Multimeter Reading Question

You don't say if they are analogue or digital meters?

sorry they are digital

Sounds like you have several duff multimeters to me....

John.

well perhaps but they read everything thing else fine. Actually its about 27 VAC that it reads.

Thank you

I wonder if it might be the internal protection diodes with the high impedance giving a level shifting to the ac signal so the dc reading is about half of the peak to peak level????

Are all the multimeters the same make and model?? battery powered etc..?

John.

Oh! is it an isolated ac signal?? possibly with capacitance to ground on one side perhaps....

Just thinking aloud..... but with an isolated signal source, maybe one side capacitively coupled to ground and the multimeter being isolated battery operation..... one side being capacitively coupled through your hand to ground.... Mmmmmmmm!

Must go into the workshop and play with a few....

John...

no there is three different types of digital meters we used and they all were the same. The meters are about 50 bucks in price. They are battery operated yes.

JC,

I'm sure John is right in post #6. Try grounding one side of the output together with the negative lead of the DMM.

What is your ac supply? Is it just from a transformer? 50/60Hz? If not what is the frequency and wave shape?

the ac power supply is a plug in transformer of 24 volts 60 HZ

My guess that your transformer might have a built in rectifier so your output would be rectified AC which has both a DC component and AC component. Everything fits except when you said you reversed the leads and it reads zero. Did you check it several times to make sure you had a good connection?

Every digital meter I've used will read minus voltage, unless there is something funky in your meter leads like a diode. Check it on a battery and see if it reads both ways.

Its very nomal reading. neednt making a fussy about it

most of digital meter has a wide rqange band from 0--200khz. you can measure ordinary power supoly , even an audio frequency wave without any problem.

for the a power, the reading number is indeed differnt from DC and Ac step positon.

one has a rectifier and one is not.

but you said o v, is out of my depth. its strange. maybe your meter has something wrong inside. check up it.

besides the fact of me wanting to understand it also caused one of my coworkers to blow a bunch of CCTV cameras. He read the output one way with his meter and he inadverntly had it set to a DC reading and figured ok the output is DC voltage. He was trying to figure out the output. The cameras were of course DC cameras and because his meter said 24 vdc he thought great. He hooked power and blew the cameras because in fact it was AC voltage output. We tested this in the office and the result is of course the reason for this thread. Perhaps we should get a 200 dollar meter or something and see if the result is the same.

SO far thank you to all that have provided input :)

the camera head for cctv use 12v or 24 v genaraly. ac will burn it sometimes but as it has a protecter inside. so you can fix ti easily.

as you ssaid the best way is using a oscillagraph within the meter. fulke meter has the function.

The best answer but will cost at least 1500 USD (Fluke Scopemeter)

wow, too expensive?

although its a portable and small enough in hand, but people will select a simple digital tektonix oscilloscope instead.

in fact, some digital meter can be display ac, dc on its panel. and can measure pulse signal and has converter to equvalent rms value etc. not too expensive.

I prefer the teck THS720P for industrial variable speed drives.

Intuitive panel, fully isolated channels on all probes and meter inputs.

hello dear

i just gone through your question

i am here 2 ans u

if u measure an ac source with a dc multimeter

u will not get the correct reading

the meter will not b able to read the ac voltage

so it is showing u fault voltage

it may b any value not only 24v

ok bye

keep asking

regards

priyanka

Thanks Guist. The next step is to open and fix the Camera and be carefull next time. We need your Questions we dont want to lose you.

Jc: I would guess that the meter circuitry works with only one polarity and that it scales the readings to account for the anticipated signal. It would, then, rectify and filter AC voltages, perform A/D conversion and scale the readings to approximate RMS levels for an assumed sine wave. It may even be a "True RMS" meter that measures heating equivalence for ANY wave shape. In any case, a DC voltage of one polarity would be blocked and the reverse polarity wouldn't scale correctly. DickL

No Dick, Any digital multimeter will scale the readout as a positive or negative display. The internal A/D will accept dual polarity inputs.

John.

Hi John: You are absolutely right WHEN THE METER IS BEING USED CORRECTLY. The question posed, though, asks about the response when the meter is being misused. As stated, I was guessing at a posssible explanation. DickL

OK meter this meter that. Whats up with the power supply itself? Have you tried the meters on a different power supply? 50$ meters 10$ power supply?

I tried making a bridge once( improperly) to convert to a dc signal for powering some smoke detectors. Put a cheap bridge in that I found on the service truck. It worked for what I needed but without filtering you get ac and dc Cameras bad smoke detectors didn't care. The efficiency of the power supply is what is at fault from what I'm reading not the meter. I also think honest mishap on the actions of your counterpart.

But I do have to say a 50$ meter is probably not the professional way to go about things. Especially when you leave the realm of yea its about 110 230 bla bla .. and enter into communications and instrumentation and basic technician.

LOOK FOR FLUKE FIELD TESTED AND JOURNEYMAN APROVED!

Hi JcHammer,

I have certainly seen cheap digital multimeters spit out some really strange reading when you set them for DC and measure AC or resistance and measure any voltage but you said you used three different models of cheap meter and got the same result.

Having said that however I have the feeling I have seen exactly what you are describing before, I just can't remember when where or why for the life of me at the moment. Meanwhile, while I try and get those malfunctioning brain cells working, do you have access to an oscilloscope that you can use to look at the actual waveform from the power pack? If we know what the waveform looks like then it may hasten the memory process.

