How do you measure DC volt on the AC line? - Page 1
Forum section: electronics / beginners

author: smoothtalker
author_rank: Regular Contributor
timestamp: March 20, 2014, 03:36:54 pm
content: I know this sounds crazy, but when i poke my dmm into the mains on DC volt range it didn't read anything.May i know why can't a dmm measure DC volts directly on the mains? although the dmm is rated to 1000v but does it overload anything?

author: KD0CAC John
author_rank: Frequent Contributor
timestamp: March 20, 2014, 03:49:44 pm
content: Pretty basic .Mains voltage is ACIf your meter has a AC volt switch position , put it there and then you can read AC volts .Sounds like you need to pickup a book on basic electricity AC alternating current / DC direct current .AC mainsDC battery , or DC supply .

author: Pilot3514
author_rank: Contributor
timestamp: March 20, 2014, 03:51:34 pm
content: I kinda wonder what you where expecting to measure.If this is pure AC then the average would be zero.If the meter had a high enough display refresh rate the numbers would be all over the place and the minus sign would be flashing to fast to see it blink.My only guess of something useful is if the AC had a DC bias. Something like an AC voltage of -90 to 110. That is 200 volts peak to peak with a 10 volt positive bias. You might be able to read that with an RC filter in-line with you meter set on a DC scale.

author: BravoV
author_rank: Super Contributor
timestamp: March 20, 2014, 04:11:14 pm
content: At AC mains lines, in the "ideal" condition, the DC reading must be 0 (zero).Although there is a certain situations where can make a DC offset across the lines, posted a thread and there is a good discussion & explanation there too, also a video demo on generating a DC at AC mains line ->Mains voltage with non zero DC, how bad is it ?Becareful when dealing with high voltage main, hope this helps.

author: smoothtalker
author_rank: Regular Contributor
timestamp: March 20, 2014, 04:43:31 pm
content: thanks for all the replies!yes, i do understand some basic AC / DC.what i don't understand is why can't my dmm measure the DC component on the AC line directly? the meter is not even showing 0v, it just keeps auto rangingi used a fluke 87v to do that so i guess it pretty safe since it's rated for 1000v and the p-p for 230v is only 600+V. but please let me know if it's unsafe.

author: smoothtalker
author_rank: Regular Contributor
timestamp: March 20, 2014, 04:55:05 pm
content: Quote from: Pilot3514 on March 20, 2014, 03:51:34 pmoh, i'm trying to understand distortion in ac line. that is why i'm doing all kinds of poking around. i measured and it seems that my house ac has a got this flat top sine wave.Quote from: BravoV on March 20, 2014, 04:11:14 pmEDIT: OMG, i watched your hairdryer video then i realise how dumb i was. of course it auto ranges on AC. lolololol beginner's mistake it is.

author: michaelym
author_rank: Regular Contributor
timestamp: March 20, 2014, 06:50:17 pm
content: Quote from: smoothtalker on March 20, 2014, 04:43:31 pmIt's safe as long as you know what you are doing, i.e don't touch the mains by mistake. Or maybe I just didn't understand your question.If you want to actually "see" the signal (not just RMS you get on the multimeter) you can use an oscilloscope with a proper probe.

author: smoothtalker
author_rank: Regular Contributor
timestamp: March 20, 2014, 07:36:03 pm
content: oh, what i meant was, i have no idea if it is safe to probe the ac line with the dmm on DC range? and would the high energy AC overload the DC range on the dmm or something like that.i know its rated for 1000v, but that is for DC and the manual didn't state (if any) AC tolerance for DC range

author: michaelym
author_rank: Regular Contributor
timestamp: March 20, 2014, 07:48:15 pm
content: Since the voltage at any given moment during the ac wave does not exceed the rated voltage I think it shouldn't be a problem.