New to the Group.

[TC Chris]

There are many variations, but the central feature is an incandescent in series with an outlet that the device under test plugs into. Mine was cobbled together years ago from two very old ceramic surface-mount sockets, in series. One holds a bulb, the other a screw-in outlet to plug a device into. Fancier ones use new ceramic bulb holders, outlets, and a switch to bypass the bulb when the test looks AOK.

The bulb has two functions. One is being a resistor in series, to limit the voltage and current to the device you're testing. The other is to be an indicator. If the bulb stays bright, the device is at or near a short circuit, presenting almost no resistance in series with the bulb. A dim bulb means that the device is using power, not a zero resistance, and the device is probably functional.

I just did a Google search. There are lots of online versions. Here's a fancy one.

https://www.youtube.com/watch?v=6SXPj6HFY-Y

Chris Campbell