[SI-LIST] Re: -48v

Hi All,

This is one of the old mail from one of same grp member regarding -48V.Even
though not satisfactory answer....can be assume right one....
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Yes, I tracked the -48V "Office Battery" issue down time ago.  It goes back

to the turn of the last century in New York City.  Telephone cables were
already being placed underground.  The insulation was paper (called pulp
cables).  The rats attacked the cables so a lead wrap was added.

The cables occupied space in the streets with Edison's DC lighting systems
and the electric trolleys using 600VDC. A problem was encountered with the
lead sheath electroplating from the cables to the trolley rails. The fix
was to reverse the polarity on the telephone office battery. This was an
early form of what is called galvanic protection today.  I don't know if
Edison had a hand in the fix but it sounds like something he would do.

Fred Townsend

----- Original Message -----
From: "Paul Levin" <levinpa@xxxxxxxxxxxxx>
To: <mahesh@xxxxxxxxxx>
Cc: <si-list@xxxxxxxxxxxxx>
Sent: Wednesday, March 24, 2004 3:07 PM
Subject: [SI-LIST] Re: -48v


> Dear All,
>
> The high dc voltage was necessary to keep the powdered carbon microphone
> capsules happy. Banging them against the table helped, too. Ring voltage
> was a completely different phenomena, 90 Vrms/20 Hz, and either centered
> around ground or superimposed on the -48 Vdc, depending on the circuit.
> If you had a hand-cranked magneto ring generator, life could get even
> more exciting; I once measured 1200 Vpp. (BTW, old Army field phones were
> great for inducing night crawlers to crawl out of the ground!)
>
> Regards,
>
> Paul
> ________________
>
> Mahesh Linge wrote:
>
> > Dear Santhosh,
> > A large voltage is required to generate a ringing tone in telephone
> > system.That's why it is recommended to have a 48 Volts.When phone rings
it
> > gives atleast 40-45 Volts across the line.
> >
> > Regards,
> > Mahesh Linge
> > ----- Original Message -----
> > From: "Santhosh E P" <setavala@xxxxxxxxx>
> > To: <si-list@xxxxxxxxxxxxx>
> > Sent: Wednesday, March 24, 2004 11:17 AM
> > Subject: [SI-LIST] -48v
> >
> >
> >
> >>hi all...
> >>is there any specific requirement of having -48v for telephone lines. if
> >>then whats the advantage? why we can't have other voltages to carry the
> >>signal, not even +ve?
> >>regards
> >>santhosh
> >>
> >>
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>
> --
> Paul Levin
> Senior Principal Engineer
> Xyratex Storage Systems
>
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