'Backfeeding' Metro M4 Express?

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sudo_rm_rf
 
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'Backfeeding' Metro M4 Express?

Post by sudo_rm_rf »

I would like to 'backfeed' the Metro with power from a seperate regulator via the 3v3 and 5v pins. Would this cause any trouble?

Schematic:
https://www.mediafire.com/view/ou426rlr ... 7.png/file

I would like to use these regulators:
https://www.adafruit.com/product/4739
https://www.adafruit.com/product/4683

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Re: 'Backfeeding' Metro M4 Express?

Post by adafruit_support_bill »

That should work. Although unless you have additional circuitry requiring the 5v, it is not needed by the Metro. The board itself can be powered just from 3.3v.

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Re: 'Backfeeding' Metro M4 Express?

Post by sudo_rm_rf »

If I were to power it this way AND plug in the usb cable to a computer, would the two power supplies fight or is there circuitry to prevent this?

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Re: 'Backfeeding' Metro M4 Express?

Post by adafruit_support_bill »

That would result in a battle for control between the on-board and external regulators. In general, it is not a good idea to connect two regulated outputs in parallel like that.

The power source selection circuitry on the Metro will automatically switch between inputs from the USB-VBUS and the VIN pin.

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Re: 'Backfeeding' Metro M4 Express?

Post by sudo_rm_rf »

Is there any way to prevent this? Like for example, cutting the VCC wire on a usb cable and using that?

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Re: 'Backfeeding' Metro M4 Express?

Post by adafruit_support_bill »

You could cut the VBUS line on your USB cable. Or you could just power with the 9v via the VIN pin. (5v via the VIN pin would also work to power the board. But the output of the 5v pin would not be stable)

What is the rationale behind using the external regulators? Better efficiency?

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Re: 'Backfeeding' Metro M4 Express?

Post by sudo_rm_rf »

I want to use the external regulators for more power output. Thanks!

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Re: 'Backfeeding' Metro M4 Express?

Post by adafruit_support_bill »

You can still have the external regulators there to provide more power for external circuitry. No need to directly couple them to the 5v or 3.3v outputs of the Metro. But you should make sure all the grounds are connected.

However, if extra power is the goal, the other issue would be your primary power source. A common rectangular 9v battery is not capable of supplying very much current to begin with.

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