Case 2

A Defective Wall-Wart

I needed a wall-wart to power the router just installed to distribute the local intranet to a number of different PCs. Upon plugging into an outlet, it failed.

First Step: Verify that there is power at the outlet where the wall-wart was just plugged. Tests out OKAY.

Note bene: One needs to test for the presence of power with the outlet loaded. Not intoxicated, but with something that draws >1/2A because if there is a high-resistance in series with the power-supply line, the voltmeter will measure it, but the amount of current flowing will be limited by this resistance. So now one might say, "nobody would install a resistor in the line!" All it takes is some corrosion between the Cu wire and the screw of the outlet to form enough resistance to fake you out!


Second Step: The primary winding of a transformer is usually a few to a hundred or so ohms. Measure across the power pins. Measuring here, there was infinite resistance across the pins. That tells us:

In this image, the case was carefully opened by placing the wall-wart in a large vice and using a hack saw, fine tooth, to slowly remove plastic until the case could be separated.


Logical Troubleshooting: