All About Circuits Forum Multimeter measuring AC voltage
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#1
10-23-2007, 09:52 PM
 cheddy Member Join Date: Oct 2007 Posts: 87
Multimeter measuring AC voltage

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
10-23-2007, 10:33 PM
 beenthere Senior Member Join Date: Apr 2004 Location: Missouri, USA (GMT -6) Posts: 15,815 Blog Entries: 10

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
10-23-2007, 10:56 PM
 cheddy Member Join Date: Oct 2007 Posts: 87

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
10-23-2007, 11:55 PM
 JoeJester Senior Member Join Date: Apr 2005 Location: Grand Prairie, TX, USA Posts: 2,355

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.
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#5
10-24-2007, 12:21 AM
 niftydog Member Join Date: Jun 2007 Location: Canberra, Australia Posts: 95

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
10-24-2007, 01:02 AM
 JoeJester Senior Member Join Date: Apr 2005 Location: Grand Prairie, TX, USA Posts: 2,355

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.
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#7
10-24-2007, 05:01 AM
 SgtWookie Expert Member Join Date: Jul 2007 Location: In the vast midwest of the USA; CST Posts: 22,032

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.
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#8
10-24-2007, 05:52 AM
 The Electrician Senior Member Join Date: Oct 2007 Posts: 1,722

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
10-24-2007, 03:26 PM
 techroomt Senior Member Join Date: May 2004 Location: pennsylvania Posts: 198

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
10-24-2007, 09:22 PM
 GS3 Senior Member Join Date: Sep 2007 Posts: 268

Quote:
 Originally Posted by cheddy 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?
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.

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