I accidentally hooked a small servo motor up to the 3.3v rail (don’t ask). I didn’t notice any issues at the time, but later when debugging an unrelated issue, I noticed that the 3.3v rail was outputting 4.7 volts, as were the GPIOs. The Photon appeared to be functioning normally for the most part other than that.
Later, hooking Vin up to a 5V source, it suddenly stopped functioning, and power consumption shot up to about 800mA, with a near-short across the input buck regulator. The only difference to the previous connection was the output impedance - I was hooking it directly to my bench supply rather than via a boost regulator.
Is it possible that too large a load on the 3.3v regulator can create a near-short from Vin?
Oh boy, I have a small servo hooked up to 3.3V… I’d better move it to an external power source.
If you measure a short I’m sure it’s there.
Can anybody comment on the failure modes of the 3.3V regulator?
Well, it’s not a direct short - 800 milliamps through 5 volts is 6.25 ohms - but the regulator is definitely fried. I speculate that it didn’t kill the Photon sooner because that resistance was enough to lower the output voltage appreciably, but I can’t be certain. The whole thing doesn’t make a great deal of sense to me, starting with measuring 4.7 volts on the 3.3v rail of an otherwise-functional Photon!
Edit: Got a new photon; not only is it working fine, but the resting power consumption is half what the previous one’s was. I can only conclude I must have done something nasty to the onboard regulator before I ever stuck it in the board that made the problems obvious.