Power to the Pooter

September 11, 2011 at 3:58 am (inspiron 1545, power cable) ()

It’s all about the electrons.  They magically come out of the wall, flow through the power adapter and into the laptop and ultimately keep the battery charged and the microprocessor happy.  Nobody gives much thought to the power adapter until it quits working and a new adapter must be purchased.

A frequently asked question about the power adapter is whether to use the 65 Watt or the 90 Watt variety.  As with most things that come from Texas, a Dell laptop wants to go with whatever is bigger.  Use a 90 Watt power adapter.  While a lot of Dell laptops will work with a 65 Watt adapter, they often slow down the processor when they sense that they have been connected to a wimpy power supply.

Another area of confusion has to do with the fact that there are three conductors at the end of the power adapter cable (the end which plugs into the laptop) even though it appears there are only two conductors.  The three conductors are marked in the photograph below.

Typical Dell Power Adapter Connector

Typical Dell Power Adapter Connector

The outer metallic surface is essentially the DC common.  This shielding surface is the “DC zero volt” reference point, and when the cable is plugged into the laptop, all of the internal metal shielding components (i.e., the chassis ground) and all of the circuit board ground planes are connected to this DC common.

The inner metallic surface, which one might easily assume is just the inside part of the DC common, is actually a second, separate conductor.  The inner metallic surface provides the main supply voltage.  The inner surface is nominally 19 volts positive of the DC common when the power adapter is plugged into a wall outlet.

So why is there also a center pin?  This center pin is used for communication between the external power adapter and the motherboard inside the laptop.  So what kind of communication is going on here?  In general, this communication signal lets the laptop know that external power is available and that it is time to start charing the battery.

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