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By satacoy
#189812
I'm having an issue when using the Sparkfun Thing Dev Board with the Sparkfun Thermocouple breakout board (MAX31855k).

I have a basic sketch working that reads the temperature of the thermocouple from the breakout board successfully. This works fine when I'm powering the Thing from the USB cord. However, when I switch to a fixed power supply, about 30% of the time I get bad readings.

I've simplified my circuit to the simplest form, but can't seem to figure out what's going on. I'm using a Sparkfun 12v/5v 2a power supply (https://www.sparkfun.com/products/11296), with the 5 volt output going to the Vin on the Thing. I'm using the 3.3v output of the Thing to power the thermocouple board.

When I'm not using the power supply and relying on the USB power, the program runs fine for hours, dutifully broadcasting the temperature of the thermocouple without error. Once I unplug the USB, and power via the external power supply, errors pop up rapidly. It's a bit difficult to debug, since my serial output is no longer available when I remove the USB. I do flash the onboard LED for various error conditions, and I'm getting the "SCG Fault: Thermocouple is shorted to GND" message most of the time, although I do see pretty much all the other error codes over time.

If I ground the thermocouple metal sheathing, I continuously see the error. I've tried multiple thermocouples, I've reflowed all my solder joints (they all look good). I've played around with the SPI speeds, thinking maybe the ESP8266 isn't up to the default speeds, but the errors don't disappear.

Regardless of that troubleshooting, it doesn't explain why things work fine when using USB, but not when using external power.

Any ideas? Am I missing some big obvious different between USB power and external power for the Thing board?
By satacoy
#189813
Nothing like banging your head against a problem all day, finally asking for help, then figuring things out a few minutes later...

It appears that the power supply is to blame. Even though it shows a solid 5.0v and 12.0v output, once I hooked my breadboard up to a regulated bench power supply, the problem went away. I'll do some more investigation tomorrow, but it appears that the power supply is dodgy. Reading the comments on that power supply page would have probably clued me in quicker.

Pete