ethernet shield causing power issues

I’m having some trouble when using my ethernet shield and powering from a wall wart. I have a circuit that is working out just fine when plugged into usb, but doesn’t work when powered off a wall wart so I started doing some testing.

I powered up my arduino from both a 9v and 12 volt wall wart and everything is fine. But when I add the ethernet shield the regulator starts getting hot and the 5v line only reads 3.1 volts. If I plug this same circuit into my computer via usb everything reads fine and the sketch runs. Could I have a defective shield? I visually checked the board for shorts and it seems fine.

Your power supplies are delivering too much voltage for the onboard regulator to handle both the Arduino and the shield

If you can find a wall wart that produces 7.5 or 8 volts DC (measured with a voltmeter) you should be OK).

Or, use a wall wart that has a regulated output of 5 volts and connect to the 5v pin and ground instead of the DC jack.

Sorry, I'm new to this, could you explain a bit more why that is? I was under the impression the arduino could safely handle between 7 and 12 volts, and the on-board regulator would take care of the voltage difference.

The voltage regulator on the Arduino board is of the simple ( = cheap ) linear type. If you feed it with a voltage X of 7.5V or more, it adjusts its resistance so the Arduino only gets 5V. This works well if the current drawn is not too big. The difference is "burned" in the regulator ( P = I*I*R ). As soon as you add the 2nd shield more current is needed and the regulator gets too hot. If you only measure 3.something volts in that situation I assume it goes into some sort of thermal shutdown, so it doesn't turn into ash.

The closer to 7V (from above 7V) you get with the supply voltage, the better for the onboard regulator. Or as suggested before, feed the Arduino with regulated 5V DC from a good power supply (bypassing the regulator).

The regulator has to get rid of the excess voltage (the difference between the output of your wall wart and 5 volts it needs to drive the chips) as heat. The more current used, the more heat it generates. The combined current of Arduino and the shield produces too much heat and the regulator shuts down

edit: was composing this when madworm posted

Thanks for the extra info. I think I'll go ahead and build a good 5v power supply.

Hello ,

I wanted to know if i could connect a 7,5 v suply with 300 mA to my ethernet shield.Would this destroy it?how many mA should i suply??


You should connect the 7.5 volts to the Arduino external power socket. The Ethernet shield gets its power from the Arduino 5 volt line.

What is the best regulator (replacement of original) to use when supplying 12VDC to the Arduino? I'm having similar issues when using an ethernet shield where the built in regulator is heating up.

What is the best regulator

There is no best regulator it depends on what you are asking it to do.

I'm having similar issues when using an ethernet shield where the built in regulator is heating up.

In that case, try a regulator with a lower voltage, however don't go below 7V because there would not be enough voltage to regulate properly.

I have an old phone charger that puts out about 8v. The standard on board regulator works fine feeding the Arduino , ethernet shield and numerous (low power) sensors powered from it, the regulator only gets mildly warm to touch. I'd measure what your 'wall wart' actually puts out, what its says on the label and reality are often entirely unrelated numbers.......