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DVM transient voltages
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I have a DVM, a Thsinde 18B+ ... not a major manufacturer, I know ... but, still, why do I get weird transient readings when set for detecting AC voltages?? When I touch the probes to each other, I get 0 volts - that's good.? When I check a household outlet, I get 110 volts (more or less) - also good.? When I set the DVM on my work table, with the probes not touching anything, I get 0.2 volts - what?? If I pick it up and hold it about 2 feet from the table, it settles down to zero...unless I wave the probes around vigorously, in which case I can get, maybe, 0.02 volts - huh?
Anyone help me understand what's going on around here? |
开云体育?Hi,
Most DVM have very high impedance (high Z or just high resistance). ? And your probes are picking up the AC signal from your wall wires in the walls. ? See this demo :
Notice how around 1 minute mark, even before he touches the red probe, the meter is already reading about 0.1 volts. ? The more he “connects” (like his body) the probe becomes longer antenna and it picks up more signal.
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Some meters have low-z setting to make sure you do NOT see such voltage.?
This type of meter have relatively low impedance to load down the weak AC (signal from the wire acting as an antenna) so you see zero voltage.
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Normal DVM inputs are designed to have very high impedance as to NOT load the circuit under test. ?They can be into 10 million ohms. If they were much lower impedance any weak signal / voltage would get LOADED DOWN (and read zero).
Translation : your meter is working as designed
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Another similar DVM “problem” : when you set your DVM on resistance (ohm or upside down horseshoe shape setting), and touch your probes together, you would think they should read zero ohms. ?But many better meters will read tiny bit of non-zero
ohm. ?That’s actually measuring the resistance in the probe wires.
I remember a guy doing a DVM review (on a video) and he touched the probes together and saw non-zero reading and he immediately declared the meter junk since it didn’t read zero ohms. ?He didn’t realize he actually had a high quality meter.?
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If you had an oscilloscope you can actually see the stray 60 hz that’s all around you when the probes are NOT touching anything. ?Here’s a guy touching the probe and seeing 60 hz (using his body as an antenna)
Jong
On Nov 30, 2021, at 8:48 AM, KI7MWA via groups.io <entilleser@...> wrote:
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I have found DMM's will pick up the high frequency signal generated by flourecent lights. If you hold the probes close to the fixture you will see a large increase is displayed numbers. Since you are not directly connected to the souce, it could be any stray voltage in the fixture. At one time I spent a great deal of time chasing a 6 KHz signal that was emitted from an indutrial flourecent fixture that was picked up on my Oscope. I had to turn off the lights to T/S my problem. Dan Kahn On Tuesday, November 30, 2021, 01:48:44 PM EST, KI7MWA via groups.io <entilleser@...> wrote: I have a DVM, a Thsinde 18B+ ... not a major manufacturer, I know ... but, still, why do I get weird transient readings when set for detecting AC voltages?? When I touch the probes to each other, I get 0 volts - that's good.? When I check a household outlet, I get 110 volts (more or less) - also good.? When I set the DVM on my work table, with the probes not touching anything, I get 0.2 volts - what?? If I pick it up and hold it about 2 feet from the table, it settles down to zero...unless I wave the probes around vigorously, in which case I can get, maybe, 0.02 volts - huh? Anyone help me understand what's going on around here? |
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A good DVM has a very high input impedance.? When the probes are not connected to anything, they respond to what we might call "static" electric fields in the space around us.? Electric fields are everywhere, around any wire that has any voltage on it.? Think of it as capacitance from those wires to your probes.? Yes the capacitance is small, but if the DVM's input impedance is large enough, the voltage divider can get a few volts across the probe leads. You may be able to induce a voltage also by holding both probes with opposite hands.? That's probably a combination of things, including the small voltage where the probe's conductors react with the junk and oils in your skin.? Battery action due to chemical?reaction. You might induce a voltage even when the probe tips touch one another, by waving them around which causes a transformer-like action where the loop formed by the shorted probes crosses through magnetic flux. Electricity is not strictly defined by conducting wires.? Ever wonder how current gets through a capacitor, or voltages get from one transformer winding to another even though they are insulated?? Welcome to the electric and magnetic fields that are all around us! Andy |
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With ohmmeters, I remember from many years back that VOMs used to have a "zero" knob that you would adjust to zero-out the resistance when the probes were touched. I think this was done mainly because the ohmmeter's indication was a function of the battery inside the VOM.? It wasn't so much to compensate for the resistance of the probes (though it did that too), as it was to compensate for the battery as it ages. BTW, a "VOM" is a Volt/Ohmmeter, so basically it's a DVM without the D = digital, and it had an analog meter.? Those good-old Simpson meters that everyone loved.? They read Amps too but that wasn't part of the name. Andy |
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