Powering arduino through common ac ground

I am currently working on a simple home automation system. I need to send power and data to 3 arduinos from 3 different rooms but i don't want to install new cables. I have telephony cable that i don't use, i am planning to use it but it has only 2 wire. I need 3 for power,ground and signal. I am planning to use common 120VAC ground as arduino's ground, this way i can use telephony wire for +5v and signal. With relays there will be around 500mA of current through ground cable continuous. Is it safe to do it this way ?

Power line ground is earth ground and you can safely use and share it. In the U.S. that's the green wire. (It's NOT safe or legal to use neutral as ground).

The standard home automation protocols use wireless RF or RF through the existing power wiring. If you want to build your own system, I recommend build your controller(s) and buy your switches and controlled outlets. For example, a [u]dimmer switch like this[/u] doesn't require re-wiring, it works locally or remotely, it fits into a standard switch box, and it looks good. It would cost you more to build something like that than to buy it.

Also see [u]SmartHome.co.[/u]

DVDdoug:
Power line ground is earth ground and you can safely use and share it. In the U.S. that's the green wire. (It's NOT safe or legal to use neutral as ground).

The standard home automation protocols use wireless RF or RF through the existing power wiring. If you want to build your own system, I recommend build your controller(s) and buy your switches and controlled outlets. For example, a [u]dimmer switch like this[/u] doesn't require re-wiring, it works locally or remotely, it fits into a standard switch box, and it looks good. It would cost you more to build something like that than to buy it.

Also see [u]SmartHome.co.[/u]

That is ALL assuming your house was and still is properly wired.

You may have problems with signaling using the house ground. Remember the signal must also make a complete circuit.

Paul

Couple of thoughts here:

In the North American system, ground is not to be used as a current carrying conductor. It's a fail safe path for current in the event of a short. The neutral wire is the current carrying conductor back to ground at the circuit breaker panel. In fact, that's what GFCI protected outlets are looking for...current taking a return path other than the neutral. E.g. ground...possibly via a human. Note: I'm not saying to use the neutral as your return path. Clearly that's not gonna work out well. I'm simply pointing out that the purpose of ground in household AC circuits is very different from ground in your typical Arduino project.

Also, 500mA return current from relays and such...from where? Sending 500mA supply down the telephone wires? Depending on the wire gauge (probably 22-26 gauge) you may be pushing the safe limits...depending on your preferred safety margins. Check out this site for some additional guidance and see how you compare for your conditions.

https://www.powerstream.com/Wire_Size.htm

That site also provides some info on my last thought...voltage drop. Copper is a great conductor, but in the real world, it's not perfect and has some resistance. This will cause a measurable drop in voltage...especially over significant distance, with small gauge wire, and 500mA of current. Again, your speciic conditions will cause this value to vary significantly. With only 5 VDC...you don't have a lot to lose. Look into it.

Interesting project! But 500mA is capable of causing fire in a wall if not done properly. And using ground as a conductor certainly won't pass any code inspections and could be an insurance liability in the event the worst happens.

Proceed with caution!

GarageGeek:
Couple of thoughts here:

In the North American system, ground is not to be used as a current carrying conductor. It's a fail safe path for current in the event of a short. The neutral wire is the current carrying conductor back to ground at the circuit breaker panel. In fact, that's what GFCI protected outlets are looking for...current taking a return path other than the neutral. E.g. ground...possibly via a human. Note: I'm not saying to use the neutral as your return path. Clearly that's not gonna work out well. I'm simply pointing out that the purpose of ground in household AC circuits is very different from ground in your typical Arduino project.

Also, 500mA return current from relays and such...from where? Sending 500mA supply down the telephone wires? Depending on the wire gauge (probably 22-26 gauge) you may be pushing the safe limits...depending on your preferred safety margins. Check out this site for some additional guidance and see how you compare for your conditions.

American Wire Gauge Chart and AWG Electrical Current Load Limits table with ampacities, wire sizes, skin depth frequencies and wire breaking strength

That site also provides some info on my last thought...voltage drop. Copper is a great conductor, but in the real world, it's not perfect and has some resistance. This will cause a measurable drop in voltage...especially over significant distance, with small gauge wire, and 500mA of current. Again, your speciic conditions will cause this value to vary significantly. With only 5 VDC...you don't have a lot to lose. Look into it.

Interesting project! But 500mA is capable of causing fire in a wall if not done properly. And using ground as a conductor certainly won't pass any code inspections and could be an insurance liability in the event the worst happens.

Proceed with caution!

I thought about voltage drop and i tested arduino with different voltages. Actually arduino runs all the way down to the 2.9v without problem. I think it won't be a problem. But you're right about risks. I want to run 7 relays on a thin wire, looks like it won't work and sadly i need to install new cables

Synchronizer:
I thought about voltage drop and i tested arduino with different voltages. Actually arduino runs all the way down to the 2.9v without problem. I think it won't be a problem. But you're right about risks. I want to run 7 relays on a thin wire, looks like it won't work and sadly i need to install new cables

Sorry to upset the project, but I just want you to be safe.

Also, the voltage required for a chip to operate properly depends heavily on the speed you're running it at. Not sure what Arduino you're using, but as an example, the classic ATMEGA328 chip used in many variants runs at 16 MHz from an external oscillator and would require more than your 2.9v to RELIABLY operate. Check out this chart from the datasheet...

https://goo.gl/images/3sTeTQ

There are methods of reducing clock speed to operate safely in those regions, but that's a whole other discussion.

Also, while you can certainly get your chip to operate at less than 5v...the 5v relays you planned to use arent going to be happy with that. Make sure all components of your circuits are compatible with your supply.

And so goes the seemingly endless design and redesign routine! Why do we all enjoy this so much!? :wink:

Thank you so much. Actually you didn't upset the project. Your comments are pretty helpful :slight_smile: