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I'm using an Arduino Uno with a Sainsmart Ethernet shield and a DF Robot shield on top. When I've got it plugged into the USB supply (to my mac), it works - connects to network, testing a sensor updates accordingly in the web xml data. When I plug it into the dc power supply, however, I get a light on the main board and a faint light on the relay shield - nothing on the ethernet shield. I'm using an - ac/dc adapter output 9v dc, 650mA.

I imagine the issue is related to my project using 3 shields instead of the combined ardunio/ethernet board in the tutorial. This is my first project, so i'm still getting oriented... anyone have thoughts on what type of power supply i need to use?

650ma is probably too low for all three boards. You might want to try another power supply with a higher amp rating.

What's the amp requirement for each board you're using ? How does that add up, compared to your 650ma power supply ? ...

Thanks for the info. I've been trying to locate the amp requirements for the three boards, but am having trouble finding useful info. Any pointers on where I can find this?
I'm using:
Arduino Uno R3
SainSmart Ethernet shield W5100
DF Robot relay shield - 4 relays (2 active), 5 sensors in use

Should I worry about connecting a power supply with too high an amp rating?
Thanks

Having a power supply with a high AMP rating is fine. Actually, the higher the better ... It simply
means it'll be able to handle higher loads. What you need to make sure is that the voltage is correct.