Low Voltage AC to DC Outdoor System?

[lchackr]

I am working on building a system that includes outdoor lighting and data gathering sensors (managed by Raspberry Pi units).  The lighting and Raspberry Pi units plus associated sensors will need 12VDC and 5VDC respectively.  For safety and ease of installation I would prefer to use low voltage AC for the distribution, especially the runs of up to 100 meters/300 feet, then convert that to DC at the destination.  If DC is reasonable for this I can consider it but I am concerned about voltage drop.

Are there some relatively cost-effective, reliable, and ready-built components that will let me do this please let me know.  None of the components I am planning should demand high amperage and I am flexible on the input AC voltage.  Thanks a lot guys.

[Someone]

Voltage drop still occurs in an AC system. But while it is easy to use a transformer to go up to/down from higher voltages for distribution, if you stick to the safety standards as limits for voltage you will get more power from DC.

[jhpadjustable]

Once you know the worst-case current your Pis will require during operation, which will probably be when first turned on, consult a wire ampacity table to find the lightest useful gauge for your run, then use the resistance per meter of your chosen gauge to sketch out your run, including the appropriate wire resistance between nodes to detemine voltage drop between nodes.

Are you sure there isn't a more appropriate, less power-hungry platform for what you want to do? I'd think ESP8266 should be plenty for light-duty control and data submission over wifi.

[lchackr]

Thank you for the advice.  After looking at the numbers I do think that a DC-DC system will probably be better for this purpose. 

Are you sure there isn't a more appropriate, less power-hungry platform for what you want to do? I'd think ESP8266 should be plenty for light-duty control and data submission over wifi.

In addition to basic sensors I also plan to collect visual data from cameras and do some near real-time data analysis so I will need the faster ARM in the Pi for that. 

Now that it looks like I will focus on DC power, do you have any recommendations for a mains AC to DC power supply that offers battery backup capability?  (I am okay with an external lead-acid battery as backup.)

[jhpadjustable]

In addition to basic sensors I also plan to collect visual data from cameras and do some near real-time data analysis so I will need the faster ARM in the Pi for that. 

Got it. From a Pi BBS users tests it sounds wise to plan for about 3 watts per Pi in the worst case, which doesn't include peripherals, cameras, or cable losses. How many nodes are we talking about on any given branch and what are the power requirements for the peripherals and cameras? Also, how many lights on a branch and how much power will they consume (assuming all lights on as a worst case)? These are important data for informing the choice of power supply and the wire you use. You will need to supply a voltage somewhat above 12V at the "hub" if you want to be able to provide the full 12V at the far end of a run.

Now that it looks like I will focus on DC power, do you have any recommendations for a mains AC to DC power supply that offers battery backup capability?  (I am okay with an external lead-acid battery as backup.)

I'd punt to a small computer UPS on the ac side instead, which you might be able to augment with larger batteries.

[james_s]

The only advantage of AC for low voltage outdoor lighting is that it is much less prone to electrolytic corrosion if you have connections that get wet. As long as you keep everything sealed it should make no difference. If you want to minimize drop, you can use the highest reasonable voltage, IIRC 48V is still considered "low voltage" and then use efficient buck regulators at the loads. It's probably not necessary to go that high though, whether it's worthwhile to do something like this depends on the amount of power you need.