Help on Isolated AC voltage measure

Discussion in 'General Electronics Chat' started by vikramgholap, Sep 10, 2009.

  1. vikramgholap

    Thread Starter New Member

    Sep 10, 2009
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    Hi All,
    I will be grateful if anybody can help me out.
    I need to convert AC line voltage from 110V to 560VAC to equivalent low voltage in 5V range either AC or DC.
    The requirement is it should be isolated measurement and should not be transformer based.
    If anyone has come across Optical Isolation circuit please.
    Appreciate your help. Preferably Analog optocoupler.

    Thanks
    Vikram
     
  2. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Very important to specify the current (or power) levels involved?
     
  3. vikramgholap

    Thread Starter New Member

    Sep 10, 2009
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    I need to measure voltage across Line and neutral. the current would be very less. Eventually the 0-5V signal needs to be given to a microcontroller
     
  4. vikramgholap

    Thread Starter New Member

    Sep 10, 2009
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    The current would be very less.Eventually, the converted 0 to 5 volt signal needs to be given to a Microcontroller ADC.
     
  5. studiot

    AAC Fanatic!

    Nov 9, 2007
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    I would go for a resistive divider, about 100:1, across the power lines.

    100M in series with 1M would achieve this.

    You would take the output across the 1M at low voltage, rectify, amplify and schmidt it. The Schmidt output could then drive an opto isolator or be capacitively isolated.

    The divider should be made with say 4x25M resistors so 4 would have to fail short before subsequent circuitry was exposed to high voltage. Of course the 100M should be connected to the high side. The 1M could be trimmed for exact divider ratio if required.

    Remember if you are working at mains voltage to take proper precautions and seek professional help if you are unsure.
     
  6. vikramgholap

    Thread Starter New Member

    Sep 10, 2009
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    Hi Studiot,
    I got your point but, after optocoupler/schmitt, we will get only High/Low. I will not get the voltage equivalent there. I also gone through Analog isolator datasheet. In that, we will need additional Opamps and probably an isolated power supply for the opamps placed at the input side.
    Is there any simple idea you know. The circuit is cost sensitive and Analog optocoupler would be an expensive business.
    Thanks for your kind reply. I would choose fail-to-open resistors

    Thanks
    Vikram
     
  7. studiot

    AAC Fanatic!

    Nov 9, 2007
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    OK so you don't need a schmidt for analog. But you will still need some signal conditioning to buffer and to get the output to appropriate level for measurement.

    In theory the opto isolator is not necessary, but it is still a wise precaution.

    You can get these with built in signal conditioning amps

    eg

    http://www.netvoxuk.com/opto-isolators.html

    Look for linear opto isolaters
     
  8. vikramgholap

    Thread Starter New Member

    Sep 10, 2009
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    The link provided looks Ok. I shall check out for price with them
    Thanks for quick response
     
  9. someonesdad

    Senior Member

    Jul 7, 2009
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    If you use a resistive divider, make sure the resistors have the proper voltage ratings.
     
  10. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Fair point, but I already said that and there is much more than just the correct voltage ratings.
    But hands are always thrown up in horror when someone suggests connecting a resistor or resistors across the mains, forgetting completely that all devices using the mains do just this. You just need to provide properly spaced and enclosed connections.

     
  11. fabius

    New Member

    Oct 13, 2009
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    Why resistive divider?
    You have to measure an AC voltage, consider using a capacitive divider.
    To attenuate for example 100:1, you need a small capacitor connected to the mains, and a capacitor 100 times larger at the low side, for example 1nF/100nF.
    After all you need voltage, not power. And capacitive divider do not waste power and do not heat (almost).

    There is plenty of safety capacitors designed and approved (so called Y type) to be connected between a net leg and protective earth, they let a very small current flow at 50 Hz, that is considered safe by all agencies.
    So no other isolation is needed, if you use one of these capacitor (for the large one there is no issue) touching the low side is as safe as touching the metal frame of a washing machine.
     
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