Hardware Reference
In-Depth Information
Negative DC Voltages
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 −12V and possibly −5V. Al-
though −12V and (possibly) −5V are supplied to the motherboard via the power supply
connectors, the motherboard normally uses only the +3.3V, +5V, and +12V. If present, the
−5V is simply routed to the ISA bus on pin B5 so any ISA cards can use it, even though
very few ever have. However, as an example, the analog data separator circuits found in
older floppy controllers did use −5V. The motherboard logic typically doesn't use −12V
either; however, it might be used in some board designs for serial port or local area net-
work (LAN) circuits.
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! In fact, −5V was re-
movedfromtheATX12V1.3andlaterspecifications.Theonlyreasonitremainedinmost
power supply designs for many years is that −5V was required on the ISA bus for full
backwardcompatibility.Because modernPCsnolongerincludeISAslots,the−5Vsignal
was deemed as no longer necessary. However, if you are installing a new power supply
in a system with an older motherboard that incorporates ISA bus slots, you want a supply
that does include the −5V signal.
Note
Theloadplacedonthe−12Voutputbyanintegrated LANadapterissmall.Forexample,the
integrated 10/100 Ethernet adapter in the Intel D815EEAL motherboard uses only 10mA of
+12V and 10mA of −12V (0.01 amps each) to operate.
Although older serial port circuits used +/−12V outputs, today most run only on +3.3V or
+5V.
Themainfunctionofthe+12Vpoweristorundiskdrivemotorsaswellasthehigher-out-
put processor voltage regulators in some of the newer boards. Usually, a large amount of
+12V current is available from the power supply, especially in those designed for systems
with a large number of drive bays (such as in a tower configuration). Besides disk drive
motorsandnewerCPUvoltageregulators,the+12Vsupplyisusedbyanycoolingfansin
the system—which, of course, should always be running. A single cooling fan can draw
between 100mA and 250mA (0.1-0.25 amps); however, most newer fans use the lower
100mA figure. Note that although most fans in desktop systems run on +12V, portable
systems can use fans that run on +5V or even +3.3V.
Systems with modern form factors based on the ATX or BTX standards include another
special signal. This feature, called
PS_ON
, can turn the power supply (and thus the sys-
