DC voltage measuring

Discussion in 'The Projects Forum' started by AFAS87, Feb 21, 2010.

  1. AFAS87

    Thread Starter Member

    Oct 9, 2008
    23
    0
    hi,,, i wanna measure the output voltage of a buck-boost converter with PIC,,

    the idea is that to reduce the voltage from [0,100] v at maximum ,to [0,5] volt to match the ADC of the pic, in addition of that i need isolation to protect the pic and control circuit

    iam to use 4n35 optoisolator.

    is that true? " can i use this optoisolator for this purpose",

    thanks,,
     
  2. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    By using the opto-isolator you will not be able to directly monitor the DC voltage from the converter. You could use pulsewidth modulation to encode the data into a logic signal. Then you could use the PIC to decode the PWM signal.

    hgmjr
     
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  3. russ_hensel

    Well-Known Member

    Jan 11, 2009
    818
    47
    No problem with direct connection to the PIC as long as the supply is isolated from the mains ( as the brits say ). Just the voltage divider is enough. If it is not isolated then it is still ok for the PIC but the circuit can be dangerous to work on unless you do the work with a bench isolation transformer in place.
     
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  4. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    The easiest approach, which is not isolating, is to use a resistor voltage divider and use Schottky diodes on the analog input of the PIC to protect it from possible over/undervoltages.
     
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  5. AFAS87

    Thread Starter Member

    Oct 9, 2008
    23
    0
    thank u..
    i wanna make voltage divider but can i use this optoisolator to isoalte DC volatge for protection.
    regard
     
  6. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    No, you cannot use an opto-isolator in an easy way.
     
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  7. AFAS87

    Thread Starter Member

    Oct 9, 2008
    23
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    why?
    what is the solution?
     
  8. MikeML

    AAC Fanatic!

    Oct 2, 2009
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  9. AFAS87

    Thread Starter Member

    Oct 9, 2008
    23
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    thank you...
    does isolation amplifier work for 0-1000 Vdc?
    can you attach aschematic for this application or alink.
     
  10. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    1,015
    69
    Normal opto-isolators are very none-linear, they are designed more as switches.

    You can get special analog signal ones, which have two matched sensor elements. The idea is you use identical loads on the two sensing elements, one of which is the actual output and the other is used as feedback with an opamp to control the LED.

    But, I don't know just how accurate that would be considering long-term drift.

    Another technique is to use a voltage-to-frequency converter to feed a conventional opto isolator, then a frequency-to-voltage converter to re-create the analog signal.
    This is much more accurate than the analog coupler.

    That may be your best approach, but doing the frequency to voltage part in software by feeding the opto to a counter input on the PIC. A VtoF converter should be far cheaper than a commercial isolation amp.
     
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