Run PC from Positive Ground Telecoms Power

[GJS123]

Hi Everyone,

Am I right in thinking I cannot practically run a convential PC mainboard from a positive ground telecoms -48V DC supply?

There are plenty of -48V ATX-type power supplies around e.g. http://www.sunpower-uk.com/ranges/DC-DC-PC-Power-Supplies/30/default.htm but if I use one of these the mounting holes in the mainboard, plus the IO sheild etc. would connect the the negative output of the PSU to the metal chassis of the PC.  This would be OK if the PC was floating in mid air and not connected to any other equipment but in reality it will be connected to other equipment and bolted into a 19 inch rack so would cause a short, right?

Most (if not all) of the -48V power supply specs say they are isolated input and output but I can't see how this helps.  On the other hand, they must sell these -48V DC-DC ATX power supplies for a reason and I cannot imagine there are motherboards designed for positive ground.

Any advice appreciated.


 

[rs20]

Most (if not all) of the -48V power supply specs say they are isolated input and output but I can't see how this helps.

Isolated means there is no galvanic connection from the input to the output. Which is exactly what you want?

Hint: if you look at the power entry point, you'll see 3 different inputs: V+, V-, and FG (frame ground). The power supply does not care whether you feed in +48V/GND into V+/V- respectively, or whether you feed in GND/-48V. FG is connected to the chassis around the power supply, and the outputs are probably referenced to that (or themselves yet again isolated).

[Neilm]

Telecom circuits are limited to a few hundred milli-amps - 300mA if I recall. I believe the spec is 1A specified as a maximum fault current - so you would not be able to run anything very powerful

[GJS123]

Neilm - Thanks for your reply, but as blueskull says, I am at the exchange (central office) not on the end of an exchange line.

rs20 - Thanks for your reply.  So you are saying that the 0V line of the output, and the 0V of the input can both be connected to frame ground because both input and output are isolated.  If the frame ground of the PSU is isolated from both the input and output terminals this makes sensense to me, kind of.  I'll think I have to get one of the supplies and prod around with the ohmeter to get my head around it.

[madires]

The DC PSUs of network elements usally got two terminals, -48V and RTN (return). The chassis has a grounding terminal for the connection to ground/earth. You want a low impedance path to earth. Sometimes very nice producers label the positive terminal GND.  Or they label the negative terminal GND and the positive 48V. That's when you hear the technician swearing, might be preceded by an exploding sound. 

BTW., for redundancy there are two separate DC "mains", mostly called something like A path and B path.