Ethernet Shield only working if USB is plugged in

Hi :slight_smile:

I have a problem with the Ethernet shield.

If I connect the USB cable (via PC or standard phone USB charger) it works perfectly.

If I connect a 9V adapter there is a problem.
The power (red) led is on, but the link, 100M (orange) leds are not on.

A'm I missing something?

A'm using:
Arduino UNO (ATMEGA328P / ATMEGA8U2)
Arduino Ethernet Shield, SD shield (w5100 version)
9V, 1000mA adaptor.

Any ideas?

Sorry if this is a noob question, I'm new here :slight_smile:

What if you press the reset button after plugging in the board?

Does the Uno work by itself when it's on the external power supply (e.g. with just a blink program)?

The external voltage regulation on my arduino (with an ethernet shield) never seemed to work correctly. When I connected a 9v battery to the external supply, the board voltage jumped to ~7v and with a 12v supply it went to ~8v+. The arduino survived, but the LEDs got brighter before I disconnected the external supply. Measure the voltage at the board 5v pin and see what it is.

Ethernet shield by itself draws 190mA. That's just sitting idle; with only 5V and GND connected. Add 20mA for the Arduino and you are at a total of 210mA for the combined.

9V - 5V = 4V
4V * 210mA = 840mW

That's a pretty large amount of power for the Arduino's on-board regulator to dissipate. It is quite possible the on-board regulator is shutting down.

I have tried the reset button, but the problem was not fixed.

I'll will try to measure the voltage and look for a solution.

I'll write back if I find one :slight_smile:

Thanks for the replies.

It is recommended in a book I have, to use an external power supply. The book stated that USB power would not always be enough.

I'll will try to measure the voltage and look for a solution.

It would be good to look at the 5V line and make sure it is actually 5V when externally powered, ethernet shield is connected, and any other hardware you are using is connected.

Just a thought, but Iโ€™m probably wrong.

  1. USB spec is 500ma

2)You say the power LED is on, double check voltages as stated

  1. If you have an aligator clip handy, ground arduino ground pin with a ground pin on computer case. (This is assuming hat you have a cable plugged in- USB is providing a ground, 9V is notโ€ฆI donโ€™t think ethernet uses a ground, but I could be wrong)

I have exactly this problem and the original topic owner didn't post their solution so I hope it's OK to reopen this thread.

Just as the OP (although with slightly older hardware) I have a Arduino Duemilanove with Arduino Ethernet shield running from a 9V 2A smoothed powersupply.

I've done all my coding with the USB cable plugged in and now I want to run it on it's the Ethernet Shield that won't connect. The Arduino itself is staying powered fine :~

I've checked the 5V and 3.3V lines which are both fine. Any ideas please? :~

Yes the key observation that I have is that the voltage was -7 volts on the 9V batt and when you plugged in the 12 v supply you measured ~8V... Where was your meter ground connected to? as I see the schematic there is no easy way for you to do that. If you reversed the battery D1 would not conduct so the -7 volt reading doesn't make sense and if your power supply was an AC output wall wart and this time the meter negative lead was at ground on the second measurement, that ~ 8V reading would be a correct voltage under load for 12 v AC in but the board wouldn't work. the reason isn't immediately clear... the dc after d1 from an ac supply would 'average' out the pulsating output of D1 and show that voltage but the regulator wouldn't work well or the board because of the 60Hz unfiltered component of the 'measured' voltage... Or you 'Bricked' the board.
which is short of connecting external power to the 5 and 3.3V sources pins (Bad Idea) nearly impossible.


Same problem here.
BUT: When i press the reset button on the shield is starting.

Someone resolve the problem?


Can someone provide a code to emulate pushing reset button because i do not want to hardwire a pushbutton on the shield?