Mega 2560 power supply

Other Hardware 3rd Party Boards
1

I have a project using a Mega 2560. It has four DS18B20 temperature sensors, data logging. voltage sensing, a RTC, bluetooth. It all controls 4 solid state relays. It ONLY works correctly if I power it with my desktop PC through USB. It will work for hours, turning the relays on and off at the appropriate times. Log to the data card and output through serial via bluetooth to my phone. Very, very rarely it will show -196.6° F on data and serial, not a big deal. I should add that the exact thing happens when I use a very basic sketch that only reads the temps of the 4 sensors and sends to serial.

The problem is I can't use ANY other power source to the Mega, laptop through USB, 7.5V at VIN pin, 12V at power jack, 5V at USB. When using these other power sources the temp readings randomly, on all four readings go to -196.6° F sometimes 4 or 5 in a row. Speed of readings makes no difference. Slowing down to 3000 mS makes no difference.

I can't see much, if any, noise difference anywhere by probing power pins with an oscilloscope with any of the power sources. I've tried adding extra grounds just to make sure

Wiring diagram

5160×3330 1.44 MB

View of project.

Two views of how secondary board is wired

3300×4800 854 KB

Any suggestions?

2

How hot does the voltage regulator on the board get when using 7.5V or 12V ? You probably are drawing more current than it can deliver.

Not sure about the 5V USB; what is the current that the source (charger?) can provide?

3

Problem solved. It was my bench top power supply. It's been running for about 5 hours now using a wall wort as 12V in to main power lug through the voltage reducer (7.5V) and powering the Mega at VIN and powering the relays. The ONLY time I had any dropped temperature readings was when I switched one of the voltage sense wires to the bench top power supply. As soon as I changed to another source for that sensor the problem went away again. Hopefully I won't have the problem when I install it in it's final resting place using the available power there.