fredag 23 oktober 2015

DIY 12V UPS for home network equipment

I have a small cabinet for modem, router, network switch and 8-port patch panel. I've managed to squeeze in a small UPS as well. When I finally got DOCSIS 100/20 Mbit Internet, the cable modem was so big that I had to make some changes to the cabinet. It is good to have an UPS for the equipment. Internet usually works during short power outages and the equipment is not damaged by hard reboots. It also protects somewhat against voltage spikes. So when all of the equipment runs on 12 V it got me thinking. Why do I need an UPS that converts 230 V AC into 12 V DC (for the battery) and then to 230 V DC again, and then at least three small adapters to convert it separately to 12 V DC again? Instead I could replace it with one larger 12 V DC power supply combined with a battery.

I found a 12 V UPS, a Mean Well AD-55A. It outputs 4 A at 13.8 V DC and a small battery can be attached (charge 13.4 V, 0 - 0.23 A). Output voltage is adjustable between 12 - 14.5V.

It has of course to be built into a box to be electrically safe. I had an old monitor switch box that was suitable. A few holes were cut to add a fuse and five "DC plug" 12 V outputs. Add a small combined voltage and current meter and we have this:

You have to find the correct DC plugs that fit into the equipment. Be sure to check the polarity!

Installed in the cabinet together with the small lead battery from the previous UPS:

In the top is the network patch panel. Middle right is the big cable modem (DOCSIS). Middle left is the router, an Ubiquiti Erlite-3. A fantastic router. I mistakenly run it on 5 V when I bought it. It didn't mind, but for sure 12 V is better. At the left on the inside of the cabinet door is a small 8 port Gbit network switch. In the bottom is the battery and UPS. Note that there is no Wifi access point in the cabinet. It wouldn't work well inside the metal cabinet. 

A closer look of the running UPS. The switch, router and cable modem together need 1.56 A DC, so there is a lot of margin to run other equipment if needed.

I mostly did this just because I liked to do it. There is a bit more space in the cabinet for air flow and it is definitely not as hot as before. Some electricity is probably also saved, although I didn't do any measurements. The cost was mostly the power supply/UPS module. It was 28 € + VAT + shipping.

It took a lot of time to make the DC plug patch cables, though. But I don't know if you can get them ready built from somewhere.

If I had a large room for the equipment, I would definitely use a large lead battery with a regular car battery charger. Then use a 12 V - 12 V DC power supply to drive the equipment.