Power Supply Oddity

  • Thread starter Thread starter Jack Gillis
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Jack Gillis

I just noticed after a long, long time that the PC Health BIOS display for
my Biostar M7VIG motherboard reports 0v for the -5v rail. The system works
just fine so I decided to look into it a little bit. I discovered the power
connector on supply does not have a wire on pin 18 ( that is where the MB
manual says it expects -5v) so of course the BIOS was reporting 0v.

No one around here knows what the -5v might have been used for nor why the
manual calls for it on pin 18 when the it apparently lives happily without
it. I remember that back in TTL days, -5v was commonly used.

Can someone clue me in here?

Thank you
 
Jack said:
I just noticed after a long, long time that the PC Health BIOS display for
my Biostar M7VIG motherboard reports 0v for the -5v rail. The system works
just fine so I decided to look into it a little bit. I discovered the power
connector on supply does not have a wire on pin 18 ( that is where the MB
manual says it expects -5v) so of course the BIOS was reporting 0v.

No one around here knows what the -5v might have been used for nor why the
manual calls for it on pin 18 when the it apparently lives happily without
it. I remember that back in TTL days, -5v was commonly used.

Can someone clue me in here?

Thank you
Click to expand...


Quoted from Upgrading and Repairing PCs by Scott Mueller, 15th Edition
Ch.21

"If you look at a specification sheet for a typical PC power supply, you
can see that the supply generates not only +3.3V, +5V, and +12V, but
also –5V and –12V. The positive voltages seemingly power everything in
the system (logic and motors), so what are the negative voltages used
for? The answer is not much! Some of the power supply designs, such as
the small form factor (SFX) design, no longer include the –5V output for
that reason. The only reason it has remained in most power supply
designs is that –5V is required on the Industry Standard Architecture
(ISA) bus for full backward-compatibility.

Although –5V and –12V are supplied to the motherboard via the power
supply connectors, the motherboard normally uses only the +3.3V, +5V,
and +12V. The –5V is simply routed to the ISA bus on pin B5 so any ISA
cards can use it, even though not many do today. However, as an example,
the analog data separator circuits found in older floppy controllers do
use –5V.

The motherboard logic typically doesn't use –12V either; however, it
might be used in some board designs for serial port or LAN circuits."
 
John Coode said:
Quoted from Upgrading and Repairing PCs by Scott Mueller, 15th Edition
Ch.21

"If you look at a specification sheet for a typical PC power supply, you
can see that the supply generates not only +3.3V, +5V, and +12V, but
also –5V and –12V. The positive voltages seemingly power everything in the
system (logic and motors), so what are the negative voltages used for? The
answer is not much! Some of the power supply designs, such as the small
form factor (SFX) design, no longer include the –5V output for that
reason. The only reason it has remained in most power supply designs is
that –5V is required on the Industry Standard Architecture (ISA) bus for
full backward-compatibility.

Although –5V and –12V are supplied to the motherboard via the power supply
connectors, the motherboard normally uses only the +3.3V, +5V, and +12V.
The –5V is simply routed to the ISA bus on pin B5 so any ISA cards can use
it, even though not many do today. However, as an example, the analog data
separator circuits found in older floppy controllers do use –5V.

The motherboard logic typically doesn't use –12V either; however, it might
be used in some board designs for serial port or LAN circuits."
Click to expand...

Thank you John for the extensive answer. That could explain why when I put
the machine together I found one of my PCI cards wouldn't work. No big
deal.

Thanks again.
 
-5 volts had a number of functions. Originally, processors such as
8080 and other chips required -5 V so that transistors on the substate
(inside chip) were electrically isolated. That - voltage is now
created inside on chips.

A second function involves op amps - analog type circuits that need
both +5 and -5 volts. However that -5 volts can also be created from
the -12 volts.

Some peripheral interfaces also required a + and - voltage. But
again, newer chips create that - voltage from the + voltage. IOW -5
and -12 volts remain mostly for legacy reasons.

TTL, CMOS, and other digital families providing same logic functions
don't use -5 volts. However other (rare) technologies such as ECL did
use - voltages.

Missing -5 volts would cause no problems to any PCI card since PCI
bus does not have -5 volt pins.

Purpose of -5 V would be obvious to those with hardware experience.
Its function makes an excellent benchmark to discover those who
actually have basic electronics knowledge.
 
I just noticed after a long, long time that the PC Health BIOS display for
my Biostar M7VIG motherboard reports 0v for the -5v rail. The system works
just fine so I decided to look into it a little bit. I discovered the power
connector on supply does not have a wire on pin 18 ( that is where the MB
manual says it expects -5v) so of course the BIOS was reporting 0v.

No one around here knows what the -5v might have been used for nor why the
manual calls for it on pin 18 when the it apparently lives happily without
it. I remember that back in TTL days, -5v was commonly used.

Can someone clue me in here?
Click to expand...

In addition to the other comments I've had an encounter with a power
supply that didn't put out -5V and the board itself wouldn't consider
booting.
 
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