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Good luck and welcome to CR4.

The best way to be 100% sure what kind of voltage you are running (AC or DC), is to use an oscilliscope. As for an explanation why you are getting readings of 12V AC and 24V DC, it's all about the sine wave and how your digital voltmeter interprets the data.

A 12V AC signal oscillates fro +12V to -12V, hence the term Alternating Current. A 12V DC signal stays steady at +12V (when connector leads are connected correctly, -12V if they are bass ackwards). When your multimeter reads AC voltage, it simply takes the magnitude, |(+12V)+(-12V)|, and divides it by 2 = +12V. The meter may interpret the AC signal as |(+12V)+(-12V)|= +24V. (|x+y| = absolute value of x+y).

You can verify what kind of voltage you are dealing with with your test data. If you readings show +12VAC regardless of how you connect your leads (corrrect or bas ackwards), then you have AC. If you read +12VDC with the leads connected correctly and -12V when the leads are switched, then you have DC.

Guest, I think your definition of an ac signal chopping between +12 and - 12 volts is a little incorrect....

Don't forget the peak value will be higher than +/-12 volts to give an rms or average reading of 12 volts...

As for your statement that the meter when set to ac volts just takes the magnitude of +12V and -12V and divides by 2 that is wrong!!

As for your statement about the meter taking the absolute magnitude.......

Ohhhhh let's just say your post is full of errors and is incorrect!!!

John.

That explains the tingly feeling everytime I turn off the light! Hahahaha!

'A 12V AC signal oscillates fro +12V to -12V, hence the term Alternating Current.'

Actually that is incorrect a 12 V AC voltage oscillates from +16.97 to -16.97.

The difference is due to AC voltages normally being the Root Mean Squared RMS voltage rather than the peak voltage. The RMS voltage is the effective voltage so that's the way we have expressed it but the peak voltage is actually the √2 times the RMS voltage.

I would consider the power supply nfg.

There has to be both a DC and AC component present, as noted with the CCT camera.

My Fluke meters (several hundred dollars each) separate out the components rather nicely and will read DC average, DC peak, AC (ripple component), and true RMS of total voltage. If you have a DC peak reading meter with a positive offset in the power supply it may read zero with the leads reversed.

If you really want to know what is going on, throw on a 'scope.

Now, why your meter doesn't read the DC +/- with the leads reversed is a good question. May be related to $50.

A 'cheap' multimeter can be simply considered as a half-wave rectifier which consists of a diode (D), a resistance (R), an inductance (L) and a meter head (M).

D,R,M and L are connected in series where D cuts off half wave of AC input, R provides resistance to both AC and DC, L provides resistance to AC only but presents 0 (or almost 0) resistance to DC, and M picks up the rectified current for display.

When you measure AC with such a 'cheap' multimeter, both R and L apply resistance (actually the resistance by L comes in form of reactance) to the circuit in reducing the through-put current, so that the M would display correct reading (12V in your case).

In case when you measured DC with the multimeter set to AC, and when the leads are connected in a way that D is happily letting the current flow, R would be the only guy who stands in the loop to provide resistance or to reduce the current, while L presents ~ 0 Ω, which results in M receive extra (double?) current, hence the extra (double) reading (24V in your case). When you switch the leads, D will block the current therefore you get 0 volt, because of 0 current through M.

Hope this explain the things.

What a load of Bollox!!!

Even cheap multimeters are based on the 7106 type chip and are quite capable of rejecting ac signals at their inputs....

I can't beleive some of the daft answers that have been put on this thread.... Only 2 so far... but it does make me wonder...??

It looks as if the meters have some sort of reverse voltage protection (Diodes) do you measure the correct voltage but as a negative - signed voltage when looking at a DC voltage with the leads crossed over. + to -? this will show up any funny business. If the voltage reads wrong. A good meter will read correctly but in reverse sign.

Try it once. Say 12V DC from a bench supply. Set the current trip to low.

Ok still not the meter............ It blew up some camras the meter is / was correct to some degree. I am concerned about polarity though dc backwards dead camm(maybe) I have a question though why did it blow the cammras ac (RMS) is equivalent to the effective dc voltage so the camm may still function to some degree on ac without blowwing up. Yep Im starting to be convinced throw the power supply away or go find an o scope and trouble shoot that thing or send it in for callibration. Better go to radio shack and buy a build your own kit and work up to one that is shop ready that you have built. Then one would have a base of bridges op amps and 555 timers ect. In meanwhile make a sign cammras are cheap but not that cheap.

Holy crap. You'll ALWAYS get reading from a DMM measuring AC with DC setting.

Nothing wrong with the power supply or the meter. With an analog meter you may see the needle moving due to the AC voltage.

Try measure a DC power supply with AC setting on the meter. You'll get the ripple/noise voltage.

Try measure AC socket with meter set to 1000V DC. You'll get a reading.

Learn how to use the meter and read the label on the power supply to see what its output is before connecting anything to it. Measuring both AC and DC would help. DC source usually have a much lower AC reading 10% or less.

Pineapple

On some cheap multimeter, when you reverse the lead, instead of displaying negative voltage, it would display zero (0) volt because it can't go beyond 0. Maybe this is how your meter show negative voltage.

MifniteFighter

You may want to calibrate the instrument. Each type, analogue or digital have their own procedure for doing it. Many digital may have you cross the leads, and press a pencil-tip button at the button of the case, while analogue may have a flat tester tip trimmer, located at the bottom, or the battery compartment.