Optoisolator switch

Discussion in 'The Projects Forum' started by Man_in_UK, Aug 5, 2008.

  1. Man_in_UK

    Thread Starter Senior Member

    May 13, 2008
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    I though I could work this one out but I am having trouble.

    I was to replace an LED with a 12v array of leds. As the source was only designed to power a single LED I figured a 4n25 opto would be ideal as the array only draws 40ma.

    Problem .... I can't work out how to wire the array through the opto. Every way I try to use the transistor it permanently conducts giving me permanent power to the array.

    I have not yet connected up the input of the opto so its not a input fault.

    How should I use the 4n25 in a circuit to switch this ?
     
  2. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Has the LED-array a current control of its own?
    You can connect the LED of the 4n25 in the place of the origenal led (pin1 anode,pin 2 cathode).
    The output of the 4n25 emitter to ground (pin 4), led array cathode to collector of the 4n25 (pin 5), the anode from the LED array (evt. via resistor) to the +12 Volts.

    Greetings,
    Bertus
     
  3. Man_in_UK

    Thread Starter Senior Member

    May 13, 2008
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    0
    Thanks Bertus.

    The array has current limit resistors so your suggestion is pretty much spot on. Something I have noticed is that the pinout diagram you got is completly different to the one I have been working with.

    I am not at work now but first thing I got to check a schematic that I was using as a starting point, something fishy is going on here ????

    Thanks again.
    What resistor from pin 5 to 12v would you go for ?
     
  4. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    The resistor I named is for current reduction if the LED array had none.
    You say that the LED array has its own current limitting resistors, so you can leave it out.

    Greetings,
    Bertus
     
  5. Man_in_UK

    Thread Starter Senior Member

    May 13, 2008
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    I thought the res would be to stop the switch going into meltdown. I guess they thought of that when they put the opto together.

    Happy days the project is on the go again.
     
  6. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    The transistor in the 4n25 can switch upto 100 mA.
    See the datasheet.

    Greetings,
    Bertus
     
  7. Man_in_UK

    Thread Starter Senior Member

    May 13, 2008
    132
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    I thought (obvously no expert me) that you always drive the gate of a transistor with a limiting resistor ?

    The data sheet does go into switch times depending on what res is used.

    As I say i'm not that clued up with the fine details, if you say I don't need one .. I won't use one.
     
  8. bertus

    Administrator

    Apr 5, 2008
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  9. mik3

    Senior Member

    Feb 4, 2008
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    You dont connect the base of the transistor to 12V via a resistor but connect it to ground if you want. The purpose of this resistor is to control the sensitivity of the transistor to the light emitted by the led. If you want maximum sensitivity dont connect a resistor but if you want to control the sensitivity connect a variable resistor.
     
  10. Man_in_UK

    Thread Starter Senior Member

    May 13, 2008
    132
    0
    Now I understand the resistor bit I wired it all up to what happens.

    It does do what it is supposed to do but the led array is not getting a good enough ground and it is not very bright. All the connections are good as shorting pins 4 & 5 makes the array work correctly.

    Shame.
     
  11. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    How much current is flowing through the LED of the 4n25?
    If this current is to low the transistor of the 4n25 will not pass enough current.
    See for more information on this page 5 of the datasheet.

    Greetings,
    Bertus
     
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