Multimeter measuring AC voltage

Discussion in 'General Electronics Chat' started by cheddy, Oct 23, 2007.

  1. cheddy

    cheddy Thread Starter Active Member

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    I bought a 29 range digital multimeter from radio shack "22-813" (Link to Owners Manual)

    1. What is a "AC voltage riding on a DC source bias" mean?

    2. The specifications say that it can measure AC 600 V RMS Maximum at 50/60Hz. Does that mean that it can't measure any voltage above 50/60Hz or it just can't measure 600V at 50/60Hz?

    I am trying to measure what should be 4.5 V AC at 58kHz and am not getting an accurate reading. Is it possible my multimeter is just a piece of crap?
  2. beenthere

    beenthere AAC Fanatic!

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    It sounds like the meter is optomized for 50/60 Hz RMS measurements. The maximum voltage is 600 VAC. I would be surprised if it did an accurate measurement at 58 KHz, though. I'm not sure my Fluke 23 would do well. It depends on the measurment circuit in the meter.
  3. cheddy

    cheddy Thread Starter Active Member

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    Thanks for the response. I would like to hear more peoples opinions on the matter. Still does anyone know what an AC voltage on a DC source bias means?

    Also, is it possible that an analog multimeter would give an accurate reading on such a high frequency that a digital multimeter just can't read?
  4. JoeJester

    JoeJester AAC Fanatic!

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    the bias is the DC offset. If you had a one volt peak signal and a 0.5V bias, the peaks would be 1.5 volts and neg 0.5 volts.
  5. niftydog

    niftydog Active Member

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    The best way to make that measurement would be with an oscilloscope.

    Multimeters, analogue or otherwise, are just not designed for measuring anything other than mains frequency AC @ 50 or 60hz.
  6. JoeJester

    JoeJester AAC Fanatic!

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    The DMM or Analog meter is limited. You would have to look at a frequency response correction factor .... they did publish them with the old Simpson 260's ... up to about 1 MHz if my memory serves me correctly.
  7. SgtWookie

    SgtWookie Expert

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    Good old Simpson 260's - and that's about what they cost back in the early 70's.

    DMM/DVM's are pretty hard-pressed to accurately measure AC waveforms above 10KHz; even there, you really need to be looking at a chart.
  8. The Electrician

    The Electrician Senior Member

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    The specifications for AC measurement say "Average responds, RMS calibrated, DC coupled". Apparently (you should check this) if you have it set to measure AC volts, the meter will respond to DC. Set it to measure 4 volts AC, and see what it reads if you connect the leads to a flashlight battery. Some voltages you want to measure may have an AC component riding on top of a DC component. If you want to measure just the AC component, you must block the DC component. That's the purpose of the .1 uF capacitor they tell you to put in series with the meter.

    This meter probably won't measure high audio frequency AC voltages accurately. Check this by connecting a variable frequency oscillator to the meter and sweep from 50 Hz up to 100 kHz; see if the meter reads the same at all frequencies.
  9. techroomt

    techroomt Senior Member

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    ac voltmeters are designed to measure the rms values of sine waves of 60 hertz. any other wave form or frerquency will provide a certain amount of error. if you measure a dc source (battery) with an ac voltmeter it will read 0 vac, as there is no ac component. if you read an ac voltage (say you duplex receptacle outlet) with a dc voltmeter it will read o vdc, as there is no dc component. an ac signal on a dc offset will read those repective values individually.
  10. GS3

    GS3 Active Member

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    Well, that is possible but not likely. As I said in the other thread, you need to understand the tools and their limitations. What you really need is a 'scope and understanding about DC and AC components and the differences between RMS, average etc and what each meter indicates and its limitations.
  11. techroomt

    techroomt Senior Member

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    i find it hard to believe the 58 hz (vs 60 hz) is providing the error you are experiencing. have you tried another emeter? and are you sure it is a sine wave?
  12. GS3

    GS3 Active Member

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    If you re-read the OP you will see its 58 KHZ. Quite a difference.
  13. techroomt

    techroomt Senior Member

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    good catch gs3, my bad. meter will have error no doubt. pretty sure your meter will read high in that instance.
  14. pccmd

    pccmd New Member

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    I concur.......frequency response is terrible for dmm's. Unfortunately, oscilloscopes cost a great deal more.....However: I recently purchased a 1970's vintage Tektronics(analog) Oscilloscope from a store in my general area for about $200 and having a ball with it, but just today driving myself crazy with discrepancy between its results and dmm's voltage readings.

