Which power supply approach for wall tablet? (USA)

Hardware
1

I am thinking a lot how to best supply my wall tablet (currently 5VDC) from mains. I have a few restrictions:

  1. I do not have an outlet close by and I cannot use a normal plug-in power brick

  2. I will run a low voltage wire from the attic to the tablet (~2m)

  3. In the attic I plan to install a junction box. The available electrical circuit has a fixed load of >50% and I am not allowed to add a normal receptacle.

  4. I want everything to be NEC code compliant in the US. This means in particular, I need a hardwired solution that is UL listed or ETL certified. DIY parts can be in the low-voltage domain but if possible, would avoid as well.

I have three ideas:

  1. USB-only outlet (this one). I install this in a metal junction box in attic and bring down a ~2m USB cable. (Remember, I cannot use an ordinary outlet because that circuit has >50% fixed load already)

  2. Doorbell transformer. I use an ordinary doorbell transformer to get 24V AC from 120V AC and bring down 24V AC. At the tablet I use a small rectifier/DC-DC converter (this one).

  3. PoE: I just do a complete run of CAT6 cable and use PoE

Efficiency is very important for me. (1) feels somewhat straight forward but I am concerned about IR drop at just 5V for ~2m.
(2) would mitigate this a bit (IR loss is nearly 5x lower for 24V AC) but I am concerned that doorbell transformers are not as efficient. I did not find data for it, but it concerns me that they feel warm, even though nothing is connected. Furthermore, running down 24V AC is somewhat more flexible (because I can change the converter to a different voltage if needed) but also more homebrew.
Finally (3) works as I have a PoE switch. But the CAT6 cable run would be long (30m instead of 2m) and again I am concerned about efficiency/IR drop (voltage is up to 50V but still half of line voltage). So I am not 100% sold by this.

What would you do?

2

I would keep it simple and probably go with option 1 or 3. Convenience, ease of maintenance and cost would be the drivers for me.

Attic temp may be an issue, the unit is only rated to 60c, could be an issue in some hot climates.

Option 2 just seems too much of a hack. I sort of doubt it would really be much more efficient in practice.

I don’t really get the efficiency worries for option 1. A 2m usb power cable is pretty standard. I’d actually be surprised if the outlet truly outputs an even 5v despite the specsheet. I would expect more like 5.1 or so as the minimum. If the worry is strictly about being green, I still can’t see stressing over loss that would amount to maybe $1 a year at 10% loss?

Why not place the USB outlet in closer proximity? Cosmetics? Not much difference in running 2m of romex vs an ethernet or USB cable.

1 Like
3

Thank you! So as I was continuing to think about it, PoE is sub optimal as well because I may want to add in a Shelly to allow for controlled charging (otherwise battery will die very quickly). Shelly is UL listed and I can put it directly into the junction box and control the USB outlet. Wouldn’t work with PoE.

Got point, but not here in the Bay Area. Very mild year around :wink:

Why not place the USB outlet in closer proximity? Cosmetics? Not much difference in running 2m of romex vs an ethernet or USB cable.

Yes, I don’t want cables on the walls. I am replacing an old ADT security panel and the hole in the wall exists already and there is a direct passage to the attic on top where I can place the receptable.

4

Maybe a recessed box like would go behind a tv with tablet over it?

5

If there was an ADT security panel there, aren’t there already wires run from a central location?

I did something similar where I replaced a couple of alarm system wall panels with tablets; I just ran 5v over two of the wires that were coming from the alarm control box elsewhere in the house.

6

Yes that would be the nice solution definitely but very complicated/ lots of work. Given that I’m not 100% sure how permanent this tablet will be at that location. I’d rather just do it a but simpler (which is tapping the existing circuit in the attic)

1 Like
7

Yes and no. Wires are there but they have been cut. They went to the basement (a phone cable). Since there is a wall cavity which is accessible from the attic, it’s easier to just run a new low voltage wire

8

Hey Scott, mind sharing more about how you did this and parts used?

In a similar situation; 3 locations with old alarm wiring.

In particular am curious about:

  1. What did you use to power the alarm wiring at the source?
  2. How did you step down the voltage at the panel locations for tablet usb power?
  3. Are you doing anything to control over charging the devices?
9

Sure!

  1. I needed 12v power at the source for the Zooz Multi-relays I’m using to monitor all the door/window sensors, so…

  2. I just added a 12V → 5V step-down transformer (specifically this one) at the source, and am sending 5V over the old alarm control panel wires. If you don’t need any 12V power, you should be able to just use a 120V → 5V transformer.

  3. The cheap Lenovo android tablets I’m using as control panels have software charging limiters, so I set them to not charge more than 80%.

Hope that helps! Here’s a pic of the final setup:

image
image1920Ă—1480 211 KB

10

Scott, this is the setup I need to use also. Thanks for the great documentation.

I need to understand one more piece and that is how you connected the 5V alarm wire to the tablet. Can you describe that?
Thanks!

11

Sure! At each tablet location, I just connected the two 5V leads to a USB connector.

The “Makes By Mike” tablet wall mounts I bought came with really nice flush-mount usb-to-bare-wire connectors, but there are lots of other options out there (just search e.g. “micro usb power lead” on Amazon).

1 Like