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!

wallwart

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.

Wallwart2

Logical Troubleshooting: