Path: utzoo!utgpu!news-server.csri.toronto.edu!rutgers!ub!kitty!larry
From: larry@kitty.UUCP (Larry Lippman)
Newsgroups: comp.dcom.modems
Subject: Re: Why do they insist on power cubes???
Summary: A word of caution about replacing power cubes with power supplies
Message-ID: <4299@kitty.UUCP>
Date: 5 Jan 91 18:22:53 GMT
References: <139957@pyramid.pyramid.com>
Distribution: usa
Organization: Recognition Research Corp., Clarence, NY
Lines: 36

In article <139957@pyramid.pyramid.com>, lstowell@pyrnova.pyramid.com (Lon Stowell) writes:
> Most vendors provide a rack mount type option which uses single
> supply for a group of modems....
> 
> Although it is not rocket science to scrap the glob of power
> cubes and just wire in a single commercial supply, I have a
> hunch the FCC would cast a jaundiced eye...both from an
> emissions/conduction view as well as (dial modems) protection of
> the Public Switched Network....

	I would not be overly concerned about an RFI emission problem
(from a *practical* and not regulatory standpoint, however) provided
that a powerline filter were installed on any common power supply.

	I have replaced power cubes with common power supplies and
transformers, but a few words of caution are in order for reasons other
than the above:

1.	Many modems depend upon electrical isolation of their power cube,
	and tying more than one modem into a common DC power supply or
	AC transformer can result in serious impairment of the modem.

	As an example, some AT&T modem racks use a common transformer
	having independent *isolated* secondary windings.

2.	Some modems have an internal rectifier and DC power supply, with
	the power cube being just a transformer. It may be a reasonable
	temptation to replace such a power cube with a DC power supply
	(say 12 volts, which is pretty common).  While this will work in
	some cases (I have done it successfully), in others the modem
	rectifier arrangement is such that AC is *necessary* in order to
	obtain both a positive and negative internal DC supply.

Larry Lippman @ Recognition Research Corp.  "Have you hugged your cat today?"
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