strange electric current tester screwdriver

What exactly does this made in China electric current tester screwdriver do?

The name is SKYLINK FH-102A 100V-500V.

The usual tester screwdriver has some kind of bulb which lights up, when the tip is in contact with any current source, and the handle end is in contact with your finger.

This one lights up easily when applied to any metal surface carrying no current, faintly or brightly. Of course, it also lights up when applied to any A.C. electric source.

Anyone with any idea about this electric current tester screwdriver?

Now read this: this Chinese tester also lights up when both ends are in contact with any of your body parts.

The guy who sold me this device told me that it is used also for detecting whether you are dead or not: if it does not light up you are dead.

Susma Rio Sep

It sounds like a non-contact voltage sensor. It’s basically a FET that switches on an LED when an electric field is applied to the gate. It glows when brought near people and large ungrounded metal objects because those things are very good at picking up electric fields of nearby equipment and power lines. If you go out away from electrical equipment, it will no longer sense a voltage on you. A dead body will also pick up electric fields–it’s not a biological field the device is sensing.

Dear QED:

Where do you get all that information and knowledge?

Thanks for your ever present goodness to reply to all kinds of technical questions.

Do you also do other kinds of questions? like the one on hell and heaven.

Anyway, thanks again for your information and knowledge.

O.K., what about this one: What is the difference between knowledge and information?

Love you, you son of your mother.

Susma Rio Sep

The reason that it can light up when both ends are touched is that there is nearly always a differance in electrical potential between diffferant parts of the human body.

This device is so sensitive it can pick the differance up and illuminates.

The electrical properties of the human body are extremely important in medicine, for example in an electrocardiograph.

The electrical signals that represent the heart muscles operating are extremely small by the time they reach the skin, where the sensor is placed.

Unfortunately the human body acts like a large antenna, and picks up all sorts of electrical noise generated by things such as flourescant lights, or even just the magnetic fields emenating from wiring installed in the electrical distribution network of any building. These recieved signals are nearly always far larger than those picked up in the skin surface that show electircal activity in the body.

The way to filter it out is to assume the whole body picks up the same electrical noise, but as you get further on the body away from the heart, the detected signal from it will diminish.

By using two sensors, one near the heart and another some way off, and then mixing them in antiphase, or opposition to each other, the unwanted electrical noise cancels itself out, and the heart signal, although reduced, still remains and can be amplified.

The reality is that in practice, five or six sensors may be used, and this all helps to cancel the excess environmental electrical noise, and also to cancel the electrical noise the nervous system itself produces.

In electronic terms, what we are doing is using common mode rejection, ie a similar signal applied to the two inputs of an op-amp will result in a zero output, since they can be configured to work on the voltage differance between them.

Your screwdriver can detect the differances in potential between points on a live human, but on a dead one there would be no potential differance between points.

Dead = no electrical field, no screwdriver lighting up.

I don’t think so. The bioelectric field of the human body is measured in millivolts. These non-contact sensors need at least several volts to trigger. They pick up the far stronger induced voltage from nearby electrical equipment that the body receives.

One other point, by the way: If you’re referring to the thingamajigs you stick into one hole of an electrical outlet, they’re not measuring current, because the current there is zero. You only have a current in an outlet when something is plugged into it and turned on. But you always have a voltage in the socket, regardless of whether anything’s plugged in.

And this has been a pet-peeve of mine.

Sorry about the hijack, but why do they call it “alternating current”? Shouldn’t it be called “alternating voltage”? After all, unless the circuit is completely dead, you always have an alternating voltage present at the outlet. While you may or may not have an alternating current through the outlet. A couple examples of the latter:

  1. When you have nothing plugged into the outlet, the current is zero. It is not alternating; it is zero. (Or nearly zero. There’s a few nanoamps due to capacitance.)

  2. Direct current will flow through an outlet when the load is half wave rectified.