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I am working on an arduino project that uses transistor switches to power on high power leds that are powered by a separate 24 volt power supply. My issue is that I have two different grounds, making it difficult to bias the transistors. Can I just connect the arduino ground pin to my external power supply's ground or will this damage the board? How can I end up with just one ground? Thanks.

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You should be able to connect your arduino ground to the 24V ground, unless the power supplies are somehow referenced to one another. If one or the other or both of your supplies happens to be a battery, then you don't have a concern. If both supplies are transformer isolated AC supplies, you shouldn't have a problem. Even if both supplies tie their DC return side to the earth-ground lug of the AC mains, you should be ok.

If you bridge the two grounds through a 10K resistor, and a DVM across that resistor shows anything more than a volt or so, your supplies probably aren't isolated enough to just connect the grounds.

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  • \$\begingroup\$ If one of your supplies is the USB cable from your computer, then it is referenced to ground. \$\endgroup\$ – Kevin Vermeer Aug 1 '10 at 4:07
  • \$\begingroup\$ @reemrevnivek- If your computer supplying the USB cable happens to be a laptop, perhaps not so much. \$\endgroup\$ – JustJeff Aug 1 '10 at 4:22
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    \$\begingroup\$ @JustJeff: True, but then it's floating, so it's OK to connect. \$\endgroup\$ – pingswept Aug 1 '10 at 4:32
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If the supplies are electrically isolated then the grounds can be connected. If you are not sure you have isolated supplies I would not connect them.

Another option is to use optical isolators and keep the grounds separate. This can be quite a few additional parts but is a very safe way to interface circuits.

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    \$\begingroup\$ Could you give an example of not electrically isolated supplies? Not sure I've ever encountered this before... \$\endgroup\$ – NickHalden Jun 13 '11 at 19:37
  • \$\begingroup\$ The majority of supplies will have isolation for safety and sometimes for noise immunity. Sometimes you will see non-isolated supplies embedded in an appliance (e.g. solid-state luminaire). I have never seen a non-isolated DC output supply as a stand-alone device. \$\endgroup\$ – jluciani Jun 17 '11 at 1:20

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