W5100 power draw on Nano


I’m building a home monitor using this Nano clone and this Ethernet module.

The test setup ran fine for a week. Then, once when I was making some changes in the program and plugged out wall charger to change to USB cable, there was a faint crack and a smell. (I might have caused a short circuit by touching something with the USB cable end, cannot tell.) The Nano still works but it behaves “weakly”, the LCD characters are dark and look funny, the backlight flickers and it does not have enough power to drive the Ethernet module. I have definitely fried something in there.

As the schematic shows, I fed the Ethernet module directly from 3.3V. Later I read that W5100 can draw up to 150mA and the FTDI chip can supply only 40mA. (I have a Dueminalove and I cannot communicate with it over USB when the Ethernet module is connected. I also had to reprogram the FTDI chip to request 200mA, otherwise my setup does not work when plugged into my PC.)

So, although it worked fine for a week, I’m worried so I thought it’s time to ask for some expert opinion. Since the PCB is already completed, I would appreciate a suggestion for an easy fix. For example, can I directly power the W5100 from 5V? Or what happens if I connect it to 5V through a diode? Or is the power consumption too much for the 5V output as well? Currently I don’t have any other power source planned, power just goes in via the USB connector of the Nano.


you are correct with the 3.3v supply of the nano. you need to really have a separate 3.3v supply (using 78L33 voltage regulator) to power the ethernet circuit. the LCD backlight itself would require up to 200ma from your 5v supply.

Thanks for the tip.

  • My LCD spec sheet says 80mA with backlight, I didn't measure it though.
  • The Ethernet module measured 120mA (spec says 150-180mA)
  • The other items (3 LEDs, 1-wire sensors, DHT22, motion sensor) consume a maximum of 100mA, even less if I turn off the lights while reading sensors
  • And there is the consumption of the Nano itself which I don't know.

The Nano power regulator seems to be good for 500mA, so I was thinking of connecting a LD1117V33 with two capacitors to the 5V leg and feeding the Ethernet module from there. Is it a very bad idea?

your'e right. I just measured my lcd (20x4 with mcp23017 i2c circuit) and it only consumes 6.5ma with full backlight on. I think your plan is fine.