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Why Does Digital LED Volt Meter Fluctuate

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  • Why Does Digital LED Volt Meter Fluctuate

    I bought a bunch of these LED volt meters. They all are accurate when put up against my digital VOM. However........ sometimes the last two digits will fluctuate.
    What causes this??? One of them will be connected to a 6 volt battery which I'm using for test purposes. My digital VOM does not fluctuate but the small LED display does. Also the LED meter deducts what it uses to power itself. This is verified as I have my VOM connected to the battery also. When I connect the LED meter the battery voltage drops by about .30 volts and both my VOM and the LED meter read the same, so it's accurate but sometimes the last two digits fluctuate all over the place and my VOM tries to keep up. What's going on???

    Does anyone understand what I'm talking about?????

    JL..................

    This is what the LED volt meters look like.

  • #2
    The ones I bought like that have a separate lead for power. Use them on my electric carts and they are pretty stable. Could be something to do with the meter measuring the same voltage as the one that is powering it.

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    • #3
      In a word, "averaging", or lack of it.

      Most hand held meters average the input so that noise, and tiny transient changes in the voltage don't quickly push the digit display up or down a number. It is also possible that the meter itself changes current draw when different digits come on ("8" may draw more than "1") and there may be a feedback effect that makes the panel meter see a change in voltage from wire resistance as current changes.

      The panel meters may not have that built-in averaging because it costs, and it takes up space.

      I second and third the idea of power separate from the measurement. Even if both come from the same battery, it is best to have a different set of wires for measurement, not the same as for power, so that power current does not affect the measurement due to wire resistance.. Obviously if the power comes from some totally different source, then the meter will have even less effect on the measurement.

      Ideally the meter would not draw any current when doing a voltage measurement. Obviously that never happens, but most hand held meters are 10 megohms input impedance, so they draw only 100nA (0.0000001 A) per volt. I do not know what the impedance of the panel meters is.
      Last edited by J Tiers; 11-27-2017, 09:23 PM.
      CNC machines only go through the motions

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      • #4
        Originally posted by JoeLee View Post
        When I connect the LED meter the battery voltage drops by about .30 volts and both my VOM and the LED meter read the same, so it's accurate but sometimes the last two digits fluctuate all over the place and my VOM tries to keep up. What's going on???


        The .3 volt drop is a LOT for a low drain design like that should be. It tells me your battery may be weak or the current drain may be high. I'm guessing that the last two digits are to the right of the decimal, is that correct?

        Dan
        At the end of the project, there is a profound difference between spare parts and left over parts.

        Location: SF East Bay.

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        • #5
          It sounds like your LED meter is pulling enough power to draw down the battery by .3volt. If you are measuring that much drop with your digital VOM when you connect it to the LED panel meter you show then I suspect that your 6 volt battery is a little older or might not be all that good a condition or there's a slightly ginky cell in somewhere in the pack. It happens....

          If the fluttering of the last digits is related to jumping between numbers then I'd agree with all that has been posted prior to me. But if it's a LED intensity thing then your panel meter has a bum joint or component in the switching element that enables the last digit. Usually these things are strobed with one line for each digit and 8 lines to run the segments and decimal point. If the line that runs the flaky digit has some manner of fault it may cause the last number to flicker in brilliance. But be aware that if the it's flickering between two or three digits that'll often have the same effect.
          Chilliwack BC, Canada

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          • #6
            Noise in the ADC circuit, and as J Tiers nailed it, no averaging of the values being read.

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            • #7
              J Tiers.............. It is also possible that the meter itself changes current draw when different digits come on ("8" may draw more than "1")
              That is a very logical explanation. The more segments lit the more the drain.

              DanLB & BC...... you guys are also right on.......... the 6v lead acid battery is old. It's dated 2004. It still holds a charge but at that age it's questionable.
              Sometimes the meter will stabilize for 5 or 10 seconds or so and then start to fluctuate again. The drain is about .25 volts not .3 but not much diff.
              I've tested a couple of these meters so far. The other one seems a little more stable. Bad solder joints somewhere???

              A closer look at the fluctuation............ about 5.41 to 5.47 hard to tell accurately but that's an approximate in the fluctuation.

              JL............

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              • #8
                If you know the input impedance of the little meter, you could potentially create an averaging filter to slow down the variations that are presented to the meter input. Basicall.y a series resistor and a capacitor shunted across the input

                You need the inout impedance to see if the resistance part of the filter is going to make a significant difference in the actual reading.

                This is really only for reading solid low impedance sources like a battery, you would not want to use the technique for high impedance sources.
                CNC machines only go through the motions

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                • #9
                  If you are reading 5.41 to 5.47 volts on a six volt lead-acid battery, it is definitely weak or mostly discharged, and it could even be noisy due to deterioration. Most digital multimeters update the reading about 3-4 times per second and probably average ten or so samples for each reading, so noise and jitter are canceled out. You might also see if the meter leads are extremely thin and possibly made of something other than copper, with high resistance. There are cheap meters and really cheap meters.
                  http://pauleschoen.com/pix/PM08_P76_P54.png
                  Paul , P S Technology, Inc. and MrTibbs
                  USA Maryland 21030

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                  • #10
                    To rule out (auto) supply issues, you could test it on a car battery.
                    Helder Ferreira
                    Setubal, Portugal

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                    • #11
                      A closer look at the fluctuation............ about 5.41 to 5.47 hard to tell accurately but that's an approximate in the fluctuation.
                      I'd try running your panel meter off a different battery than the voltage you're trying to measure. Given your readings it's quite likely the battery is on its last legs. And it's just possible that the swing you're noticing is related to the current draw of three segments being lit for the "7" and just two segments for the "1". So you might be seeing a 7, which draws a hair more current and pulls the battery down to a "1" which lets off the current and allows the battery to slowly recover up to 5.47 which the meter duly reports and that sucks it down again.....

                      I know that a segment only draws a very few milliamp and it's likely that you'll get SOME fluttering of the last number in any case. But if the battery is that old then this issue likely isn't helping any either. And it would be funny if the battery is so bad that this proves to be the issue....

                      So get another separate battery even if it's only a single cell out of a flashlight and try it.
                      Chilliwack BC, Canada

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                      • #12
                        Try putting a little .1uf decoupling cap across the power input as close to the LED meter PCB as possible.

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                        • #13
                          I did try the meter on a good 12v car battery. It was a solid 12.6v reading with no fluctuation but on 12v there is only one digit after the decimal point.
                          After a fresh charge of the 6v battery It seems to be a little more stable. It takes a while for the battery voltage to settle after the charge, I waited about 24 hours before testing the meter. I guess the condition of a 14 year old lead acid battery is questionable.

                          JL...............

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                          • #14
                            For a stable single digit power source try measuring the voltage on a USB port.
                            I get a very stable 5.05 v on mine.

                            Home, down in the valley behind the Red Angus
                            Bad Decisions Make Good Stories​

                            Location: British Columbia

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                            • #15
                              Maybe related, or worth a mention at least, I can clearly remember working on the controll system on a big powered car that carried things called tundishes for continuous casting, when going through the setup l/we could not get a steady reading using a digital meter, it fluctuated wildly and we could not calibrate the thing, one of the old technicians came and had a look, went back to the workshop and got an old analog meter (avo8 as it happens) apparently there was a low frequency component that was driving the digital bonkers, it was years ago mind, perhaps new gear isn't suceptable to that?
                              Mark

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