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[on:]

Hello,

I'm using my arduino to measure temperature and then control fans, via a transistor, depending on the temp.  I'm currently using an unregulated 9v wall wart and since this is in a small environment the heat produced by the arduino's voltage regulator dropping the ~12v to 5v interferes with the temperature readings.

To combat this I think a dual power output power supply would do the trick.  I have this guy: http://www.diytrade.com/china/4/products/3794954/Switching_Power_Adapter_with_Double_Output_5V_and_12V.html which outputs 5v and 12v which is perfect for the arduino (5v) and the fans (12v).

The issue is the transistor circuit requires a common ground with the arduino to work.  The power supply has separate grounds for the 12v and 5v.  Is there any way I can have 5v going to the Arduino and 12v to the fans while sharing a  common ground?

Any help is appreciated.  Thank you!

[Reply #1 on:]

Have you checked with a multimeter and made sure the grounds were truly separate? If they really are (ie, you don't measure a short, but rather "infinite" resistance), I am not sure if there would be harm in tying them together or not.

You might want to power up the supply, put a load on each output, then measure the voltage across the grounds; if it is anything but 0 volts, don't tie them together. For such a result, you might be able to tie them together via a couple of diodes, such that voltage can't flow from one to the other, but can still act as a common ground sink.

Maybe someone else here has better ideas...?

 

[Reply #2 on:]

Hmmm the grounds should be tied together.

If you really can't tie the grounds together why not drive the transistor that controls the fans through an opto isolator.

Connect the LED of the optoisolator to through a current limiting resistor to the Arduino output, and ground the LED on your Arduino's ground.

Then use the output of the optoisolator to drive the transistor and that can connect to the other ground for the 12V.

Check the web otherwise wikipedia has a nice intro http://en.wikipedia.org/wiki/Opto-isolator

Otherwise even simpler would be to use a relay  

Just remember one thing, maybe you know it already, when supplying the Arduino with 5V, the regulator on board has a drop out voltage, even though its low you will still need slightly over 5V of you are going to supply power into the regulator, otherwise supply the 5V directly to the 5V "bus" if you are going to use your adapter.

[Reply #3 on:]

There is no issue with connecting the grounds together in this supply. It is quite safe to do.