connected to a host machine, the power lines in the USB cable provide (as far as I know) regulated 5V. For powering the unattended board I need an autonomous 5V power supply, there are heaps of plugpack units on the market that accept a USB cable, but all of them market as ‘chargers’, or if they aren’t so specified, the specifications tend to be vague.

Unfortunately, no schematic of the board is available. Whether there are devices that may be damaged by voltages above 5 I can’t tell.

Is there anything in the specification of these devices that ensure that if they are plugged into a straight USB port they won’t do any damage?

Title: Re: Powering target device using a USB charger
Post by: sleemanj on May 28, 2022, 05:01:46 am

USB supplies are 5v by default, they can, putting aside failures and complete non-compliance, only output higher voltages with specific active negotiation between the device being powered and the supply.

A "USB Charger" is a laypersons name for a USB power supply.

Note that the supply might limit current in the default mode to 500mA or even less, but in practice, for your "usb charger" bought from a retail shop, it is incredibly unlikely to do so and you'll be able to draw the full rated current of the supply should your device need it.
 

Title: Re: Powering target device using a USB charger
Post by: tunk on May 28, 2022, 10:12:24 am

IIRC, the original USB specs said that "5V" could be 4.75-5.25V (or was it 4.75-5.5?).
Many of the cheaper USB power supplies have questionable safety and high ripple,
check out https://lygte-info.dk/info/indexUSB%20UK.html or watch some of DiodeGoneWild's
videos on youtube.