Hot Raspberry Pi!

I recently took the plunge and purchased the 4.3.3 kit to use with an RPi 3. I am running the snapshot and have elected to go with the thermocouple option and a micro damper. I had a couple head-scratching moments, but nothing terrible. When I plugged in the RPi with the 5V jack, everything powered up, I could get online, my thermocouple and probes measured pretty close, etc. So far, so good.

I finally found a 12V, 1A power supply and plugged it in (only the 12V, not both). Again, everything seemed to be good until I noticed that distinct hint of frying electronics in the air. I immediately unplugged it and noticed that the RPi was extremely hot somewhere around the power jack. I rechecked all my solder joints and verified voltages per the HeaterMeter433BaseV drawing. No issues were found, though my eyesight isn't what it used to be. After verifying that it still worked when powered by 5V, I thought I'd try 12V again. I was able to measure that the two 5V pins on the rPi were now reading a bit less than 3V before it began to get hot again and I had to pull the plug.

Now when I try to plug the RPi into 5V, I get the blinking green ACT(ive?) light, but the heatermeter doesn't seem to power up. The LED's blink and then go out again. Unfortunately, I'm not at home and I don't currently have the means to see if I can remote into it. Oh, and the HM by itself with 12V seems to be okay.

Sorry for the wall of text, but I'm trying to be as descriptive as possible. Does anyone have any thoughts? Do I maybe have a defective RPi?

Update. I was able to get a few minutes with a thermal imager at lunch. I took a couple quick snaps with the RPi plugged in to 5V by itself and with the SD card removed. The pics aren't great, but they show a lot. I took these with the unit plugged in for a total of less than 1 minute.

http://imgur.com/AUnJwN7

http://imgur.com/o6CyGJF

The highest temp I read on the front was 319.3 degrees F. I'm not thinking that's normal. Has anyone seen anything similar before? I'm thinking I may need to hit an RPi-specific board for this one...

It does look/sound like the Pi is fried. The risk of powering the Pi via the 5v PIN is that there isn't any protecting like there is on the micro usb port. I'd verify that the 5v regulator on the HM board is only sending 5v.

Power up the HM Board without the RasPi attached and measure the red +5V points show on this diagram.

Thanks for the reply! Sorry if I worded it funny, but I did check the voltages you mentioned. The two pins on the lower right side of the diagram have 5V when I have a 12V power supply plugged in. They drop to ~3V if the RPi is hooked up.

Follow-up. I went back through my build, step by step and realized that I had one of the 100uf 10V caps swapped with the 100uf 25V cap.

These two: http://imgur.com/dopqY1V

I would think that the 10V cap would be able to withstand 12V for a short period of time before failing, but I'm no expert. I did test it and got 105uf. I forgot to test the 25V cap before re-soldering it.

I have a replacement RPi on the way, but am very nervous about plugging it in until I'm sure it wasn't something I did that fried it in the first place. Does anyone know if having those caps swapped might have caused a problem like that? Any thoughts on other items I should check? Again, many thanks!

Okay, so I jumped in and did some figuring on my own. I wanted to post my results just in case someone else makes the same boneheaded mistake I did. I found that the FQU11P06TU MOSFET was fried. I was getting 12V out of the gate and source, but nothing out of the drain. I swapped it out and, as an added precaution, I replaced the previously mention capacitors as well. As I write this, I've been running 30+ minutes on 12V without issue. I'd have a beer to celebrate, but I think I've already had too much to drink tonight. Though I didn't get much direct assistance in this thread, the wealth of information on this forum has been very helpful and I want to say thank you to all who have made it what it is!