    I found the answer today:
    http://groups.google.com/group/sci....7a658?hl=en&ie=UTF-8&q=dmm frequency response
  15. someonesdad

    someonesdad Senior Member

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    Answer to 2: it means don't measure more than 600 volts RMS at 50 to 60 Hz. The allowed voltage will reduce as the frequency increases, but most manufacturers don't specify how to derate the meter (if it's given, it's usually a V*Hz product that you must stay under).

    Answer to last question: No, your meter isn't a piece of crap. It's just not capable of making the measurement you want to make. Rest assured that most other hand-held digital multimeters won't make a decent measurement at 58 kHz (unless they're pretty expensive).

    I looked at the manual for this meter, but it doesn't specify the bandwidth. Given that it is a $30 meter (quite inexpensive), it's unlikely that it will have a significant AC bandwidth. Manufacturers usually specify bandwidth using the 3 dB points, but I find that pretty meaningless for measurement equipment. Instead, I like to measure the frequency at which the output will drop by 0.1% or 1%.

    I have a Radio Shack 22-812 meter (note it is a different model number than yours). When it is set to read a 1 V RMS sine wave at 60 Hz, the measured value drops by 0.1% at 300 Hz, 0.5% at 700 Hz, and by 1% at 1.25 kHz. The 3 dB point is at 13.57 kHz. The output drops by 0.1% when the frequency falls to 29 Hz.

    Since the AC accuracy is given as 0.5%, I would (personally) rate this meter useful over the frequency range of about 30 - 400 Hz. Of course, if you know you're measuring sine waves and you can calibrate the meter, it then can be used over a much wider range.

    Your meter will likely behave approximately in the same fashion, as it's probable that Radio Shack buys them both from the same Chinese manufacturer.

    Inexpensive digital multimeters typically are average-responding. This means they are calibrated to measure the RMS value of a sine wave. Note they will not read the RMS value correctly for other waveforms. For this, you'll need a true RMS meter. Even then, the typical hand-held true RMS digital multimeter has a fairly limited bandwidth, usually (at best) over the audio range of 20 Hz to 20 kHz.

    Measuring the RMS value of waveforms over large bandwidths (here, large may mean around 1 MHz) usually requires more expensive instrumentation. A popular analog RMS meter for this was the HP 3400, both the A and B versions (the B version extended the upper bandwidth limit from 10 MHz to 20 MHz). You can still find them for sale on ebay for under $100 (I remember they were around $2000 new in the 1980's). I have a 3400A model. If I set the amplitude of an HP 3326 synthesizer to 1.024 V RMS at 60 Hz, my 3400A reads exactly 1 volt RMS (I can detect about a 0.2% change on the meter at full scale). I can then increase the frequency to 1 MHz before the 3400A's measured value drops by 1% (1 division on the 1 V scale). In truth, I don't know whether this is the RMS meter or the synthesizer's drop, but as the drop occurs roughly linearly with increasing frequency, I'm guessing it's the 3400A. The 3400's reading keeps falling with increasing frequency, falling to 0.5 volts at 10.936 MHz. Since 10 MHz is the stated bandwidth of the 3400A, I'm reasonably confident the changes I'm seeing are due to the frequency response of the 3400A meter, not changes in the synthesizer's output (besides, I monitored the output on a scope and didn't see any amplitude changes).

    These HP 3400's were remarkable designs. Based on the knob style and instrument panel, I'd estimate mine was made in the mid to late 1960's. It's a vacuum tube instrument, yet it draws only 7 W of power sitting on my bench (costs me a penny a day). Its rated measurement accuracy is 1% for frequencies between 50 Hz and 1 MHz. Personally, I rate it as nearly the ideal general-purpose AC voltmeter for an engineer or technician because it has measuring ranges from 1 mV full scale to 300 V full scale.
  16. t06afre

    t06afre AAC Fanatic!

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    As a thumb rule standard DMMs is only god for measuring AC related to the mains. This information should be avilible in the manual. Some DMMs like the Fluke true RMS series are able to measure other waveforms and give a correct result.
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