Hardware Reference
In-Depth Information
PC/XT/AT and LPX Power Switches
The earliest systems had power switches integrated or built directly into the power supply, which
turned the main AC power to the system on and off. This was a simple design, but because the power
supply was mounted in the rear right of the system, it required reaching around to the right side near
the back of the system to actuate the switch. Also, switching the AC power directly meant the system
couldn't be remotely started without special hardware.
Starting in the late 1980s, systems with LPX power supplies began using remote front panel switches.
These were still AC switches; the only difference was that the AC switch was now mounted remotely
(usually on the front panel of the chassis), rather than integrated in the power supply unit. The switch
was connected to the power supply via a four-wire cable, and the ends of the cable were fitted with
spade connector lugs, which plugged onto the spade connectors on the power switch. The cable from
the power supply to the switch in the case contained four color-coded wires. In addition, a fifth wire
supplying a ground connection to the case was sometimes included. The switch was usually included
with the power supply and heavily shrink-wrapped or insulated where the connector lugs attached, to
prevent electric shock.
This solved the ergonomic problem of reaching the switch, but it still didn't enable remote or
automated system power-up without special hardware. Plus, you now had a 120V AC switch mounted
in the chassis, with wires carrying dangerous voltage through the system. Some of these wires are hot
anytime the system is plugged in (all are hot when the system's turned on), creating a dangerous
environment for the average person when messing around inside the system.
Caution
At least two of the remote power switch leads to a remote-mounted AC power switch in an
AT/LPX supply are energized with 120V AC at all times. You can be electrocuted if you touch
the ends of these wires with the power supply plugged in, even if the unit is turned off! For this
reason, always make sure the power supply is unplugged before connecting or disconnecting
the remote power switch or touching any of the wires connected to it.
The four or five wires are usually color-coded as follows:
Brown and blue —These wires are the live and neutral feed wires from the 120V power cord
to the power supply. These are always hot when the power supply is plugged in.
Black and white —These wires carry the AC feed from the switch back to the power supply.
These leads should be hot only when the power supply is plugged in and the switch is turned on.
Green or green with a yellow stripe —This is the ground lead. It should be connected to the
PC case and should help ground the power supply to the case.
On the switch, the tabs for the leads are usually color-coded; if not, you'll find that most switches
have two parallel tabs and two angled tabs. If no color-coding is on the switch, plug the blue and
brown wires onto the tabs that are parallel to each other and the black and white wires to the tabs that
are angled away from each other. If none of the tabs are angled, simply make sure the blue and brown
wires are plugged into the most closely spaced tabs on one side of the switch and the black and white
wires on the most closely spaced tabs on the other side (see Figure 18.18 ) .
 
 
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