Documentation says:
VIN: Connect an unregulated power source here with a voltage between 3.6V and 6V to power the Core. If you’re powering the Core over USB, this pin should not be used.
Is there a possibility to log data to USB while using Vin to power core and peripherals? Would be nice to debug my firmware and logging data from sensors.
Sure - there’s a diode on the Core that protects the USB port from higher voltages coming from VIN.
Hi @zach, does the power coming in from the USB go through exactly the same regulator / circuitry as the VIN pin? Tried to look for a circuit diagram but couldn’t find it.
In other words; could i attach a micro-usb plug to a 6V power supply and plug that in to the Spark without any problems? (for the sake of convenience)
Thanks!
R
It’s the same so don’t plug both usb and Vin unless you power with 5v external source 
On second thought, i doubted it and check the schematic:
There’s a protection diode there to prevent a back feed into the USB so I guess it’s fine 
Thanks again Kenneth!
Zach did mention the diode in the 2nd post of this thread, but was good to see the diagram!
Cheers.
Just to make sure, the core (and the photo eventually) will be able to support power coming from the Vin and from the usb at the same time ?
William
It works on the core and photon or even the electron. Nothing special 
Just a simple diode doing it’s job to keep them both in place
@tchinou1, as @kennethlimcp says, it will work just fine. That’s because there is a protection diode that prevents any problems when powering the Core/Photon/Electron that way
Great, thanks !
What about powering a Spark via the 3v3 pins? I am attempting to load a custom firmware on the Choosatron and want to troubleshoot using the serial interface while powering the printer etc via the 9V powerpack
Yes? What exactly is your concern in that case?
You can power via the 3v3 pin but only with a 3.3V pin. For the choosatron, you won’t want to do that unless you take the core out from it.
More common to power it via USB though!
however, i remember chosatron being powered by a 9V supply and all you need to do is to power up the choosatron and plug in the usb cable 
In the case of the Choosatron it appears they regulate the 9V down to 3.3V which is then powering the Spark Core via the 3.3V pin
My concern was if plugging in USB (effectively putting 5V into the VIN) would damage the core, or provide feedback back on the USB to damage the host?
I dont think so since a LDO sits between USb 5v and 3.3v.
If you want to be safe, only power through USB but the printer will not work…