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

  • Thread starter Thread starter Herbert Chan
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Herbert Chan

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
 
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.
 
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
 
Herbert Chan said:
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?
Click to expand...
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
 
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
Click to expand...

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
 
"Roger Hamlett" said:
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
Click to expand...

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
 
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
 
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