What is -12V for? Usually -12V shows no voltage at startup

G

Guest

Guest
Archived from groups: alt.comp.periphs.mainboard.asus (More info?)

Hello,

What is it for? I think CPU fan runs on +12V. How about -12V? My computer
always reports that there is no -12V at startup. There will be -12V (like at
this moment, it is -12.27V) again when the computer has been on for a while.

I want to know what components actually run on this voltage. Does it mean
that if there is no -12V, those components won't run during startup until
there is a voltage?

Thanks.

Herbert
 
G

Guest

Guest
Archived from groups: alt.comp.periphs.mainboard.asus (More info?)

"Herbert Chan" <herbert@chan.com> wrote in message
news:4110d693$1_1@rain.i-cable.com...
> Hello,
>
> What is it for? I think CPU fan runs on +12V. How about -12V? My
computer
> always reports that there is no -12V at startup. There will be -12V
(like at
> this moment, it is -12.27V) again when the computer has been on for a
while.
>
> I want to know what components actually run on this voltage. Does it
mean
> that if there is no -12V, those components won't run during startup
until
> there is a voltage?
The -12v rail, was historically used for two things. On early machines
some memories required a -ve bias (hasn't applied for perhaps 10
years...). The other user, is the RS232 ports. A few PCI cards use the
supply too, but these are now very rare.
Many modern boards use a chip like the MAX232 to supply the RS232, and no
longer use this rail at all.
Even if the rail is used by such drivers, they will still wake up and
'run', but just be unable to pull the line properly 'down' when sending
RS232 data.

Best Wishes
 
G

Guest

Guest
Archived from groups: alt.comp.periphs.mainboard.asus (More info?)

RS-232 serial ports require it. On some motherboards, that voltage is
generated on the motherboard for that purpose only, and -12v from the
power supply isn't used.


Herbert Chan wrote:

> Hello,
>
> What is it for? I think CPU fan runs on +12V. How about -12V? My computer
> always reports that there is no -12V at startup. There will be -12V (like at
> this moment, it is -12.27V) again when the computer has been on for a while.
>
> I want to know what components actually run on this voltage. Does it mean
> that if there is no -12V, those components won't run during startup until
> there is a voltage?
>
> Thanks.
>
> Herbert
>
>
 

Paul

Splendid
Mar 30, 2004
5,267
0
25,780
Archived from groups: alt.comp.periphs.mainboard.asus (More info?)

In article <DG6Qc.184$S%2.148@newsfe2-gui.ntli.net>, "Roger Hamlett"
<rogerspamignored@ttelmah.demon.co.uk> wrote:

> "Herbert Chan" <herbert@chan.com> wrote in message
> news:4110d693$1_1@rain.i-cable.com...
> > Hello,
> >
> > What is it for? I think CPU fan runs on +12V. How about -12V? My
> computer
> > always reports that there is no -12V at startup. There will be -12V
> (like at
> > this moment, it is -12.27V) again when the computer has been on for a
> while.
> >
> > I want to know what components actually run on this voltage. Does it
> mean
> > that if there is no -12V, those components won't run during startup
> until
> > there is a voltage?
> The -12v rail, was historically used for two things. On early machines
> some memories required a -ve bias (hasn't applied for perhaps 10
> years...). The other user, is the RS232 ports. A few PCI cards use the
> supply too, but these are now very rare.
> Many modern boards use a chip like the MAX232 to supply the RS232, and no
> longer use this rail at all.
> Even if the rail is used by such drivers, they will still wake up and
> 'run', but just be unable to pull the line properly 'down' when sending
> RS232 data.
>
> Best Wishes

I have just discovered to my shock and horror, that my P4C800-E Deluxe
uses a TI GD75232 for at least the COM2 port, and that is not a MAX
type device. The MAX232 style has a charge pump built in, and converts
a motherboard voltage to make the necessary negative voltage. The charge
pump uses external capacitors to work. It seems the GD75232 is one
of the old style parts, running off +12 and -12V. So, there are
still boards that need the -12V supply.

http://www-s.ti.com/sc/ds/gd75232.pdf

For the following, see pg.17 and C1, C2, C3, C4, which are parts of
the voltage doubler and voltage inverter. +5V makes something like
+10 and -10V.

http://pdfserv.maxim-ic.com/en/ds/MAX220-MAX249.pdf

When a GD75232 costs $0.50 and a MAX232 type costs $1.85, it is not
difficult to see which they will buy.

