Doublecheck wiring PC, Modem, Phone - Page 1
Forum section: electronics / projects

author: con-f-use
author_rank: Unknown
timestamp: August 26, 2015, 11:16:28 am
content: So I noticed, that my media server, my cable modem (internet), my phone (connected to the cable modem), my Wii, my cooling fans for all of this and my external disks all run of 12 VDC. They are also physically close together, seeing they "live" in the same closet. I figured it might be better for my electricity bill and general tidiness to run them all from a big 12 VDC, 20A supply. That would also give me the option to put a car battery in there and have a poor man's UPS.My concern is, when I connect them all, I might create unwanted potential difference, i.e. between the ethernet on the media server and die cable modem connected to, well, the cable network. Or phone and cable network. Is that a real concern? What should I measure and check before I do this? Am I missing something?

author: TheElectricChicken
author_rank: Frequent Contributor
timestamp: August 26, 2015, 03:47:51 pm
content: leave the phone and modem on one island and put the computer and media center on the 12v island. Phone systems work very differently and on different voltages and different circuits than the house wiring. You don't get a 0v on a telephone line into the home as a general and widespread rule.Good idea btw. I also suggest not to give up the chance to add a 12v solar panel or two to the setup

author: con-f-use
author_rank: Unknown
timestamp: August 26, 2015, 04:00:51 pm
content: Okay, thanks that's what I thought. I don't want an extra UPS battery for the modem, so I guess in one of the rare power out's I'll have to live without internet.As far as I know ethernet jacks (should) be isolated with magnetics. I wonder why modem's and phones aren't. If every ethernet switch can do it, it shouldn't be that much of cost factor.

author: TheElectricChicken
author_rank: Frequent Contributor
timestamp: August 26, 2015, 05:20:43 pm
content: Quote from: con-f-use on August 26, 2015, 04:00:51 pmdo you mean to type optics or magnetics ?Can you make an extra UPS for the modem ?

author: con-f-use
author_rank: Unknown
timestamp: August 26, 2015, 06:07:19 pm
content: I could, but then I'd defeat the purpose of having less cables and stuff in my closet.No, I meant magnetics, most decent quality ethernet jacks incorporate little transformers to remove any offset in potential.

author: Ian.M
author_rank: Super Contributor
timestamp: August 26, 2015, 06:16:02 pm
content: Most dialup or DSL/ADSL modems also isolate the line interface. However don't count on a cable modem having its line interface isolated from its PSU. Phones may also not isolate the line interface from the PSU.IMHO it would be unwise to use the same supply for the phone and modem as that may bridge across the isolation resulting in either immediate failure or serious damage in the next thunderstorm.

author: SeanB
author_rank: Super Contributor
timestamp: August 26, 2015, 06:34:09 pm
content: ADSL modem is isolated from the line by either a pair of 2kV capacitors or a transformer with 2kV isolation ( or both) so it will be fine connected to 12V. Cordless phone power supply however will sit at -48V below ground when the phone is on hook, and will swing either side by 90V with the ring signal.Cable modem likely the same, as there is no guarantee the cable shield is not going to have a voltage difference to the house side. Simplest is to check continuity from cable shield to any of the power pins, if there is no continuity then there is isolation. Ethernet in any case ( aside from POE stuff) the cable is DC isolated with 10n 2kV ceramic capacitors and 1M resistors at each end from ground, to minimise common mode distortion of the differential data.Only issue is the external HDD drives, they will have a common terminal with the USB cable shield, so you can get ground current flowing there.You might want to buy a few isolated DC-Dc converters ( appropriate ratings for each device) and power them from these instead. There will still be an efficiency gain even with a converter for each device, just from the lower losses in the UPS transformer.

author: con-f-use
author_rank: Unknown
timestamp: August 26, 2015, 06:46:45 pm
content: USB ground current would surprise me, seeing the media server doesn't have a ground connection, it is a strange model that runs from an ACDC 12V adapter.

author: con-f-use
author_rank: Unknown
timestamp: August 27, 2015, 08:29:11 am
content: Is there such a thing as a good (relatively) cheap, efficient and isolated 12VDC to 12VDC converter with 2 Amp output current rating? I image there should be something for cars.

author: con-f-use
author_rank: Unknown
timestamp: September 04, 2015, 09:25:19 am
content: Okay, it's not the USB ground that's dangerous, it's that my HDMI shielding was grounded in the projectors side. Luckily I measured everything.