need info AC current meters

Discussion in 'General Electronics Chat' started by raffter, Nov 25, 2008.

  1. raffter

    Thread Starter Active Member

    Feb 28, 2008
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    Hi all,

    WHY do current meters have monitoring on BOTH lines (live,neutral)?? Im talking about current transformers... then both "conditioned" signals are sent to 2 ADC inputs(post processor) ....

    how does it measure current?? getting the average value of the two? etc??

    tia

    -Ralph
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    Can you cite a source for that setup? That is usually confined to protective devices like ground fault interrupters, where a difference in hot and neutral implies a current leakage and forces a trip on the breaker.

    In a circuit, the current in the hot lead is the same as the current in the neutral, so measuring one is the same as measuring the other.
     
  3. raffter

    Thread Starter Active Member

    Feb 28, 2008
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    here is one such source:

    http://www.atmel.com/dyn/resources/prod_documents/doc2566.pdf

    now I have made my own CT for this(metal core of course) 1:1000 turns ratio with a 100R burden resistor... I have just made the front end , to make things much simpler.. I can make the PIC/firmware later....

    my problem is this: measuring resistive loads IS OK (like incandescent bulbs and soldering iron<ceramic heater element>) now with reactive loads(like a TV set, I get erronous readings..registering MORE than its actual consumption...

    IS there a trick on HOW to measure with CTs and reactive loads?? or what math equations are involved?
     
  4. The Electrician

    AAC Fanatic!

    Oct 9, 2007
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  5. leftyretro

    Active Member

    Nov 25, 2008
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    Yes, reactive loads can skew a measurement that assumes a sine wave RMS type load. There were chips avalible from Analog Devices that would take a low level AC voltage and output a RMS value. It's the same as used on the better DMM that have can read true RMS voltage. So short of adding this kind of input conditioning I can't think of another method for you.
     
  6. raffter

    Thread Starter Active Member

    Feb 28, 2008
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    can RMS value be "CODED" ?? say a PIC to process RMS value??
     
  7. beenthere

    Retired Moderator

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    Easily, if you can make the conversion at peak voltage. RMS is pretty close to .707 of peak. It's also pretty simple to make the absolute value circuit from the National Semiconductor op amp collection and present the PIC with a voltage also very close to RMS.
     
  8. raffter

    Thread Starter Active Member

    Feb 28, 2008
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    so uhmmm ADC input will suffice as input for RMS conversion??


    regarding NatSemi op-amp collection, what specific circuit should I be looking at?
     
    Last edited: Nov 25, 2008
  9. leftyretro

    Active Member

    Nov 25, 2008
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    Not very easily, would take lots of processing resources. The input AC voltage wave shape, (actually representing the AC current being consumed) would have to be sampled many times per single AC cycle and then some calculus performed before you would have the data needed to do an accurate conversion to true RMS power being consumed. External special function IC circuit is much more practical I think.

    PS: Just answered another thread on a similar topic and found this product. Seems it is possible but I suspect they are using a analog devices RMS converter chip.

    http://www.sparkfun.com/commerce/news.php?id=204
     
    Last edited: Nov 26, 2008
  10. leftyretro

    Active Member

    Nov 25, 2008
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    That's only true when the AC wave form is a sine wave, which only happens with a pure resistive load. When there is a reactive load the AC current wave shape is no longer a sine wave and the .707 Vs peak rule does not apply. Voltage and current are no longer in phase with each other when there is a reactive load.

    Lefty
     
  11. The Electrician

    AAC Fanatic!

    Oct 9, 2007
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    How do you know that your readings are erroneous? How do you know what the true reading is?

    Are you trying to measure only current, or are you trying to measure power?
     
  12. raffter

    Thread Starter Active Member

    Feb 28, 2008
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    my analog front end "connects" to a Digital VOM(in DC mode) ...output in mV equals watts... my reference "loads" are a ceramic heater element soldering iron(20W) and a 100W incandescent bulb....

    reading of 100mV is 100W...
     
  13. raffter

    Thread Starter Active Member

    Feb 28, 2008
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    Leftie,

    is THAT an LC filter?? :eek: I can see maybe a 0.001R shunt...

    EDIT:

    RC filter.. sorry..
     
    Last edited: Nov 26, 2008
  14. leftyretro

    Active Member

    Nov 25, 2008
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    Yea, it's a shunt for sampling the current flow.
     
  15. raffter

    Thread Starter Active Member

    Feb 28, 2008
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    oh I think its NOT an RC filter... more-or-less its an RC line "reducer" :rolleyes:

    to supply power to the circuit... now I get it..

    so "sensing" is purely an ohmic shunt
     
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