Optocoupler 4n25 relay circuit

Discussion in 'The Projects Forum' started by espanholito, Oct 12, 2010.

  1. espanholito

    Thread Starter New Member

    Oct 12, 2010
    3
    0
    Hello,

    I hope someone can help me this circuit. I have some 4n25 optocoplers and 2n2222 NPN transistors and I nedd to activate a 24VDC relay coil.

    I have tryed this circuit, but haven´t manage to activate the coil.

    I'm using two computer DC sources connected in series to get the 24VDC.



    Thank you
     
  2. blueroomelectronics

    AAC Fanatic!

    Jul 22, 2007
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    Why bother with the opto-coupler? Relays by their very nature are isolated.
     
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  3. gerty

    AAC Fanatic!

    Aug 30, 2007
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    Looks like he only has a 1.5 volt signal, not enough to turn on a relay, or the 4n25 for that matter.
     
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  4. SgtWookie

    Expert

    Jul 17, 2007
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    If your input signal is actually only 1.5v, then change R1 to 30 Ohms.

    Remove R2; it'll cause problems.
    Replace R3 with a 2.2k resistor.
    Do you have the emitter of the 2N2222 connected to -12v? You didn't differentiate between collector and emitter (the arrow is missing), but the emitter has to go to -12v in order for it to work.
     
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  5. espanholito

    Thread Starter New Member

    Oct 12, 2010
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    Thank you all very much for your help,

    I forgot to remove the R1 resistor, actually i'm using 1.5V without any resistor connected to pin 1.

    Yes, the emitter is connected to the -12V. I will now test the circuit, after that i'll give my feedback.

    Tonight I have also found a pdf from electus distribution talking about the theory of optocoplers and connection examples.

    I've attached the pdf for everyone who needs it.
     
  6. espanholito

    Thread Starter New Member

    Oct 12, 2010
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    0
    I have managed to complete the circuit, I attached the final schematic.

    If you see something that can damage the circuit, I would appreciate that you tell me.

    Thank you very much for your support
     
  7. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    I do not see any reason for the opto. But if u want to use it, u can.
    Use a 30 ohm resistor (R1) in series with signal and opto input or u might burn the opto LED as Sgt suggested

    Second what is the relay coil voltage. If it is not rated for 24V, u will have trouble.

    Third , pin 6 is an input, do not use it, Connect R3 to pin 4.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    You do need a current limiting resistor for the input side.

    The datasheet states that the IR emitter has a typical Vf of 1.15v@10mA current.
    You're using a 1.5v signal to turn it on.
    (1.5v - 1.15v) / 10mA = 35 Ohms

    If you omit the resistor, you will probably have excessive current through the IR emitter and it won't last very long.

    As Rifaa mentioned, you have connected R3 to the output transistors' base, which won't work. If you want to connect something to pin 4, make it a 100k resistor that connects to -12v; that will help it turn off more quickly.
     
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