Splitting 12V DC Power Output

[mbelanger]

In my home all my CAT 6 wiring terminates in one of those Leviton in wall enclosures, so space is at a premium. My modem, firewall and switch all are 12V, but unfortunately have wall-wart transformers, but the enclosure only has a 2 AC outlets with no room for an extension cord. Given that all three devices have 12V 1A draw, is it safe to assume that I can split a quality 12V 4-5A power supply output into three DC barrel plugs?

I'm so happy to have found this forum and greatly appreciate that there is a section for newbs.

Thanks,
-MB

[David Hess]

That will work as long as all of the devices do not share a common as will be the case with ethernet connected devices.

[ebastler]

That will work as long as all of the devices do not share a common...

Am I misunderstanding something, or was this just a typo?
Thanks for confiming or elaborating!

[Bratster]

It should work just fine unless one of the devices is doing something stupid with the power supply input.

You can use an ohm meter to check between the negative on the barrel jack and the case of the device to see if they are connected together as ground. In in which case everything's good.



Sent from my Pixel 2 XL using Tapatalk

[mariush]

You'll be fine.

Pretty much everything in those three devices runs at maximum 5v ... most likely 3.3v-3.6v for the main networking IC and 2.5v or less for Flash memory / RAM / configuration eeproms if any.
They'll all have some switching regulator inside to convert 12v to a lower voltage like 3.6v or 5v and then use LDOs (linear regulators) for smaller voltages.

The 12v is chosen because it's a common voltage that makes the wallwart adapters bundled with devices cheap, mass produced, easy to source.

Being switching regulators, the devices will probably work with as little as 7.5v on the input but I can't say this with absolute certainty and with higher voltage you get less current (ex modem may have a 12v 1A adapter but only consume 0.7A max ... so around 10w ... a 5v 2A adapter would have worked just as well but it may be more expensive and you'd need thicker wires due to wire resistance, so they just went with 12v)

[Ian.M]

OTOH you will loose a lot of the protection your network currently has against external high voltage transients.   

The current lineup is:

Code: [Select]

{ADSL or Cable}--- ][Modem][ --- ][Firewall][ --- ][Switch][ ---{your LAN}

Each of the units has isolated inputs and outputs (shown as ][ above), and its own isolated PSU.

Your proposal is:

Code: [Select]

{ADSL or Cable}--- ][Modem][ --- ][Firewall][ --- ][Switch][ ---{your LAN}
                       |              |               |
                       +--------------+---------------+
which bypasses the isolation provided by the magnetics on the Modem's ethernet port, both ethernet magnetics on the firewall and the magnetics on the upstream port of the switch.

That could get real expensive if there is a lightning strike anywhere near the phone line between your house and the exchange.

[David Hess]

That will work as long as all of the devices do not share a common...

Am I misunderstanding something, or was this just a typo?
Thanks for confiming or elaborating!

Usually someone would use the term "ground" instead of "common" but it is more properly refereed to as a common.

By replacing all of the separate floating power supplies with one, you are creating a common between all of the powered devices.  This will work as long as the devices are not otherwise connected in a way which creates a different common.

Usually this is not a problem because most devices these days use a negative ground (1) but there are exceptions sometimes like if the designer included a reverse protection diode in the negative side, included a full bridge rectifier so that a DC or AC power input could be used, or chose to regulate the negative side of the power input for some reason instead of the positive side.  If everything is connected using Ethernet, then it is irrelevant because Ethernet provides galvanic isolation.

So it will most likely work fine but if there is a problem, sharing common through some other path is likely the issue.

(1) But this is not a guarantee since if the devices uses a negative ground and have some other galvanic connection between them, this creates a ground loop which may be problematical.  Again, if only Ethernet connections are made between devices, this is irrelevant because Ethernet provides galvanic isolation.  This is *why* Ethernet provides galvanic isolation.