Paul
 
G

Guest

Guest
Archived from groups: alt.comp.periphs.mainboard.asus (More info?)

Actually my CPU fan is not running, and I think it runs on +12V.

There is a reading of 11.308V for +12V according to Asus probe. So I
suppose the power supply to this voltage is working. So it must be
something wrong with the CPU fan itself, right?

My board is Asus CUSL2 and CPU is Intel III 733MHz.

Thanks.

Herbert

"Roger Hamlett" <rogerspamignored@ttelmah.demon.co.uk> ¦b¶l¥ó
news:DG6Qc.184$S%2.148@newsfe2-gui.ntli.net ¤¤¼¶¼g...
>
> "Herbert Chan" <herbert@chan.com> wrote in message
> news:4110d693$1_1@rain.i-cable.com...
> > Hello,
> >
> > What is it for? I think CPU fan runs on +12V. How about -12V? My
> computer
> > always reports that there is no -12V at startup. There will be -12V
> (like at
> > this moment, it is -12.27V) again when the computer has been on for a
> while.
> >
> > I want to know what components actually run on this voltage. Does it
> mean
> > that if there is no -12V, those components won't run during startup
> until
> > there is a voltage?
> The -12v rail, was historically used for two things. On early machines
> some memories required a -ve bias (hasn't applied for perhaps 10
> years...). The other user, is the RS232 ports. A few PCI cards use the
> supply too, but these are now very rare.
> Many modern boards use a chip like the MAX232 to supply the RS232, and no
> longer use this rail at all.
> Even if the rail is used by such drivers, they will still wake up and
> 'run', but just be unable to pull the line properly 'down' when sending
> RS232 data.
>
> Best Wishes
>
>
 

Ed

Distinguished
Apr 1, 2004
1,253
0
19,280
Archived from groups: alt.comp.periphs.mainboard.asus (More info?)

On Wed, 4 Aug 2004 22:13:50 +0800, "Herbert Chan" <herbert@chan.com>
wrote:

>Actually my CPU fan is not running, and I think it runs on +12V.
>
>There is a reading of 11.308V for +12V according to Asus probe. So I
>suppose the power supply to this voltage is working. So it must be
>something wrong with the CPU fan itself, right?
>
>My board is Asus CUSL2 and CPU is Intel III 733MHz.
>
>Thanks.
>
>Herbert
>

Try a different known working fan on the CPU fan header, it's probably
just a dead fan and needs to be replaced, or it could be the fan header
on the mobo went bad.

Ed
 
G

Guest

Guest
Archived from groups: alt.comp.periphs.mainboard.asus (More info?)

The power fan header and the chassis fan header are free. (I do have a
power fan within the power supply unit, but nothing to plug onto the
motherboard. And I do not have a chassis fan.) The power fan header is
within reach of the CPU fan wire, while the chassis fan header is not. I
have tried plugging the CPU fan wires to the power fan header, and the fan
still does not turn.

So I can suppose the fan has gone dead? I don't have any other fan to try.

I had better lift the lever and take the CPU + heatsink + CPU fan off to
replace the CPU fan, right?

Thanks

Herbert

"Ed" <nomail@hotmail.com> ¦b¶l¥ó
news:1q12h05ncl5e1h90d5ari6mbmble1i9fh1@4ax.com ¤¤¼¶¼g...
> On Wed, 4 Aug 2004 22:13:50 +0800, "Herbert Chan" <herbert@chan.com>
> wrote:
>
> >Actually my CPU fan is not running, and I think it runs on +12V.
> >
> >There is a reading of 11.308V for +12V according to Asus probe. So I
> >suppose the power supply to this voltage is working. So it must be
> >something wrong with the CPU fan itself, right?
> >
> >My board is Asus CUSL2 and CPU is Intel III 733MHz.
> >
> >Thanks.
> >
> >Herbert
> >
>
> Try a different known working fan on the CPU fan header, it's probably
> just a dead fan and needs to be replaced, or it could be the fan header
> on the mobo went bad.
>
> Ed
>
>