Does multimeter measure Vrms or (Vripple)rms?

[savvej]

In lab experiment on DC power supply,we had to connect output of brigde rectifier to lsection,pi and capacitor filter,and calculate the ripple factor using (Vrms)ripple,which we measured directly by putting Multimeter in AC mode.Our lab assistant(who is just 1 year senior to us) told us that we are measuring Vrms ripple.Is it correct?wont t measure Vrms itself?As in does multimeter have any automatic dc offset adjustment so that it measures only ac component..?
Coz I dont agree with it.
When I connect a 1.5 volt dry cell to multimeter in dc range it measures around 1.58V and when I connect it to AC range it gives amazingly 2.7 volts ??? how is this possible???
definitely there is no dc offset adjustment...also how come it measures 2.7 volts ac for 1.58 v battery??
My multimeter is MASTECH MAS830L

 

[savvej]

you can try it yourself for dc battery ...how is it possible?

 

[MikeMl]

Any meter worth of the name has a "dc blocking Capacitor", which is switched in in the AC Volts mode. I own several Flukes, and they do that. I don't own a Mastec?

 

[crutschow]

Normally a multimeter blocks DC on the AC range by capacitive coupling on that range. The spec for your meter says its AC response goes down to only 40Hz so don't know why your particular meter measures a voltage from a battery on the AC range. Sounds like a bad meter.

 

[savvej]

U tried measuring voltage across 1.5 volt battery in ac mode?

 

[MikeMl]

U tried measuring voltage across 1.5 volt battery in ac mode?

Yep, It reads "0.000Vac"

Mastech is cheap Chinese Crap!

 

[savvej]

bazooka:
it read 0v for my li-ion battery but
for dry cell battery its reads 2.7V.
just confirm this for me..with ur meter if u have a li-ion battery

 

[hexreader]

If you know that your source is DC then select the DC range on your multimeter and read the corrrect value.

If you know that your source is 50Hz or 60Hz pure sine wave, then select the AC range on your multimeter and read the RMS value.

If neither of the above apply, then you need a good quality oscilloscope to see what is really going on. Forget about using a mutimeter - especially a cheap one.

Just my opinion...

EDIT: I own a Mastech multimeter, and I love it. But I do not expect much accuracy, or true RMS readings from it.

 

[savvej]

So the multimter doubt is done.My multimeter is cheap chinese and its ac range is faulty.

In lab we we told to get value of (Vripple)rms using multimeter(Some hi-fi autoranging one).wont it give (V)rms and not (Vripple)rms? btw can somone just define what according to you is difference between two in simple words?

 

[Reloadron]

I'll bite.

Agilent U1252A reads 1.602 VDC in DC mode and 0.000 in AC mode.
Fluke 87 Series V reads the same.

That was measuring across a fresh 1.5 volt AA alkaline battery. Both are true RMS hand held DMMs.

I thought even cheap hand held DMMs used a cap for coupling in AC mode?

Ron

 

[savvej]

i checked for carbon zinc batteries not the alkaline type.
and as i said when i measure li-ion batteries it reads zero.

 

[savvej]

our lab manual notes reads:

(Vrms)^2 = {(Vripple)rms}^2 + (Vdc)^2

so does my multimter measure Vrms or (Vripple)rms ?? (The main doubt)
As in do u mean to say that the dc component is blocked so that my multimter in ac mode will measure (Vripple) rms ??

 

[hexreader]

can somone just define what according to you is difference between two in simple words?

RMS is the equivalent AC voltage as if it were DC, if I remember correctly. I am not sure how this helps you though. The "hi-fi autoranging one" might indicate an RMS value of 'Vripple', but you would need to read it's user manual very carefully to be sure.

I still think you need an oscilloscope.

bazooka:
it read 0v for my li-ion battery but
for dry cell battery its reads 2.7V.
just confirm this for me..with ur meter if u have a li-ion battery

You do not say whether you measured using AC range or DC. Either way - this makes no sense unless your lion battery is (very) flat, or your dry cell battery has some strange oscillation.

An oscilloscope will give a much better story, but sorry to harp on..

 

[RCinFLA]

Unless it specifically states that it is a true r.m.s. meter (which are more expensive) then it is an averaging AC meter. What that means is it produces an output of the average of the rectified signal. The meter calibration for AC volts is raised to read the average as equivalent r.m.s. UNDER THE ASSUMPTION THAT THE INPUT WAVEFORM IS A SINEWAVE. If the input waveform is not a sinewave then the reading will be incorrect. If you want to read out AC rms ripple then use a true RMS voltmeter.

As to reading the 'dry cell', I have never had a multimeter that reads out a voltage from a pure DC source when on AC mode. There is the possibility of the coupling cap retaining a charge but the bleeder load resistor on output of filter should dissipate it after a short period of time. While holding connection on the battery try switching it from DC to AC and wait to see if the reading decays down.

 

[Reloadron]

My head hurts. This should be your meter. I believe it is a Mastech 830L which is a pretty inexpensive meter that retails in the US for about $14 USD. The meter is an average responding RMS indicating meter and as I read it only has two AC ranges of 200 VAC and 600 VAC with resolutions of 100 mV and 1 volt respectively. You are not going to measure AC ripple with this meter. Now is that link the meter you have?

The schematic can be downloaded here on this page. There is no AC coupling that I can see?

Ron

 

[Reloadron]

Alas, home from work I was able to duplicate your results. Well pretty much anyway. Several years ago Omega (who I buy quite a bit from) gave me a free little DMM, my trusty Omegaette Model HHM93 which is likely about a $10.00 USD DMM with about the same ranges of your meter. They could be cousins from the same factory in China. My 1.5 volt AA alkaline cell reads 3.1 volts on the 200 VAC range. There you have it. My guess is no AC coupling cap and the guts of this thing likely resemble the schematic I linked to earlier.

Ron

 

[ModemHead]

In addition to the DC coupling, note that it also uses a single diode for rectification. Full-wave RMS is roughly two times half-wave average IIRC. Which pretty well explains the bogus readings of ~3Vac on a 1.5V battery. The diode voltage drop also leads to severe non-linearity at the low end of the ACV scale.

You get what you pay for!

 

[savvej]

Thanks Reloadron for all the efforts.

 

[KeepItSimpleStupid]

I'll bite too. Some meters, RARE have the ability to select AC or AC+DC for the AC measurement. MOST meters are TRMS reading, average responding and have limitations usually defined by the "crest factor" of the waveform. There are, or once was, thermal TRMS meters. They don't have that limitation.

Average responding means, filter, precision rectify and multiply by a fudge factor which is only valid for a sine wave.