Power Question Arduino Mega

Projects General Guidance
1

I have a small project where I'm using LED buttons to send TCP/IP commands to another processor. The processor reports back an LED state. I have 10 LEDs being driven off 10 PWM pins on an Arduino mega.

It works perfectly well when I have USB connected. With no USB connected I'm using a 12V 1A POE splitter to power the board and provide communication with a single network connection. The POE switch is providing standard POE at 15W. If I disconnect the USB, all the LEDs blink on and off slowly. Sometimes it is fine for about 5 minutes and then it starts flashing. Communication is down so I assume it's a power issue. When on USB power, I'm reading 0.13V between the PWM pin and the ground pin on the board when the LED is on. When the LED is off, it's set to bet very dim and the voltage is 4.8V . If I disconnect the USB, I'm reading 3.5V or 0V on the same pins (as it flashed on and off).

The buttons are Adafruit RGB momentary buttons wired with the 5V from the board to all of the annode pins of the LEDs. The individual color LED cathodes are wired through a 220 ohm resistor (actually measuring at 270) to the PWM pin to sink the current and complete the circuit.

Next to the DC plug on the Arduino board is the power rectifier I believe. It gets very hot with the 12V input. I'll see if I have another power supply that can output 5V to see if that works. I'm also going to track down a POE+ switch or power injector. There are also POE splitters with USB 5V output instead of the 12V DC plug.

Thoughts?

2

If you put 12V to the power jack (or Vin) the 5V power is by the 5V regulator on the board (what you called the power rectifier). The regulator must drop 7V to get the 12V down to 5V. That voltage is dissipated as heat. The regulator is not well heat sinked so cannot provide much current before its output voltage drops off and it overheats and (if you are lucky) shuts down. The answer is to provide a well regulated 5V to the 5V pin, bypassing the, weak, 5V regulator. The power jack and Vin can, really, only be used if there is little current drawn from the on board regulator. Do not power the Mega from the 5V pin and USB at the same time.

3

Are your ground lines connected?

4

I can get a POE splitter that outputs 5V so I’ll look at doing that. That would prevent the regulator from working so hard.

Thanks for a clear and easy to understand answer

Scott

5

If you input power (5V) into the 5V pin, you bypass the on board 5V regulator completely. It does no work.

What is the current capacity of the POE splitter power supply? How much peak current does the project require? Measure with a DMM or calculate.

Post a schematic.

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