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Bargain Optocouplers

Discussion in 'Electronic Basics' started by Chris, Dec 18, 2004.

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  1. Chris

    Chris Guest

    I picked up some optocouplers cheap from the bargain bin. I was
    expecting to find 4 useful pins (2 in, 2 out, 2NC), but these turned
    out to be 4N32 units with a mysterious pin 6 connected to the base of
    the output phototransistor. Can I ground this pin to get the device
    to work as a 4 pin coupler? Connect it to supply voltage?

    I just want to switch the output side on/off without an electrical
    connection to the circuit on the input side, no amplification,
    inversion, etc required.

    Thanks,
    Chris
     
  2. I cant speak with experience but would imagine you could use them '4 pin'
    and should the need arise the base would give you an alternate switching
    method ??
    (when in doubt experiment :)
     
  3. CBarn24050

    CBarn24050 Guest

    Subject: Bargain Optocouplers
    You can sometimes just leave it unconnected but it is better to connect it to
    the emitter via a 47k resistor.
     
  4. Chris,

    You can use the 6-pin optocouplers just like the 4-pin ones if you leave the
    transistor base unconnected. In 4-pin optocouplers, the phototransistor
    turn-off time is usually quite a bit longer than the turn-on time. This limits
    the maximum repetition rate and distorts the waveform. For higher speed
    operation, the base connection of the 6-pin optocoupler is tied to the emitter
    via a resistor. This reduces the transistor turn-off time (drains charge from
    the base quicker). There are some good application notes on the
    Vishay-Telefunken web site that explain this and can help you figure out how to
    size the resistor if you are trying to speed up your optocoupler or reduce the
    distortion.
     
  5. Rich Grise

    Rich Grise Guest

    Excellent answer. I've wondered this myself on occasion.

    Thanks!
    Rich
     
  6. Ah. This was probably Jagman's problem when he was using the same part...

    --
    Regards,
    Robert Monsen

    "Your Highness, I have no need of this hypothesis."
    - Pierre Laplace (1749-1827), to Napoleon,
    on why his works on celestial mechanics make no mention of God.
     
  7. John Larkin

    John Larkin Guest


    The b-e resistor trades CTR for speed. It also increases the breakdown
    voltage of the transistor, which is sometimes useful.

    John
     
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