I got my first Particle Photon Shield Shield and set it up using my iPad and powering the Shield Shield using a USB cable from my PC. Everything worked just fine. I then disconnected the USB cable and attempted to power the Shield Shield using a 9 volt battery connected to the VIN terminals. The Photon went bezerk. The LED flashed weirdly and I couldn’t get the Photon to work at all. I then disconnected the 9 volt battery and reconnected the USB cable. Everything again worked just fine.
Since the USB cable supplies 5 VDC, I then decided to try powering the Shield Shield by connecting a 5 volt regulated wall-wart to VIN. Everything worked just fine. Here’s my question. The spec on the VIN is 7 to 15 VDC. According to this, the 9 volt battery should have worked just fine and the 5VDC wall-wart shouldn’t have worked. Is the VIN spec on the Shield Shield incorrect or what?
I’ve tried the same with a fresh and an old one and with the old/drained one I also see some “funny” colors (white “breathing”, dim white, red, off, cycle).
The Shield Shield uses the MC33269D-5.0 regulator. The VIN is fed to this regulator through a reverse polarity protection diode (which has a voltage drop of around 0.7 volts). The minimum input voltage for the regulator to operate properly is around (6.1 = 5V+1.1V) and with the diode voltage drop included, it comes to around 6.8VDC when you are pulling 500mA. This is why we decided to be a little conservative and put the minimum at 7VDC.
Now if you are pulling less current, the input voltage number can be much lower. This explains why you are still able to power the Photon with 5VDC on the input.
The 9V battery has very poor energy density and I would not recommend using them to power the Photon or the shield. Go for a 6xAA rechargeable pack instead. The only reason why a 9V wouldn’t work is that it must be drained and cannot provide enough current.