rookie question - ambiant ac voltage (air voltage) - what the hell - Page 1
Forum section: electronics / beginners

author: irishire
author_rank: Newbie
timestamp: November 19, 2016, 08:55:19 am
content: so ive two different multimeters which are displaying small ac milli-volt readings simply been exposed to the air. one of the meters is a cheapy and the other is decent. its possibly a coincidental fault but unlikely with both of themis this a normal occurrence for multimeters? (when i say normal im mean do you or others that you know generate these types of readings with any sort of frequency)what readings have you detected?is there such a thing as a constant-variable air voltage?

author: Paul Moir
author_rank: Frequent Contributor
timestamp: November 19, 2016, 09:13:56 am
content: They're mostly radio waves that are being transmitted by all the AC power wiring that surround you. Your meters have a (typically) 10 megohm input impedance which following ohms law means that 1mV = 1e-11 amps. This is a tiny current, amounting to a mere 60 million electrons per second.

author: irishire
author_rank: Newbie
timestamp: November 19, 2016, 09:55:59 am
content: thanks for the replys - humm not sure what to do now. yeah im generating similar results, 0.15-0.4 ac volts from air readings. the problem is i have this beginners experiment that requires reading the mV readings from an electret. the readings are supposed to be around the lower range of the readings that are generated by ambient conditions where i is.if this is normal then how can you detect the actual mV readings from the experiment given that the environment readings are in excess to the expected readings from the experiment? - a controlled environment?the experiment is from the make more electronics book. the dude who wrote the book explains things well and well id be surprised if these types of readings were normalish and for him not to point this out

author: Zbig
author_rank: Frequent Contributor
timestamp: November 19, 2016, 11:21:23 am
content: Just connect it to the actual thing you were going to measure - it should sort itself out. Doesn't the stray voltage drop then?

author: irishire
author_rank: Newbie
timestamp: November 19, 2016, 11:57:59 am
content: @ z-big - the stray ac voltages remain when meter is plugged into the breadboard@ blueskull - twisting the leads doesnt effect the readings. also the readings are at their lowest when the tip of the leads are closest and at their highest when the leads are greater than a foot apart

author: Zbig
author_rank: Frequent Contributor
timestamp: November 19, 2016, 12:19:32 pm
content: What do you mean by "plugged into the breadboard"? Have you actually (electrically) connected the meter to anything? What happens when you touch the probes together?

author: David Hess
author_rank: Super Contributor
timestamp: November 19, 2016, 03:40:30 pm
content: Quote from: irishire on November 19, 2016, 09:55:59 amThe meter integrates the input for a whole number of power line cycles so when making DC or ohms measurements, any AC cancels out.50Hz = 2 x 5 x 560Hz = 2 x 2 x 3 x 5The least common factors are 2 x 5 = 10Hz so if the integration time is 1/10th of a second which is 5 x 50Hz cycles or 6 x 60Hz cycles, then both 50Hz and 60Hz and any harmonics are rejected. Some precision bench meters phase lock to the AC line so they can cancel the AC interference more exactly.AC and capacitive coupling from the test leads into a sensitive circuit node can be a problem. Shielded test leads may be needed. If capacitive loading which is typically in the 100s of picofarads is a problem, then a resistor can be added to the tip of the test lead; a 100k resistor in series with the 10M input of a meter adds 1% error.