Author Topic: Confused on safe VIN for Mega-USB  (Read 612 times)

ProfessorChaos

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Confused on safe VIN for Mega-USB
« on: April 24, 2021, 02:19:42 PM »
Hi -

I'd like to power a Mega-USB installation from a 12v line. The specification for the Moteino Mega states that 3.3 - 16v may be applied to VIN, although it seems like >5v would be an issue if a powered FTDI adapter is also attached.

What about the Mega-USB?  The product page states that "You can however power the device through VIN from a battery pack" but does not state a maximum voltage. The schematic seems to show VIN connected to VCC of the FT231XS, so I'm wondering if this would cause damage.

Could you please clarify whether it's safe to power a Mega-USB with 12v on VIN if:

  • The Mega is connected to a computer's USB?
  • The Mega is disconnected from USB?

Thanks

Felix

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Re: Confused on safe VIN for Mega-USB
« Reply #1 on: April 30, 2021, 10:16:25 AM »
Should be fine, there is a blocking schottky diode on the USB.
The FTDI chip is only powered from USB.
The VIN powers the LDO.

ProfessorChaos

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Re: Confused on safe VIN for Mega-USB
« Reply #2 on: April 30, 2021, 12:24:32 PM »
Great, thanks Felix.  I wasn't sure if the resistor shown parallel to the Schottky was an issue or not.

Felix

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Re: Confused on safe VIN for Mega-USB
« Reply #3 on: April 30, 2021, 01:10:56 PM »
That is actually redundant and not populated.