I had a repairman in today to fix my dishwasher. The first thing he did was pull out a device that looked like what Dr. McCoy uses to check his patients. He just waved it in front of the control panel for the dishwasher and in front of nearby electrical switches and it lit up and tweedled to indicate power was available. Now, I can probably guess how this thing works, but what exactly is it? Can anyone provide an online link to one?
Something like this?
Inductive or magnetic pickup; nothing at all Star Trek about them.
Actually, that more of a “Hare and Hound” circuit tracer.
This is probably closer to what the repair guy used.
Yep, that is very much what he appeared to be using.
While I recognize that this isn’t real Star Trek technology, it was a device I had never seen before and was very impressive (and Star Trek-like) in its operation.
When I worked briefly in auido-video installation, I learned that those are very useful in finding the power wires in the wall before you cut in, and for finding the end of the right coaxial cable, if you attach a signal-maker thingy to the first end. The second end then makes the tracer warble distinctly when you get them close together.
I have a couple of them. They detect the electric field around energized wires; no current is necessary to flow. They work by passing the probe end near the wire under test and the electric field apples a small voltage to the gate of a field-effect transistor (FET). When the voltage exceeds the FET’s turn on threshold it, well, turns on and activates a light and/or buzzer.
Does that mean that the probe must be in motion to detect the field?
Not for AC - with AC, the field itself is in motion.
Er… 'scuse my ignorance, but if no current is flowing, in what sense are the wires “energized”?
There’s still voltage present. Okay, technically there’s still a minute quantity of current due to capacitive leakaged and dielectric imperfections, but even if there weren’t, the detector would still work.
The device is ground-referenced (through your hand). It will detect the field surrounding the “Hot” wire in a circuit, even if the switch is open. Think of a radio transmitter - the radio can detect the emissions without requiring a complete circuit from the antenna to ground.
Thanks, guys.
:smack:
Thanks, all.
Also, if you rub the end of the detector back and forth across your forearm (seems to work better on hairy guys) it will go off!
–FCOD
Also cats, upholstery, carpets, TV sets–lots of things.
It really isn’t; more accurately, it doesn’t have to be. The device itself has sufficient capacitance that enough current will flow at 60 Hz to trigger it, even if it were floating free in the vacuum of space next to a live voltage source. Your hand (and the attached body) does add some additional capacitance, but not much at all–and it’s not needed for operation.