Very Computer

by **Kristian** » Sat, 23 Nov 2002 00:27:40

I have Netra V120 with DC Power Supply.
But i confuse about the value of voltage input required, such as :
a) -48 V
b) Ground
c) -48 V Return
Any one know what is mean -48 V Return ?
Is it the same with +48 V ?

Thanks
Kristianto

by **Chris Lawrenc** » Sat, 23 Nov 2002 01:15:38

Quote:> I have Netra V120 with DC Power Supply.
> But i confuse about the value of voltage input required, such as :
> a) -48 V
> b) Ground
> c) -48 V Return
> Any one know what is mean -48 V Return ?
> Is it the same with +48 V ?

Did you want a Netra with a DC power suppy? What kind of power supply
did you want to plug it into? The DC Netras are supposed to be
installed in telco environments and telco environments will have the
necessary power supplies already installed to connect to the DC Netras.

--
Regards,
Chris Lawrence

by **Thomas Schu** » Sat, 23 Nov 2002 02:01:31

>> I have Netra V120 with DC Power Supply.
>> But i confuse about the value of voltage input required, such as :
>> a) -48 V
>> b) Ground
>> c) -48 V Return
>> Any one know what is mean -48 V Return ?
>> Is it the same with +48 V ?

>Did you want a Netra with a DC power suppy? What kind of power supply
>did you want to plug it into? The DC Netras are supposed to be
>installed in telco environments and telco environments will have the
>necessary power supplies already installed to connect to the DC Netras.

>--
>Regards,
>Chris Lawrence

Ah, that explains the odd voltage and polarity. The -48 V Return would
be the positive lead of the 48V supply who's negative lead is connected
to the -48 V connection. It is NOT +48 to ground. It is possible (likely?)
that the -48 V Return would be connected to Ground in the machine. If
the -48 V Return is not connected to Ground in the machine, it is likely
that the machine expects it to be connected to Ground externally.
--
Tom Schulz

by **Logan Sha** » Sat, 23 Nov 2002 16:35:25

> Ah, that explains the odd voltage and polarity. The -48 V Return would
> be the positive lead of the 48V supply who's negative lead is connected
> to the -48 V connection. It is NOT +48 to ground. It is possible (likely?)
> that the -48 V Return would be connected to Ground in the machine. If
> the -48 V Return is not connected to Ground in the machine, it is likely
> that the machine expects it to be connected to Ground externally.

So in other words, potentially (pun intended, yes) a valid configuration
would be to have four 12-volt batteries wired in series. The negative
terminal of one battery would connect to "-48 V", and the positive of
another would connect to "-48 V Return". Then, a real earth ground
(i.e. wire ultimately connected to a metal stake stuck in the dirt)
would connect to "ground".

Or, to put it another way, there are two things that are important:
1. the potential difference between "-48 V" and "-48 V Return"
is 48 volts (and adequate current can flow between them).
2. "Ground" is connected to the earth.

Note that, the above two conditions don't *necessarily* preclude
"-48 V Return" being at a potential of +48V relative to ground,
as long as "-48 V" is at basically the same potential as ground.

Of course, as you mention, I would measure/research the impedence
between "-48 V Return" and "Ground".

By the way, does anyone else find it ironic that the model in
question is called the "Netra V120"? :-)
^^^
- Logan

--
I'm currently looking for work as a Unix/Solaris
administrator, or Perl/C++/Java developer. Resume
at http://home.austin.rr.com/logan/resume.html.

by **Thomas Schu** » Sun, 24 Nov 2002 00:43:45

>> Ah, that explains the odd voltage and polarity. The -48 V Return would
>> be the positive lead of the 48V supply who's negative lead is connected
>> to the -48 V connection. It is NOT +48 to ground. It is possible (likely?)
>> that the -48 V Return would be connected to Ground in the machine. If
>> the -48 V Return is not connected to Ground in the machine, it is likely
>> that the machine expects it to be connected to Ground externally.

>So in other words, potentially (pun intended, yes) a valid configuration
>would be to have four 12-volt batteries wired in series. The negative
>terminal of one battery would connect to "-48 V", and the positive of
>another would connect to "-48 V Return". Then, a real earth ground
>(i.e. wire ultimately connected to a metal stake stuck in the dirt)
>would connect to "ground".

>Or, to put it another way, there are two things that are important:
>1. the potential difference between "-48 V" and "-48 V Return"
> is 48 volts (and adequate current can flow between them).
>2. "Ground" is connected to the earth.

>Note that, the above two conditions don't *necessarily* preclude
>"-48 V Return" being at a potential of +48V relative to ground,
>as long as "-48 V" is at basically the same potential as ground.

>Of course, as you mention, I would measure/research the impedence
>between "-48 V Return" and "Ground".

The key to understanding this is the fact that these machines were designed
for telephone company use (from an earlier posting). In a telephone
installation, the power may well be a 48 volt battery with a line powered
battery charger to supply the power when AC power is available. At least
in the United States, the positive lead of the battery is connected to
ground (and yes this could be a metal stake stuck in the dirt).

What we don't know is whether the designers of the Netra rely on the
"-48 V Return" being connected to ground or not. It could be designed
such that the only requirement is that there is 48 volts between the
"-48 V" and the "-48 V Return". It could also be designed such that
the "-48 V Return" must be within 0.5 volts or so of ground.

Measuring the impedence between "-48 V Return" and "Ground" could give
you a hint. If there is a very low impedence, then the two are connected
in the machine and an external connection is probably not required, but
would not hurt. If there is a medium impedence, then most likely an
external connection between "-48 V Return" and "Ground" is expected and
needed. If you measure an open circuit, then most likely the "-48 V Return"
can be any voltage to ground (I would still tie to ground anyway).

Quote:>By the way, does anyone else find it ironic that the model in
>question is called the "Netra V120"? :-)
> ^^^
> - Logan

>--
>I'm currently looking for work as a Unix/Solaris
>administrator, or Perl/C++/Java developer. Resume
>at http://home.austin.rr.com/logan/resume.html.

--
Tom Schulz

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