This is my first post here so please have patience with me:

I understand, there are at least two kinds:

  1. a direct contact tester which basically is a neonnlight, and a resistor

  2. a non contact AC meter where a capacitor is involved somehow

I believe in both cases some current, less than a mA has to flow through our body for these to work. I was wondering exactly how much in each case, with an example of a design.

I guess I could buy some for a couple of bucks, and try to figure it out, but I throw the question out here first

Thank you

EDIT: Progress so far: I/We have found 3 designs so far. I still don’t know how to tell which ones work with magnetic induction and which ones with capacitive coupling. Furthermore, I don’t know where the human body comes into the picture/designs. Here are the designs (from the comments section):

  1. DIY Jameco: https://www.jameco.com/Jameco/workshop/jamecofavorites/non-contact-AC-voltage-detector.html

  2. EEVblog #267 (thank you @isdi): https://www.youtube.com/watch?v=AWlRGLxm7nc&app=desktop

  3. Someone here already opened one up: Why are non-contact voltage detectors sensitive to vibration?

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    \$\begingroup\$ You mean "less than a mA", not mV. Current is measured in amperes, A. Hit the edit link under your question ... \$\endgroup\$ – Transistor Dec 9 '18 at 17:03
  • \$\begingroup\$ If these devices had to pass medical regulations, the current would have to be less than 10uA. But I doubt it’s that low, I would look into which regulations apply. \$\endgroup\$ – Edgar Brown Dec 9 '18 at 17:47
  • \$\begingroup\$ for the contactless, google "EEVblog #267" for a quick teardown, schematic and the original patent info. It's basically a quasi electrostatic field detector for 50/60Hz. The "screwdriver" type -does- use your body to complete the circuit (both types are subject to false readings). There are teardowns for the latter as well. \$\endgroup\$ – isdi Dec 9 '18 at 22:04
  • \$\begingroup\$ Thank you, it was really interesting video blog. It seems like non-contact detectors are of two kinds as well. I only had read about the ones operating on the basis of capacitive coupling. I believe this design works on magnetic induction instead (?) The patent only describes the shape and connection, not why it works. Here is a text mentioning the two types: homeinspect2020.com/uploads/… \$\endgroup\$ – AbsolutelyFreeWeb Dec 10 '18 at 17:24

See this answer on DIY for a complete details on a wide variety of testing equipment

A good quality Non-Contact Voltage (NCV) detector will NOT make you part of the circuit and is perfectly safe. It does require a battery and has a small but significant range of both False Positive (showing power from a nearby wire even when the wire you are really interested in may be "dead") and False Negative (not detecting a live wire). But very safe to use.

A direct contact tester that is, essentially, a neon bulb inside a screwdriver, is NOT very safe. Because it effectively makes you part of the circuit, if things don't work right there is a possibility that used as designed and without any worn out insulation or other "red flags" it can be dangerous.

  • \$\begingroup\$ thank you. any idea about the designs? \$\endgroup\$ – AbsolutelyFreeWeb Dec 9 '18 at 21:12
  • \$\begingroup\$ I doubt that a self-powered voltage tester does not make you part of the circuit. It is just that it can afford to use much smaller currents and rely on capacitive coupling. Thus keeping you galvanically isolated and thus much safer. \$\endgroup\$ – Edgar Brown Dec 9 '18 at 21:42
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    \$\begingroup\$ I found this design: jameco.com/Jameco/workshop/jamecofavorites/… But it is saying, it is detecting the current by its magnetic field, rather than by capacitor coupling. see ecmweb.com/content/… \$\endgroup\$ – AbsolutelyFreeWeb Dec 9 '18 at 21:55
  • \$\begingroup\$ A proper non-contact tester is truly non-contact - it is not making you or the wire (which can even be insulated) part of a circuit. But as a result it is a bit "fuzzy", which can lead to False Positives. \$\endgroup\$ – manassehkatz-Reinstate Monica Dec 9 '18 at 22:27
  • \$\begingroup\$ According to this paper, a non-contact voltmeter still uses your body as the large capacitator: powerlogic.com.au/Attachments/ProximityTesters.pdf \$\endgroup\$ – AbsolutelyFreeWeb Dec 10 '18 at 18:23

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