IBM Personal Computer AT
Power supply
### Power supply

The IBM PC AT came with a 192\-watt [switching power supply](/wiki/Switching_power_supply "Switching power supply"), significantly higher than the 130\-watt XT power supply.

According to IBM's documentation, in order to function properly, the AT power supply needed a load of at least 7\.0 amperes on the \+5 V line and a minimum of 2\.5 amperes on its \+12 V line. The power supply would fail to start unless these minimum load requirements were met, but the AT motherboard did not provide much load on the \+12 V line. To solve this problem, entry\-level IBM AT models that did not have a hard drive were shipped with a 5\-ohm, 50\-watt resistor connected on the \+12 V line of the hard disk power connector. In normal operation this resistor drew 2\.4 amperes (dissipating 28\.8 watts), getting fairly hot.


Why did entry-level IBM AT models without a hard drive have a resistor connected to the +12 V line?