Vin Voltage When Photon is Powered By Battery?

Hardware
1

Hello, on the Photon documentation it states “When the Photon is powered via the USB port, VIN will output a voltage of approximately 4.8VDC”. I was wondering what the VIN voltage would be if the Photon was running off just battery? Would VIN change depending on the battery voltage? I plan to have the SparkFun Battery Shield installed.

Some background: Right now I am using USB to power the Photon and a sensor requiring 5V is connected to VIN. It works fine now, but I would like to add the battery for more portability, hence the question.

Many thanks,
Ben

2

Yes, if you are using a battery, you will be connecting to Vin and GND. If there’s anything powering off Vin, it will be dependent on the battery voltage.

3

Hi Kenneth,

Thanks for the reply. I just wanted to make sure: for example if I connected a 5V battery to the Power Shield, could I expect about 4.8V from VIN?

Ben

4

No, it’s actual voltage on VIN. The reason is that there is a diode between VIN and USB power, so power can flow from USB to VIN but not the other way. The diode causes a voltage drop of around 0.2 volts from the 5V USB power, which is where the approximately 4.8 volts comes from.

I’m pretty sure it works mostly like this when using the power shield:

  • If the power shield power input is > 6.0V Photon VIN will be 4.4V (BQ24030 regulator used)
  • Otherwise the VIN is equal to the power shield power input
  • Unless it falls below some minimum, in which case it will go to power shield USB power, if available. VIN will then be the actual USB power 5.0V, not reduced by a diode since the BQ24030 controls this, not a diode as on the Photon itself.
  • Finally, if there is no external power, VIN will be equal to the battery voltage.

It’s more complicated than that, I suggest reading the BQ24030 data sheet and prepare to be confused.

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