Connecting second power supply's ground to GND pin

Hi, I'm planning to play around with an addressable LED strip and my Netduino, and having read some tutorials I've learned that I will need an external power supply to provide enough current to the LEDs. According to the tutorials an old PC power supply suffices since one of the 4 pins on the molex connectors provides the 5V required, and the power supply will be able to provide more than enough current. All well and good.

 

Now, a couple of tutorials (this one and this one, for instance) mention that I should connect the strip's 5V input and GND pins to the 5V and GND pins on one of the power supply's molex connectors, which makes sense. However, the tutorials also mention that I should connect this ground pin to one of the Netduino's GND pins as well, which is the part that worries me.

 

If I power the Netduino via USB (during development, for instance), wouldn't that mean we have two different grounds (namely, the PC's ground and the external power supply's ground) in the same circuit? I'm no expert when it comes to electronics but it seems to me like this might be a recipe for disaster.

 

I'm probably missing something though, so I thought I'd check it here. What would happen if I connected the netduino to my laptop via USB but also connected an external power supply's ground to the netduino GND pin? Would it make a difference if I powered the laptop via battery as opposed to plugged into a wall outlet?

Hi basp,

Good question.

Voltage is the difference in electric pressure. If your signal line is outputting a 3.3V signal, the signal needs a return GND path (and a 0V baseline for that 3.3V to make sense to the other device). Tying the ground from both power sources together solves that issue.

In some cases (especially when working with high voltage gear--which is outside the scope of this post) you'll want to look at opto- or magnetic isolation of signals. Or filters to reduce power supply noise.

Chris