Hardware Reference
In-Depth Information
Toalleviateproblems,whenIBMusedtoshiptheoriginalATsystemswithoutaharddisk,
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
ampsat+5V,and0.5-1.0ampsat+12Visconsideredacceptable.Mostmotherboardseas-
ilydrawtheminimum+5Vcurrentbythemselves.Thestandardpowersupplycoolingfan
drawsonly0.1-0.25amps,sothe+12Vminimumloadmightstillbeaproblemforadisk-
less 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 situ-
ation. 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 document-
ation 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.
