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Metro M4 Express - Power via Header?
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Metro M4 Express - Power via Header?

by Dudenski on Fri Jan 18, 2019 5:57 pm

I'm very new to this sort of thing, so my apologies for the simple questions.

I have a regulated 5V rail that I would like use to power a Metro M4 Express. I think I can use the Vin/GND pins to do this. It will be connected to a PC via USB, but I only want to use USB for communications, not power (it will not always be present). Can I safely connect this way?

Thanks!

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Re: Metro M4 Express - Power via Header?

by adafruit_support_mike on Sun Jan 20, 2019 5:10 am

You can power a Metro from an external 5V supply, but the connections are a little different.

The Vin pin connects to the input side of the Metro's 5V voltage regulator, and the regulator needs about 1.5V between its input and output to do its thing.

Vin is also connected to an input power selection circuit that compares half of Vin to the voltage on the 3.3V pin, and uses the result of that to open or close a connection to the USB jack's 5V pin. So as long as Vin is higher than about 6.6V, power from the USB cable is cut off. If Vin is below 6.6V, power from the US cable is connected.

You can connect an external 5V power source to the Metro's 5V pin instead of Vin, but it's a good idea to put a diode between the external 5V supply and the Metro's 5V pin.

A diode is basically a one-way valve for current. As long as the voltage at the diode's positive side is about half a volt higher than the voltage at its negative side, current can flow through. No current can flow if the voltages on both sides are equal, and current can't flow from the negative side to the positive side at all.

That's handy for swapping power connections. If the Metro is connected to the external 5V supply and nothing else, current will flow through the diode and run the board at about 4.4V (which is just fine). If you plug in a USB cable, the 5V pin jumps up to 5V, and the diode automatically cuts off the external 5V supply. When you unplug the USB cable, the diode will automatically let power from the external 5V supply flow through again.

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Re: Metro M4 Express - Power via Header?

by Dudenski on Tue Jan 22, 2019 11:17 am

Thanks, Mike!

Dudenski
 
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Please be positive and constructive with your questions and comments.