Power Supply Issue

Hi all. I’m having what I believe to be a power issue with an Arduino project. I’ve tried 3 different power supplies (all separately):

  1. My computer via USB B
  2. A 5V USB “outlet” via USB B
  3. A 5V USB “outlet”, through a 5V->9V boost converter, via the arduino’s barrel jack

I can run the sketch without an issue when connected to my computer (supply 1), even if the computer is sleeping and the Arduino program is closed. However, when using supply 2 or 3, everything works EXCEPT reading the pilot light thermocouple through the MAX6675 module. It runs without throwing errors, but always reads 32.0 degrees F from the pilot light thermocouple. The other thermocouples run and update as expected.


  • Arduino Uno (bought ~November 2016)
  • Running Arduino 3 for programming (downloaded October 2017)
  • Code and Wiring Schematic attached

FloShackThermostat_v3.0.ino (6.51 KB)

Is it safe to say that most of those items attached to the board have thier own positive rail from an independent supply ?
More especially the relays.


You're right. The "swtiching" side of the relay is totally isolated from the "power" side.

As for the other components, they're hooked up exactly as shown in the diagram attached to the original post. Does that answer your question, or was something still unclear?

The only thing that stands out to me is the Arduino power supply.
A USB adaptor is only capable of providing 500mA MAX.

I would be looking for something in the 7 to 10 volt range with a higher output.
You may have peaked at the limit of the supply so some things may operate sporadically if set to use the boards supply.

Sorry for late reply was away for a while.


I tried using a boost converter to power the Arduino via the barrel jack:

Same results - thermocouple with the MAX6675 chip reads 32 F all the time.

Most "boost convertors" are quite low current capacity and I would not use one unless I knew it was more than capable in that area.
I would also ensure it stays BELOW 12 Volts (7 to 10).

Never used one of those and it didn't say what the actual spec was on the link.
Not my first choice but who knows.

I would be aiming at supply the components external to the board with thier own power supply with a common ground back to the Arduino. For most current hungry applications it is the only safe route.