Why do they bleep when...

i think they measure the electric field thats created by the AC supply

>even open circuits can trigger them
thatsthepoint.mpg

>>1022560
Op is tarded

these devices are pretty shit for detecting presence of a voltage coz, as you've discovered, they have a ton of false positives. nothing you can do about that except get a proper contact device, like a voltmeter or one of those neon bulb screwdriver gadgets. even voltmeters get false positives coz they have very high input impedances. an old analog meter is better in case you're confused by unexplained wild readings.

>>1022565
There is no electric field generated by an open circuit.

>>1022572
No, that is not the point, there is no electron movement in an open circuit

>>1022574
Ok

>>1022576
i have a DMM and that bulb screwdriver thing but that screwdriver can give a lot of false negatives in my opinion. (basically it works by shorting a small current of the live wire to the ground through your body but if you wear shoes with good insulation or the material you stand on is insulated well then it wouldn't light up even when there are voltage in the measured object)

>>1022582
electrons are always moving.

You need to realize that there are two different things: voltage and current. Voltage is present in an outlet even if nothing is connected to it (therefore you can have a voltage and no current). in those wires.

Now go to your fuse box and turn off the power at the main source, then try the pen on those same wires again.
The results will be different.

>>1022576
>nothing you can do about that except get a proper contact device
>contact device

You do realise you are basically telling op to stick a contact voltmeter in to a live power outlet.

That is some dangerous advice.

>>1022591
How do you sense voltage?
Static current flow can induce a magnetic field, variable current can induce even more interesting stuff but how would you detect simple voltage?

>>1022608
I've been putting my multimeters into outlets for years

>>1022621
With capacitive voltage sensors

>>1022560
>Why do these [Non Contact Voltage Indicator] bleep and flash when i put them close to an unplugged socket?

To demonstrate that the 'unplugged' socket still has a live electrical connection - thus aiding troubleshooting, and preventing you from electrocuting yourself needlessly. This is expected behaviour, a sign of correct functionality and a GOOD THING.

>Why does my 'small screwdriver, with a test bulb built in' sometimes work near a live connection, sometímes not?

Because it is a cheap and evil piece of junk that should be tossed down your nearest main sewage drain, before it kills you. If using normal household waste receptacle? please, break into pieces before disposal, to prevent some other poor unfortunate 'recycling' this useless and dangerous piece of shit. tldr - if you rely on one of these? - you already dead.

>>1022560
>It seems like that even open circuits can trigger them or am i wrong?
>What is going on?
They are "Non-Contact Voltage" detectors, they don't measure current at all.
They use a method to detect presence of voltage, which is present in a light switch even if the switch is turned off.
You can (it should?) detect voltage in an extension cord, even when there's nothing plugged into it at all. Just having it plugged into the wall is enough.

I dunno if they work on DC? Have not tried that yet.

Some devices have a relative indicator--such as they beep faster for more volts present, or they show a bar-chart or toss up relative numbers. None I've seen are meant to give any actual value of volts tho.

I bought a PeakMeter pen DMM on aliexpress for $11, just for low-voltage/continuity checking use,,, and I found that the NCV on it works amazingly well.

>>1022608
while being dumb is perfectly safe

>>1022608
Yea. You can stick meters into outlets. Unless I'm missing something, I've been doing this for over a decade.

>>1022636
This is right.

>>1022582
You don't need current to create an electric field you need current to create a magnetic field

>>1022644
If you were retarded and had a shitty meter and plunged it into a socket in amperage mode you might die. Otherwise you should be fine.

File: maxwell.png (3KB, 185x43px) Image search: [Google]

3KB, 185x43px

>>1023131
Incorrect, you need a changing electric field or a charge flux (aka current) to produce a magnetic field. See: what is light.

>>1023142
If you have a shitty meter in ANY mode it shouldn't go anywhere near mains. Those 5-10USD cheapies are passable if you are doing low voltage work (<50VDC), but mains work is just asking for shock or pieces of a meter embedded in you.

If you want to do anything with mains, please, spent ~50USD and get something with a proper CAT rating from a reputable brand. Check for HRC ceramic fuses, a deep groove in the case joints, and proper input protection. You don't need to get a 400$+ Fluke, but 50$ for a decent DMM can save your life.

>>1023143
And a current still does what I said... I was just just saying that to emphasize that he may have been thinking about that when he mistakenly said you need a current to create an electric field.

>>1022560
An ' open' ac circuit still carries line voltage. Line voltage changes 50/60(etc) times a second. A changing voltage means a changing current means an electromagnetic field which can be detected.
A circuit under load cannot be detected by current alone(direct current) it requires the voltage(and current) to be changing because of the ac frequency.
If you want more information look up how radio works because its literally a transmitting antenna. For more information try deriving maxwells equations from scratch.

The other day I was using my shitty Stud finder that has an ac alarm built in (beeps when it detects power behind the wall) and it was going crazy for a 2 foot radius around a socket that I had a little DC wall wart plugged in to, but would stop when I unplugged it and wouldn't do it if I plugged in and ran something else like my drill press aside from where the wires actually were. Do little switch modes put out that huge of an em field or what was happening?

>>1023260
They are pretty noisy when it comes to EM emissions. They operate at higher frequencies (kHz ranges) and the transformer is going to emit and EM field (plus they generally have an inductor for filtering).

>>1023264
thats crazy, i had no idea they would make that big of a field with enough energy to make a stud finder think there was ac current 2 feet away

>>1023213
>If you want more information look up how radio works because its literally a transmitting antenna.

Not a bad point. Just had an idea for a product. A safety plug that blocks/reflects these transmissions from open outlets. I could sell it as a WiFi or phone low antenna signal booster, by removing the noise. Would also have ones for $500 for the audiophiles.

Now where do I get some funds for this?

>>1022582
there is electron movement in an open AC circuit, dipshit

>>1022582
Parasitics and finite speed of light, breh

>>1022560
>Or even when i get close to a turned off light switch
learn how a light switch works

>>1023142

I did this, it went poof and popped the lead out of one of the probes. Still works fine.

>>1022560
>Deathstick....
Nuff said

>>1023284

>Remove 50Hz noise from your audio!
>Get cleaner bass!

That sounds amazing!

I prefer these to have false positives every so often to let me know it's working. Rather have a false positive than a false negative. That being said, I rarely have false positives with my fluke. My harbor freight special I keep in my glove compartment is a different story, lots of false positives but they're easy to difference from actual live lines This isn't the be all end all of detection, but it's nice for installing fixtures and working in boxes that have more than one line coming in. They've always done just what I needed or expected them to do.

File: TOS-TEL-FVDP3_280_280.jpg (21KB, 280x280px) Image search: [Google]

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>>1027185
This one has a little step up circuit that runs up to 240v when you turn it on (push the button) then it falls back to normal.

So you know: the battery is good, the sensor is good, the display is good, the audio is good.

It even tells you voltage, which is pretty accurate.

It sucks when it's near highvoltage though when parasitic voltage is putting 70v on to grounded shit and the only way to clear it is to just touch the shit you know is grounded.

>>1022582
the point of these is not to tell you if you have a closed circuit, they only tell you if you have voltage at the point you are testing... so you don't have to touch it... which is why they're called ... wait for it... non contact testers...

and they let you know if the breakers are labeled right without having to fuck around with meter leads.

>being an electrician and reading this thread.
Don't kill yourselves anons.