calculating optocoupler resistor values

Discussion in 'General Electronics Chat' started by yusim, Jun 10, 2009.

  1. yusim

    Thread Starter Member

    Apr 27, 2009
    10
    0
    hello all
    this is my first post i think

    i'm trying to build a circuit that isolates two PIC16F877As using a quad optocoupler - pc847

    this is a sample of the circuit i built
    [​IMG]

    i was wondering if someone could explain to me what the ideal value for resistor R2 would be

    thanx in advance,
    yuda
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    You may have some trouble with your circuit. It will only allow a few milliamps through the emitter. IR emitters are rated up to 50 ma. You would be better off using a Darlington array for enough current.

    I always use a fair sized resistor in the collector circuit of the detector - around 47K, with the emitter straight to ground. The transistor does not get much drive from photons, and CMOS likes to see the input switch between Vcc and ground, which is hard to do in an emitter circuit.
     
  3. hgmjr

    Moderator

    Jan 28, 2005
    9,030
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    Another parameter that would be helpful to know is what frequency do you plan to use in driving the photo-coupler?

    hgmjr
     
  4. yusim

    Thread Starter Member

    Apr 27, 2009
    10
    0
    thanx for the promt replies :)
    beenthere said:
    the ir led can be lit by as low as 5mA with a ctr of 120% giving up to 6mA output - in my circuit (~4.7v-1.1v)/560 = ~6.5mA
    being that the output goes to the input of a PIC i thought that would be enough

    beenthere said:
    i tried the circuit i made and it seems to work fine
    i tried 1K ohm Voh was about 4.88v which was ok but this takes about 4.7mA
    using 13 of these in a low power circuit is a bit much
    i then tried 1M ohm but i got a VoL in the hundreds of millivolts - too high

    what im looking for is a way to calculate the ideal value for R2

    hgmjr said:
    a very low freq. maybe a few times per second
     
  5. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    Why do you need this isolation?
     
  6. yusim

    Thread Starter Member

    Apr 27, 2009
    10
    0
    this is all for a biofeedback system that outputs a composite video signal
    one microcontroller does al the calculations and the other creates the video signal
     
  7. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    Ok but why you need this kind of isolation?

    Does the microcontrollers have different power supplies?
     
  8. yusim

    Thread Starter Member

    Apr 27, 2009
    10
    0
    mik3 said:
    the user/patient must be galvanicly isolated from the mains power supply of the tv sceen

    mik3 said:
    Yes, the PIC that takes care of the calculations is battery powered and the PIC that creates the video signal is powered by an AC adapter.
     
  9. yusim

    Thread Starter Member

    Apr 27, 2009
    10
    0
    ok, back to the original question

    is there a formula to calculate the best value for the pull-down resistor R2?
     
  10. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    The PICs' pin has a small amount of capacitance. Whatever resistance you chose will drain the charge from the pin when the transistor turns off.

    I suggest using a 10k pull-down resistor for R2. It's a decent compromise between power consumption and pin discharge time.
     
  11. Zenock

    Active Member

    Jun 1, 2009
    36
    1
    Have you considered putting a small resistor or possibly a diode on the ground pin of the second PIC?

    I don't know, there's \sout{possibly}\  probably some reason why you shouldn't do this.

    Z
     
    Last edited: Jun 16, 2009
  12. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    You wouldn't want to do either of those things. Interfering with the ground path of a uC just would be a bad idea.
     
  13. yusim

    Thread Starter Member

    Apr 27, 2009
    10
    0
    SgtWookie said:
    i cant seem find the spec for the input pin capacitance in the datasheet
    i found the "Capacitive Loading Specs on Output Pins" in the data sheet but nothing about the inputs

    where might i be able to find it ?
     
  14. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    Take a look at parameter D101 in the PIC16F87X datasheet. It appears that the capacitance at all but the three SPI pins is 50 picoFarads.


    hgmjr
     
  15. yusim

    Thread Starter Member

    Apr 27, 2009
    10
    0
    hgmjr said:
    that parameter is talking about the cappacitance loading of output pins
    - im looking for the specs for an input pin - or is it the same thing??
     
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