Universal Power Adapters: How Do They Work?
Universal power adapters are unlike the power adapter that ships with your notebook. The most obvious difference is that the aftermarket adapters include a variety of tips. Each tip has a unique fitting to accommodate as many manufacturers' hardware as possible. Vendors use a range of voltages, and varied sizes help prevent connectivity between incompatible components.
Even within a single brand you'll find notebooks that use different voltages and plug sizes. For example, Dell has a standard plug size, but the Inspiron Minis don't follow it.
Here's another difference: the stock power adapter that ships with your notebook supplies voltage at a fixed setting. This is not the case with universal power adapters. In order to satisfy the multiple voltages required of different devices, they must be capable of outputting different voltages as well.
The switching mechanism is in the tips. Inside any given tip, there is a sense resistor that has variable resistance to the current supplied by the power adapter. This resistor drops the voltage as the current crosses its path. The small voltage drops are then fed to a simple comparator circuit, which then is used to further control the DC output from the power supply.
|Tip # (Targus)||Voltage||Pin 1-3 resistance (kΩ)||Pin 1-4 resistance (kΩ)||Connector ID (mm)||Connector OD (mm)||Connector length (mm)||Systems|
|L105||16.5||13.9||168||2.5||5.5||11.1||IBM (older T4X connector)|
If you take measurements from the tips themselves, you'll notice the difference in resistance output. Pins 1 and 2 supply power to the computer, and we've numbered the pins from the center pin in the connector. Pin 1 is negative and pin 2 is positive. These measurements are unique to the tips, not the adapter.
With the technical explanation out of the way, let's compare three universal power adapters.