Hardware Reference
In-Depth Information
Some power supplies have minimum load requirements for both the +5V and +12V sides. According
to IBM specifications for the 192-watt power supply used in the original AT, a minimum load of 7.0
amps was required at +5V and a minimum of 2.5 amps was required at +12V for the supply to work
properly. As long as a motherboard was plugged into the power supply, the motherboard would draw
sufficient +5V at all times to keep those circuits in the supply happy. However, +12V is typically
used only by motors (and not motherboards), and the floppy or optical drive motors are usually off.
Because floppy or optical drives don't present +12V load unless they are spinning, systems without a
hard disk drive could have problems because there wouldn't be enough load on the +12V circuit in
the supply.
To alleviate problems, when IBM used to ship the original AT systems without a hard disk, it plugged
the hard disk drive power cable into a large 5-ohm, 50-watt sandbar resistor that was mounted in a
small metal cage assembly where the drive would have been. The AT case had screw holes on top of
where the hard disk would go, specifically designed to mount this resistor cage.
Note
Several computer stores I knew of in the mid-1980s ordered the diskless AT and installed their
own 20MB or 30MB drives, which they could get more cheaply from sources other than IBM.
They were throwing away the load resistors by the hundreds! I managed to grab a couple at the
time, which is how I know the type of resistor they used.
This resistor would be connected between pin 1 (+12V) and pin 2 (Ground) on the hard disk power
connector. This placed a 2.4-amp load on the supply's +12V output, drawing 28.8 watts of power (it
would get hot!) and thus enabling the supply to operate normally. Note that the cooling fan in most
power supplies draws approximately 0.1-0.25 amps, bringing the total load to 2.5 amps or more. If
the load resistor were missing, the system would intermittently fail to start.
Most of the power supplies in use today do not require as much of a load as the original IBM AT
power supply. In most cases, a minimum load of 0-0.3 amps at +3.3V, 2.0-4.0 amps at +5V, and 0.5-
1.0 amps at +12V is considered acceptable. Most motherboards easily draw the minimum +5V
current by themselves. The standard power supply cooling fan draws only 0.1-0.25 amps, so the
+12V minimum load might still be a problem for a diskless workstation. Generally, the higher the
rating on the supply, the more minimum load that is required. However, exceptions do exist, so this is
a specification you should check when evaluating power supplies.
Some switching power supplies have built-in load resistors and can run in a no-load situation. Most
power supplies don't have internal load resistors but might require only a small load on the +5V line
to operate properly. Some supplies, however, might require +3.3V, +5V, and +12V loads to work; the
only way to know is by checking the documentation for the particular supply in question.
No matter what, if you want to properly and accurately bench test a power supply, be sure you place a
load on at least one (or preferably all) of the positive voltage outputs. This is one reason it is best to
test a supply while it is installed in the system instead of testing it separately on the bench. For
impromptu bench testing, you can use a spare motherboard and one or more hard disk drives to load
the outputs.
Power Supply Ratings
A system manufacturer should be able to provide you the technical specifications of the power
 
 
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