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Old 3rd December 2011, 01:11 AM   #11
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An excellent explatation of RMS conversion may be found in the Linear Technology LTC1066 data sheet.

cds.linear.com/docs/Datasheet/1966fb.pdf
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Old 3rd December 2011, 05:32 PM   #12
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(I have posted somethig very similar in another thread but I think it is relevant here as well)

If one or two of your multimeters are of the very-cheap type they can give you very unreliable values at low AC voltages as described later in this post

True RMS DMM will show the true RMS voltage for most waveforms within specified frequency range. Remember this is often only up to a few kHz or less for not-so-expensive TRMS meters..

Some (most?) "Semi-cheap" multimeters often used the rectified average AC voltage (measured after a input cap to remove the DC component) using a OP-based precision rectifier. The average voltage is then multipled a with 1.11 to calculate the RMS voltage. 1.11 is the ratio between the RMS voltage and the rectified average for a sinus wave. This means that these DMMs are good only for clean sine-waves at one frequency. They will show wrong values for all other waveforms, except pure sine waves. You can easily test this by applying a squarewave with 1V amplitude at a low frequency 50-100HZ to the DMM. For a square wave the RMS voltage equals the average voltage, and hence the multimeter will show ~1.11V if it is of the averaging type. The measured voltage could be one or a few hundreds of a volt lower due to that the normally low bandwidth of these DMMs cuts out some of the harmonics of the square wave from the measured voltage.

I have also investigated several "dirt cheap" DMMs ( up to10-12) of different types. If you measure a pure 9V DC voltage in AC-mode with these, in one polarity they will show 0V as expected, in the other direction the ones I have tested show 20V(!). I haven't checked the actual PCB, but to me this means two things. First the input DC-block capacitance is omitted. Second, the multimeter uses only one diode to make a half wave rectification, measures the average and multiplies with ~2.22. I do not think they even take care of the voltage drop of the diode if looking at the numbers carefully. For the main use of these multimeters it does not matter very much. (i.e., measuring if 12V or 6VDC is properly connected in cars, boats, bikes.... and measuring if 230V is connected at home). The first time I discovered this was when measuring the output from a voltage converrter with the multimeter by accident put in AC-mode. to my surprise I got almost the double voltage I had expected. since then I have seen it in several different types of cheap multimeters

However, I would be very reluctant to trust the cheapest multimeters for measuring any low voltage AC-signals, especially non-pure-sines or signals with a DC level. You will get what you pay for.
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Old 3rd December 2011, 11:52 PM   #13
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I think you are confusing what you suspect with what is true.

1. No meter I've ever worked on (literally thousands from crap to highest state of art) has ever used a feed through DC-blocking cap. No input caps.
2. Virtually all multi function DMMs in use today use the same basic desigh of a DC measuring circuit and all other functions feeding that DC measuring circuit, predominantly passively on cheaper meters. None of the DC A/D circuits multiply anything. They just read DC. Ohms uses a constant current source across leads and measures DC. High end meters may contain math functions, but they all work on output end of DC measurement.
3. You are correct that most cheap DVM's use passive, simple rectification, and quite often only half wave for AC readings. They all are frequency relative. The AC response is stated in the specs.
4. You can't make generalities based on just a few samples. "You will get what you pay for." doesn't really apply without qualifiers. I have seven digit meters that resolve tenths of a microvolt that I only paid $50 for. I own a handful of cheap $2.99 DMMs (and keep one in every car). I also own multiple Flukes, HP's and other known brands. For under $10 you can make one of those $2.99 DMMs read as acurately on AC as a 3.5 digit Fluke or HP (assuming DC is accurate). Just add an AD536,636,736 or LTC1099 in place of AC rectifier. There are probably several other chips that would work as well.

