RMS measurements

Discussion in 'General Electronics Chat' started by renegoutte, Mar 26, 2006.

  1. renegoutte

    Thread Starter New Member

    Mar 25, 2006
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    I am using a battery charger controlled by thyristors. The voltmeter and ammeter I have read anything but the true rms.How can I calibrate the instuments to read the true rms? Also I would like to find articles about this subject. Thanks guys and ladies.
     
  2. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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  3. beenthere

    Retired Moderator

    Apr 20, 2004
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    Hi,

    National Semiconductor has some RMS circuits in one of its op amp applications guides. Linear technology sell several ic's that have a true RMS output - see their special functions stuff.

    The question is - why are you concerned with the true RMS measurements of the charging process? As long as the battery charges and the float voltage is good, all that is necessary has occured. Lead-acid cells are pretty insensitive to charging rate.
     
  4. Papabravo

    Expert

    Feb 24, 2006
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    "...a battery charger controlled by thyristors." doesn't tell me a great deal so I don't know how to be more helpful.

    An instrument which is not designed to read RMS voltage or current cannot be "calibrated" to do so. If your meter represents that it can measure AC volts, what measurement do you think it is making? If you stick the probes into a wall socket in the US and it measures about 110 VAC then you have a meter which is making an RMS measurement. I don't understand your problem.

    What exactly are you trying to measure?
     
  5. renegoutte

    Thread Starter New Member

    Mar 25, 2006
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    Instruments which are not labeled "true rms" give only an mrs reading if what you measure is a perfect sine wave. The output of an SCR is a chopped up sine wave which will fool the "non true rms" instrument.
     
  6. Papabravo

    Expert

    Feb 24, 2006
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    Yes I agree that the RMS measurement of a typical voltmeter makes an assumption about the input waveform. I also agree that the chopped up sine wave can be measured with a True RMS device. I'm still not sure that I understand the usefulness of the true RMS value of the chopped up sine wave.

    If the chopped up sine wave has a non-zero average over a single cycle because for example the thyristor turns on at some angle between 0 and pi (positive half cycle) and turns off at the zero crossing, and remains off for the negative half cycle. Then using either the DC or AC measurement might provide some insight. If the chopped up sine wave covers parts of both the positive and negative half cycles then getting a "true RMS" instrument may be the only choice.
     
  7. n9352527

    AAC Fanatic!

    Oct 14, 2005
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    Buying a true RMS meter would certainly meet your requirement. But it is quite an expensive option. I've previously used conversion circuit based on Analog Device AD737 and normal meter for true RMS measurement because I needed four of them at the same time and it was much cheaper to build them than to buy four true RMS meters. Of course, you would need to calibrate it before using it.

    Edited:
    You can use a known waveform that is easy to calculate the RMS to calibrate the circuit.
     
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