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Ac voltage indicator using optocoupler

  1. Dec 28, 2013 #1

    I want to design an AC voltage indicator. So i designed the circuit attached, but now i am trying to calculate the value of the resistor R1.

    I have the following problem:

    1- The voltage should be given at the terminals of the optocoupler.
    2- The minimum current that should flow through the optocoupler in order to turn on the photo transistor.
    3- How can i find these info. in the datasheet i tried to open the data sheet for PC-217 opto coupler but i wasn't able to find them.


    Attached Files:

  2. jcsd
  3. Dec 28, 2013 #2


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    hi there hisham

    just remember that the LED in the opto-coupler is pretty much a standard LED
    your R1 will need to provide voltage dropping and current limiting to keep the current through the diode below maximum
    most average LEDs are ~ 2V forward voltage and ~ 20 mA. So depending on what the max AC voltage you are applying will determine the value of R1 to keep a max of 2V drop across the LED and a max of 20mA through it

  4. Dec 28, 2013 #3
    Hi davenn,

    But drawing droping the voltage through the resistor from 354V to 2 V at the terminals for the optocoupler will result is a high resistance required (This is not a problem i can use many resistors in series), but if you take 20 mA current the power for the resistor to be selected will be very huge. So i don't think this is the case & that the minimum current that can be used in order to open the photo transistor is much less than 20 mA.
  5. Dec 29, 2013 #4


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    use a series capacitor for a high reactance without power dissipation. Google images show a few circuits. search for "AC line voltage indicator LED"
  6. Dec 29, 2013 #5
    No i want to use the resistor & optocoupler configuration, but the value of 20 mA is not the minimum current that can be drawn through the optocoupler to open the photo transistor.

    My question is what is the minimum current that can be drawn through the optocoupler to open the photo transistor (its a general question not nessary to this project).
  7. Dec 29, 2013 #6


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    You are better off using a capacitor and resistor to drop the voltage. Your sims will show how well that works. I'll let you play with the capacitor value. There is no good reason not to do it that way. The examples show an LED, but an optocoupler is essentially an LED driving a transistor. Just replace the LED with the optocoupler.

    You need to read and understand the data sheet to determine the current transfer ratio for the part you are using. Is is different for every part number. And it varies with load current. This part http://www.avagotech.com/pages/en/o...gton_transistor_output_optocoupler/hcpl-073a/ is very different than a 4n25.
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