For 50-60Hz <500 VAC -sine- measurements, virtually any handhelp DMM ought to read fairly accurately. I'd toss or repair any that varied by a volt at twelve.
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Last edited by thaumaturge; 3rd December 2011 at 11:55 PM.
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Old 4th December 2011, 06:55 AM   #14
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If you measure a DC voltage in AC mode most meters I have used show 0V. Hence DC blocking somwhere down the line. A cap would be the cheap solution but ther might be others

Even today there are still many non-TRMS meters sold (check specs carefully) and if you are not measuring True-RMS internally in the meter you have to measure using another method. The cheapest way is to measure some rectified voltage. The ratio between RMS and the average for a sinewave is 1.11. If the peak is mesured you have to divide by 1.414. Since some non-RMS meters show 1.11 on the suggested 1V amplitude sqare wave I tend to belive in the former. It might be done internally in a AC measuring chip but some conversion must be made

I agree that DC on the cheapest meters are accurate enough and that adding a AC measuring circuit is a good idea to get good reading on these. Depending on your demands att low frequency sines AC accuracy might be good enough if you are sure you have no DC.

When referring to prices I thought of new instruments. I guess you can get second hand ones at much better prices.
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Old 4th December 2011, 08:21 AM   #15
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A meter will read DC on the AC funtion if it is configured to read AC+DC, most as you say are not.

Unless a meter specifically states it reads TRMS, it almost certainly doesn't

Setting meter on DC when measuring an AC signal will read net offset. Likewise Measuring AC on a DC signal will read ripple.

Here is a complete RMS adapter circuit that will cost about $10 to build:
RMS-TO-DC_Adapter_For_DVM
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Old 4th December 2011, 08:36 AM   #16
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I agree.

Still I am not sure the thread starter has got a good answer to his question about the large differences between different meter. The output of the tranformer should be pretty close to a clean sine wave if the transformer is properly built, with no or very small DC voltage. The output impedance is quite low so loading from the meter is neglible, any meter should show fairly correct values no matter what type of AC measurement they are using. Was any of the meters of the TRMS type? I guess at least one of them is not working properly
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Old 4th December 2011, 09:22 AM   #17
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My "guess" would be the one that reads lowest only uses half wave rectification. The one that reads highest uses full wave rectification but is not scaled to read RMS. Most non true RMS AC meters are average reading with AC output scaled -by components- to read the same as RMS. This is fine but unpredictable on non pure sine signals.

The only way to resolve it is to test all three on a known good pure sine calibrator or directly compare them to a known good true RMS meter.
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Old 7th December 2011, 07:34 PM   #18
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Quote:
Originally Posted by thaumaturge View Post
I think you are confusing what you suspect with what is true.

1. No meter I've ever worked on (literally thousands from crap to highest state of art) has ever used a feed through DC-blocking cap. No input caps.
Well I don't know if it is accomplished with a blocking cap or not, but my B&K 5360 handheld DMM (True RMS) has an "AC" mode, and an "AC+DC" mode. Feed 10 VDC to the AC mode, and it reads... 0 V. Feed 10 VDC to the "AC+DC" mode, and it reads 10 V. Likewise, superimpose a 1 VRMS AC onto that 10 VDC, and the AC mode will read, surprise, 1 V. DC blocked.

I also have another True RMS bench meter (Keithley 194A) which uses high speed sampling and mathematical calculation in order to come up with the RMS value. It has selectable AC or DC input coupling, accomplished with a DC blocking input cap, listed only as 0.1 uF in the service manual. Another of my True RMS bench meters, Keithley 199, always uses AC coupling in AC mode, also accomplished with a input cap (0.1 uF 630 V polyester according to the service manual). My Thurlby 1905a (with true RMS option) uses a DC blocker in its AC rectifier section, so technically that one is not an input cap. The two Keithley meters definitely use input caps.
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Old 7th December 2011, 07:49 PM   #19
Mooly is online now Mooly  United Kingdom
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Does this help, post #121 which kind of got on to something like this.

High efficiency speakers - how much power do they really need?

High efficiency speakers - how much power do they really need?

High efficiency speakers - how much power do they really need?
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Old 7th December 2011, 10:53 PM   #20
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All above true, but you need a load across the transformer.
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