Using two power supplies, but I need a common ground somehow?

My Arduino is controlling a circuit with an external power supply, as well as measuring from a circuit with an external power supply. Obviously the Arduino needs to know what ground is on these, but I take it I should connect the two?

I am using a Duemilanove; my two power supplies are a Lab DC power supply, and a 5V DC power brick. I tried plugging ground into another analog pin and subtracting it, but I got a crazy, unstable signal.

There must be a standard way to do this.

(Also, I did try plugging the three grounds together, and it worked, but my partner is convinced that that was dangerous and a fluke)

Ondross: (Also, I did try plugging the three grounds together, and it worked, but my partner is convinced that that was dangerous and a fluke)

Did you see any smoke come out? Did anything get too hot? How long did you let it run for? If the answers are "no", "no", and "for a while" - then your partner is likely FOS (I won't expand that out - this is a family forum, ya know!).

Now - it'd be a different case if you were trying to parallel the positive sides of things (this can be done with proper power supplies and a small bit of external circuitry, if you were needing more current handling capability, for instance).

In certain situations, there could be "floating ground" issues - but I don't think that is the case here, especially as you have already tried it, and you (and your Arduino) are still among the living, so to speak...

Then again, maybe I'm FOS - and somebody around here (Grumpy Mike or RuggedCircuits - most likely ;) ) will correct me...

Thanks for the responses! I can post a circuit diagram in a few hours. I measured the voltage difference between the negative terminal of the Lab Power supply and the ground pin for the DC Power Brick, and there was .7 volts difference. I think the power brick should stay at it's voltage, but I don't know much about lab power supplies. It has a negative, ground, and positive port. Does the negative port perhaps move to meet what you put on it? Like an "adjustable reference"?

The lab power supply is a "gpc-1850d" and the power brick is a wm-113s. I'm currently researching how the (-) and ground terminals differ on the lab power supply.

Have a read of this:- http://www.thebox.myzen.co.uk/Tutorial/Power_Supplies.html It tells you why you must common the grounds.

Cool. Yeah I knew I had to common the grounds, just didn't know if it was safe with 2 different power supplies. I have them all connected and it works fine, due to, like you said, the negative terminal not being grounded. The only forced voltage on the whole circuit is the ground from the power brick, and everything references that. Thanks!