Author Topic: Using a digital multimeter to measure AC voltage  (Read 6328 times)

Richt81

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Using a digital multimeter to measure AC voltage
« on: October 30, 2014, 10:20:38 PM »
Hi Martin,
I have just finished watching your excellent tutorial on using a digital multimeter (part 1, voltage).
I think I understood most of the content, but I have a couple of questions:
When measuring AC voltage, how do you know which way round to plug in the probes?
Also, how does this differ for a UK plug with three holes?
I have it in the back of my mind that the top hole is the earth, but which way round are the live and neutral? How does this correspond to positive and negative?
What are the consequences of plugging in the wrong way around?
I am only just starting out with my first multimeter, having last used one in school over 15 years ago. I hope to be able to test musical equipment such as guitars and amplifiers, and general electricals such as light fittings and plug sockets when I have gained more experience.
I would be very grateful for any tips anyone could offer me on the forum.
I will look forward to watching more of your tutorials!
Thanks
Richard

Mr Eastwood

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Re: Using a digital multimeter to measure AC voltage
« Reply #1 on: October 31, 2014, 07:06:04 AM »
Hi,  welcome to the forum.   I'm not trying to be clever but if you're asking how to test a uk mains socket and you not even sure of the pinout of a 3 pin plug you'd be very wise to avoid the mains for now, until you've gained more experience. 

Maybe get some simple books on home wiring but in the meantime you can use your multimeter to check lots of guitar things or even start making you're own guitar fx pedals.

Last weekend I did a modification to my brothers vintage 80's fx pedal to reduce noise by swapping out some caps and opamps but things like this are very rewarding and involve no danger! ;-)






« Last Edit: October 31, 2014, 07:22:42 AM by Mr Eastwood »
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steve30

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Re: Using a digital multimeter to measure AC voltage
« Reply #2 on: October 31, 2014, 04:17:07 PM »
To measure the mains, connect the common to the Neutral pin, and the voltage measurement pin to the Live pin. The Earth pin is more for safety and you wouldn't typically use it for measurements.

However, for measuring an AC voltage, it didn't really ought to matter which way round you connect it.

On a UK plug/socket: Looking at the socket, right hand pin is Live, and the left hand pin is Neutral.

Regarding "positive and negative", The live pin is the one that has voltage (potential). Because it is AC, it rapidly alternates between +300V (ish) and -300V (ish), whereas the Neutral pin has no voltage on it, so with AC, there is no such thing as a positive pin and a negative pin, as such.

You'll probably figure it out eventually  8). Just be careful when measuring mains.

Richt81

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Re: Using a digital multimeter to measure AC voltage
« Reply #3 on: November 03, 2014, 05:41:21 AM »
Thanks for your replies folks.
From watching the tutorial again, and examining a plug more closely, I have established that the left plug hole is the neutral and the right is the live on a UK three plug socket.
I decided to have a go at measuring the AC voltage across a multigang extension plug socket. Gotta learn sometime. So, I checked the probes were connected to the correct sockets of my multimeter, switched the dial to AC voltage (max 500V), and very carefully inserted the probes, black one into neutral and red into live.
Imagine my disappointment, when I got a reading of 0.00V.
Yes the plug socket was switched on.
Does anyone have any idea what I was doing wrong?
I have the option of switching to a more sensitive voltage measurement on the multimeter, but I didn't think this would be a good idea, as it says 200V max and I am expecting the mains voltage to be around 230-240V.
Is it possible that my plug socket has some kind of safety guard inside to prevent accidents happening due to things being inserted into it?
I am sure that the zero reading is not due to the plug socket being dead, as it seems to work fine otherwise, and I tried two of the four sockets on the extension multigang.
Any ideas?
Thanks
Richard

Mr Eastwood

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Re: Using a digital multimeter to measure AC voltage
« Reply #4 on: November 03, 2014, 06:50:40 AM »
Is it possible that my plug socket has some kind of safety guard inside to prevent accidents happening due to things being inserted into it?
spot on! - they have things called "shutters" wiki

Quote
BS 1363 sockets must have shutters on the line and neutral contacts to prevent the insertion of a foreign object into the socket.[64] Many sockets use the original method of shutters opened by the earth pin (or plastic ISOD), longer than the other pins and hence opening the shutters before the other pins engage, alone. Alternatively, shutters may be opened by simultaneous insertion of line and neutral pins. Some later designs require all three pins to be inserted simultaneously.
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Richt81

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Re: Using a digital multimeter to measure AC voltage
« Reply #5 on: November 03, 2014, 07:06:40 AM »
Thanks Mr Eastwood!
Looking at the plug socket more closely, there are clearly plastic shutters present behind the live and neutral plugs, with a kind of latch device on the earth.
Going to leave it alone now until I have learnt some more.
Cheers
Richard
« Last Edit: November 03, 2014, 07:29:31 AM by Richt81 »

SeanB

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Re: Using a digital multimeter to measure AC voltage
« Reply #6 on: November 03, 2014, 10:45:58 AM »
For a little more testing use an IEC mains lead, there you do not have shutters. Just be careful, as the fuse inside will not protect you against shock ,even if it is a 3A one. Looking at it the middle upper one is earth, and the left is neutral and the right live, though this may vary as often these are not wired correctly other than the earth lead.

TechJunkie

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Re: Using a digital multimeter to measure AC voltage
« Reply #7 on: December 10, 2014, 02:17:36 PM »
Here are a couple devices that I made for AC Line and Power Measurements. One of them is for basic DMM mains measurements or variac and the other specialized for my Gossen Metrawatt Energy DMM. Both are fully potted internally to provide durability and insulation.

Caution!!! I recommend that you have a good understanding of mains voltages and current and uses devices like this with the proper safety precautions. There is a connect/disconnect procedure for both of these devices and that MUST be followed to maintain safety.

Eric Haney
Systems Programmer, EE, MCSE, DMC
SilverCore Labs

Botrous

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Re: Using a digital multimeter to measure AC voltage
« Reply #8 on: June 11, 2015, 01:06:48 PM »
Hi Martin,
I have just finished watching your excellent tutorial on using a digital multi meter, it is awesome thank you.
but I have a couple of questions as I am new with this:
I am start working with CCTV installation.
How can I use the multi meter to check the out voltage from the power supply or on the cable before connected to the camera.
normally the power supply should produce 12V to each channel, should I connect it to power then check +/- with the same way you explain in your video.
Thank you