Arduino Mega2560 - power consumption confirmation

Projects General Guidance
1

Please allow me to preface my question with the fact that I am a mechanical engineer, not an electrical or software engineer. I understand inputs, outputs, etc., but am a little lacking on the circuit side of the equation...

I am building a datalogging module using an Arduino Mega2560. I am logging (4) thermocouples to an SD card and displaying data on an LCD. Here is a summary of what components I am using (and how I have them connected):

Thermo1 (MAX6675) (link)

  • VCC from pin22 -- digitalWrite(thermo1_vcc_pin, HIGH)
  • GND from pin23 -- digitalWrite(thermo1_gnd_pin, LOW)
  • GND from pin24 -- digitalWrite(thermo1_so_pin, LOW)
  • GND from pin25 -- digitalWrite(thermo1_cs_pin, LOW)
  • GND from pin26 -- digitalWrite(thermo1_sck_pin, LOW)
    Thermo2 (MAX6675)
  • same as above, using pins 27 -> 31
    Thermo3 (MAX6675)
  • same as above, using pins 32 -> 36
    Thermo4 (MAX6675)
  • same as above, using pins 37 -> 41
    SD Card Reader (link)
  • VCC from 5v pin on shield (near pin53)
  • GND from Gnd pin on shield (near pin53)
  • DO from pin50 (MISO)
  • DI from pin51 (MOSI)
  • CLK from pin52 (SCK)
  • CS from pin53 (labeled as "SS"?)
    I2C LCD (20/4) (link)
  • VCC from 5v pin on shield near reset
  • GND from Gnd pin on shield near reset
  • SDA from pin20 (SDA)
  • SCL from pin21 (SCL)

I tested all of the components individually and developed the code step-by-step until I had everything functioning as expected.

Yesterday, I assembled the "complete system" (including all four thermocouples) on a board for final confirmation and the thermocouples are now acting "wonky" (one reads ~2.5x what it should, one reads ~0.5 what it should) and the SD card isn't opening properly.

According to the datasheet that I was able to find for the MAX6675 (link), to my non-EE eye, it appears as though the SO pin can draw 50mA and the VCC can draw 0.7(1.5max)mA - am I reading the datasheet correctly? If yes, then I am attempting to pull 50mA through all (4) SO pins (24, 29, 34 and 39), totaling 200mA for the SO's alone. I was unable to (easily) find any power consumption data on the other components.

Am I looking at the current consumption all wrong? Should I be wiring this thing up differently?

I have two more brand new Mega2560 boards here, but before I start swapping components around, I wanted to know if I am pulling too much power through the board. Should I be using an external (additional) power supply for some of the components before I "let the smoke out" of the other Mega's that I have sitting here?

Thanks!

2

It's possible that the sdcard module might need its own dedicated DC power supply.

Maybe you can test the system without the sdcard module .... eg. write data to serial monitor window .... temporary testing. And if it is stable..... then it might give clues about what to do.

3

Your connections to the MAX6675 are looking weird. VCC and GND have to be connected to Arduino 5V and Gnd, and no output signal should ever be connected to SO. In detail all signals except CS can be shared, it's a standard SPI interface.

If you want reliable power, connect a 5V DC power supply to the Mega.

4

Well, I now have the circuit functioning exactly as I need it to. Here's what I found:

  • Found a short in my soldering (which is why the two thermocouples were "wonky")
  • Fond out that the SD Card reader doesn't like to have long leads (~18"?) leads between itself and the arduino
  • The SD Card reader likes to have it's own power supply (I used a buck-converter to knock down the 12v Vin down to 4.8v

Thanks!

5

I have a nasty suspicion that you were thinking of the Mega 2560 as a "power source" and that having powered it from 12 V on the "Vin" or "barrel jack", you could use the 5 V pin to supply other components.

An extremely common blunder that presents here. No, you can not. The on-board regulator is - barely - capable of powering the microprocessor itself and a couple of indicator LEDs. For anything else, you must provide a regulated 5 V power supply - such as a buck converter - and when you do that, you might as well use the buck converter to power the Mega 2560 as well through the 5 V pin, and forget about the "Vin" or "barrel jack" which is really, only ever for experimental or testing purposes of the basic board and should never be used for serious applications.

The SD card draws much heavier current when it writes. Put a 200 µF or 1 mF capacitor across the 5 V and ground terminals of the board, especially if it is any distance from the main board.