How can I tell if an AC adapter is compatible with my laptop? [duplicate]

Possible Duplicate:
charging laptop with a diffrent manufacture charger

How can I tell if an AC adapter is compatible with my laptop? For example, does the voltage output have to bee the same, or just close? What power measurements matter?

Any AC adapter is considered compatible with any device if it has all the following traits:

• The voltage of the adapter has to be identical.

  It's possible that a voltage can be "close enough" because many devices are built to work within a certain voltage range. But there's no way to know reliably how your device will handle a different voltage.

• The amperage (amps) of the adapter has to be equal or greater than the device.

  A device draws amperage from an adapter so there has to be at least as much amperage available as the device needs.

• The polarity of the adapter has to be identical.

  The polarity of your adapter (and device) is usually indicated by a diagram with two circles showing a plus (+) and a minus (-) pointing to the outer plug and the inner plug.

Of course, the shape of the plug in your device has to match the shape of your adapter so you can plug it in. If you can match all four criteria, you should be okay.

This has been covered before, but you need to ensure:

• the voltage is the same
• the polarity of the tip (of the laptop plug) is the same
• the size and shape of the tip is the same (shape isn't usually an issue as they all tend to be round, but the size might be different).
• the current is greater than or equal to the requirements of the laptop