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Topic: CPU USB power vs. charger/converter USB power (Read 158 times) previous topic - next topic


I'm sure there's some simple explanation for this...

I have a script for a Nano that receives a 433 MHz signal. When an appropriate rf command is received, an if statement turns digital pin 8 High. That High digital pin closes a normally-open 5 V relay, allowing an 18 V DC supply to trigger a shutter. If I power my Nano via the USB port connected to the computer, the script runs fine and the relay functions as intended.

If, however, I power my Nano via the USB port connected to a AC-to-USB adapter, the script runs...and the relay doesn't. Neither does the relay run when the Nano is powered by an 18 V to 5 V DC/DC converter.

Using a multimeter to check supplies: the 5 V Nano pin is at 4.7-4.8 V depending on the AC adapter, or 4.5 V when connected to the computer.

When the logic is triggered High, the digital pin reads 4.7 V - if the Nano is powered by an adapter. If the Nano is powered by the computer, the pin reads 3.1 V because some of the power is used by the relay magnet. The Low reading is the same in any power case.

The power supplies can be swapped back and forth for reproducible results. Why doesn't the relay draw power when the Nano is powered by a USB converter instead of a computer's USB port?

Note: this is not an issue of rf reception. I'm testing with a computer and AC adapter right next to each other. The circuit literally stays in the same place while I change where the USB power supply is connected.


I am usually so far out of the box that most people don't know what I am talking about.

Please do not ask for help by PM. I will not respond.
If you need help, post a question on the appropriate forum.

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