questions about NCVT (non contact voltage testers)

[Kracin]

i do enough electrical at work but im no electrical engineer.

i understand the concept as to why the testers operate (i think) but don't understand why they sometimes can read DC voltage even though they are only supposed to read AC voltage.

i'll put it anywhere near a 110v cord and i can get it to read about 3 inches away (new klein ncvt-3 with light, real sensitive).

and if i put it anywhere near a 480v 3phase load it will read almost 9 inches away. not bad for sensitivity. i mainly use it for small stuff though, when im doing quick troubleshooting for cords and cables that seem to have "lost power" or so the operators tell me, and it just makes some troubleshooting go quicker.

the same for solenoids as well, ac solenoids can be diagnosed without testing for magnetism or pulling wires out if you use the tester.







here is the oddball thing, output of a phone charger says it is 4-5v DC. and the tester will read the voltage in the cord going to the phone, even though its supposed to be 12-1000VAC reading. and some DC lines will cause it to turn on. some 24v DC solenoids have made it turn on consistently as well. anyone have any kind of reason that this stuff happens? it would be great to be able to use something like this on DC, but from what i understand its not supposed to work at all on DC. maybe these DC voltage are strong enough that its registering?

 

[gte718p]

If it is true DC the meters will not read it because there is no electro-magnetic field generated. Most DC is not true DC due to the way it is generated. It is just a very small sine wave centered around the DC voltage. If your meter is sensitive enough it will detect the fluctuations.

The transformers, diode bridges, and capacitors that are used to convert AC to DC are also extremely noisy. If you are near one you may be detecting the transformer no the cord.

 

[Kracin]

If it is true DC the meters will not read it because there is no electro-magnetic field generated. Most DC is not true DC due to the way it is generated. It is just a very small sine wave centered around the DC voltage. If your meter is sensitive enough it will detect the fluctuations.

The transformers, diode bridges, and capacitors that are used to convert AC to DC are also extremely noisy. If you are near one you may be detecting the transformer no the cord.

makes a alot of sense when you consider that i can get readings from small things like a wall charger. but when trying to detect 24v through a power supply it doesnt want to.

 

[erswill]

It's the ac component riding on the dc called ripple that you are measuring. With an oscilloscope you will be able to see the amount of ripple on the dc.
One would require extensive filtering and regulation with chokes, capacitors and semiconductors to eliminate or minimise this ac component.

 

[gte718p]

makes a alot of sense when you consider that i can get readings from small things like a wall charger. but when trying to detect 24v through a power supply it doesnt want to.

You are never going to be able to use it reliably for DC. Better power supplies have less ripple and will be less detectable. Your standard wall wart cell phone charger makes very little attempt to smooth ripple. In many cases they don't even have a smoothing capacitor which is the bare min to smooth ripple currents.

Your better supplies will have large capacitor banks, chocks, and the power supply will be physically shielded so that you don't have extraneous electro-magnetic fields floating around.

 

[MBfreak]

Hi.
I thought I was pretty good at electromagnetic theory. Been at it for 50 years.
2nd post states "If it is true DC the meters will not read it because there is no electro-magnetic field generated."

Please explain or retract.
Cheers!
Ola

 

[brook41]

If there is no magnetic field on dc then how does a clamp on meter work ?

 

[Old Engineer]

Hi.
I thought I was pretty good at electromagnetic theory. Been at it for 50 years.
2nd post states "If it is true DC the meters will not read it because there is no electro-magnetic field generated."

Please explain or retract.
Cheers!
Ola

There is a electromagnetic field generated for DC, but if it is "pure" DC, a NCVT will not be able to pick it up. NCVT detectors work by picking up the signal capacitivly coupled. If it is pure DC, the NCVT will not allow that signal into it's detector circuit. If there is "some" sort of AC component (like the ripple that others have mentioned) that will make it through to the detector in the NCVT.

As others have said, it comes down to how "clean" the DC is. Pure DC signals will not be picked up by a NCVT, so they should not be used to detect DC.

 

[Kracin]

You are never going to be able to use it reliably for DC. Better power supplies have less ripple and will be less detectable. Your standard wall wart cell phone charger makes very little attempt to smooth ripple. In many cases they don't even have a smoothing capacitor which is the bare min to smooth ripple currents.

Your better supplies will have large capacitor banks, chocks, and the power supply will be physically shielded so that you don't have extraneous electro-magnetic fields floating around.

right, i never planned on it for dc i just noticed that it would pick up some sources.

hard to find people who will trust a ncvt to even do something simple. now, i won't trust anything but a good meter to tell me that a 480v box is de-energized. but it's one of those things of, how fallable are they if you use them to tell you there is no voltage in a box? this one is fairly sensitive so you don't even need to be on a wire to know there is anything above 90v in it. unlike my fluke tester which requires you to almost be on top of the wires to get a reading.

 

[erswill]

If u wind numerous turns of insulated copper wire around a nail (not stainless) and apply a dc voltage across the 2 ends u have an electro magnet.

 

[doan]

Pure DC does not have a CHANGING magnetic field. The NVCT detects the changing field.

Not all clamp on amp meters work with DC - the cheap ones only work on AC. The DC clamp meter is active (vs passive) in order to measure DC current.

 

[Old Engineer]

unlike my fluke tester which requires you to almost be on top of the wires to get a reading.

But, your Fluke tester is probably spec'd to start detection at voltages above 90V, where this Klein starts as low as 12V, so it is designed to be way more sensitive. You may like this increased sensitivity, where other users may find it annoying. It all comes down to how the unit is designed, and what the end user wants. No product is perfect for everyone.

How good is the flashlight on that NCVT-3? It is hard to tell from the video I watched.

 

[Kracin]

But, your Fluke tester is probably spec'd to start detection at voltages above 90V, where this Klein starts as low as 12V, so it is designed to be way more sensitive. You may like this increased sensitivity, where other users may find it annoying. It all comes down to how the unit is designed, and what the end user wants. No product is perfect for everyone.

How good is the flashlight on that NCVT-3? It is hard to tell from the video I watched.

that was why i wanted the other tester, because there were times when i had used the fluke i just happened to be just enough away from a wire that it didn't read and so far the klein is able to get at least some tiny reading (bar scale instead of lighting tip) at a good distance, so sticking it even near a 4square box means you'll be able to see fairly quickly and easily if theres any ac voltage at all).

the light is decent, not a standalone flashlight but definitely useful for it's intended purpose of lighting up breaker panels, LB type pullthroughs and anything else where you might need a little light while probing to do a quick check. honestly i'd prefer if they had one without a flashlight anyway, i always have a flashlight and it's one of those things where size and design can be affected by added features. make a tool do one thing great, not two things well.

i would put the brightness at the oldschool pocket maglite brightness, but its a very concentrated circle since they use a magnifier over an underpowered LED (to preserve battery life). so you'll have a lit up circle that is very defined, instead of a more flooding light that lights up things around it also.