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Puzzled by opto-coupler.

Discussion in 'Electronic Basics' started by [email protected], Nov 5, 2006.

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  1. Guest

    I was toying with an opto-coupler from damaged machinery and
    am puzzled by its behaviour. It has the SHARP logo, and was
    used as a tachometer with a slotted wheel revolving in a slot
    between IR LED and photo-transistor. It has three terminals
    which I've deduced to be LED anode with a series resistor,
    phototransistor collector, and a common terminal for LED
    cathode and emitter.

    When I apply an adjustable current source at the LED input,
    and a 3V supply with a series resistor at the phototransistor
    collector, the transistor current rises with LED current.

    However, until the LED current reaches about 6 mA, it makes no
    discernible difference to the transistor current whether it's
    shielded from the LED or not. That is, the transistor current
    stays high even when it's completely blocked.

    When the LED current exceeds this critical level, the transistor
    current is switched on and off by blocking and unblocking the
    IR light from the LED.

    At first I thought the device was defective, or that I'd
    incorrectly identified the terminals. But this doesn't seem to
    be the case. Can anyone please explain this behaviour ?
     
  2. Baron

    Baron Guest

    Since you don't give a part No:, I'll hazard a guess that the transistor
    has a built in Schmidt trigger !
     
  3. Guest

    Unfortunately, the part No. is no longer legible. I thought
    of some hysteresis effect too. But why should that cause it
    to refuse to turn off even when the IR beam is completely
    blocked with a metal plate ? And it does turn off when the
    LED current supply is cut.

    Also, when I gradually increase the LED current, the photo-
    transistor current increases in approximately direct proportion.
    This suggests that there is no Schmitt trigger effect.
     
  4. Baron

    Baron Guest

    That suggests a common circuit !
    Still guessing ! Can you positively identify the connections on both
    sides ? You said that the LED has a series resistor ? Is it built in
    or an external part ?
     
  5. So something like this:

    : adj. +3V
    : current |
    : source |
    : | \
    : | / R1
    : \ \
    : / R2 /
    : \ internal |
    : / series +---> to voltmeter
    : | |
    : | |
    : D1 --- ~~ |/c Q1
    : LED \ / ~~ -| NPN
    : --- ~~ |>e
    : | |
    : '-------------+
    : |
    : gnd

    There is no pin for Q1's base, I take it, and you are providing an
    external R1 but not R2 (which is internal to the device.) You are
    monitoring the collector voltage, right?? Or are you directly
    observing the current through R1?
    Okay. Clarity issue. You wrote earlier, "the transistor current
    rises with LED current." Here just now, you say "no discernable
    difference to the transistor current" AND you say, "the transistor
    current stays high."

    I think you are confusing terms, for one thing. I am going to go out
    on a limb and guess that you are measuring the voltage at that
    junction I show above as "to voltmeter." And that when you write "the
    transistor current stays high" that you really mean that the
    "transistor VOLTAGE stays high." In other words, that there is no
    appreciable current flowing through R1 and you are actually measuring
    voltages and not currents.
    By this, I take it to mean that you see the voltage at the collector
    of Q1 readily changing.
    Are my guesses correct as you understand it?

    Jon
     
  6. Baron

    Baron Guest

    Hi Jon. That makes sense to me. Not realised that he was measuring
    voltages ! His mentioning 6ma LED current didn't help.
     
  7. Guest

    Hi, Baron and Jonathan. Thanks for your interest, but no, I
    did not mix up current and voltage in my description.

    First, when I talked about 6mA and current source, I used a
    simple regulated current source I designed and constructed
    a long time ago to test zener diodes (it has served me well
    for a variety of purposes I never envisioned). It can be
    adjusted to supply 0.5 to 15mA up to a max of 50V.

    I deduced that the LED has a series resistor by measuring
    the voltage drop at different current levels. By subtracting
    an estimated LED voltage, I deduced that the series resistor
    is about 220 ohms.

    Jonathan, your ASCII diagram is correct except that I
    measured collector current, not voltage. Well, I did also
    monitor the collector voltage with a 'scope, but when I
    mentioned current, I do mean current as displayed by a meter
    in series with the collector supply.

    Regarding collector voltage, the CRO showed that it was
    driven down to saturation, and stayed down even when the
    phototransistor window was completely blocked, until the LED
    supply was removed.

    One possibility is that the common pin does not go directly
    to cathode and emitter, but through a small resistor for
    some hysteresis effect. I have not yet worked out the rest
    of the circuit in my head that will fully explain its
    behaviour. Maybe the 3V supply was too low for proper
    operation. I'll try again with a higher voltage, and note
    down numerical values.
     
  8. Accepted. I guessed wrong.
    In that case, you are probably in a better position to examine your
    problem (with us watching) than any of us. You know enough to design
    an adjustable current source for diodes (which can set overly simple
    feedback designs into oscillation.) And you have the part, too.
    I'm glad you are adding in fuller details, as you understand them
    currently.
    So no ambient light getting in, scope properly ground referenced, no
    hot lead of a reversed, non-ground-prong plug placing hot side AC
    through your weak device and ruining it, etc. Assume valid
    procedures. Got it.

    Have you considered the possibility that the IR LED emits light that
    can pass through your block? (For example, I think steel passes
    around 20cm wavelengths and longer about like glass and can be used to
    lens those wavelengths.) I know it is a longshot... but you haven't
    said what you know about your barrier and how certain you are that it
    blocks the emitted wavelengths. (I assume this is a near-IR LED, so I
    suspect that your block works okay... but I have to ask.)

    Do you have a way of observing the IR LED, more directly? A separate
    photodetector, for example, that is wired to a simple transimpedance
    amp?
    Well, there are various possibilities. Including Baron's hint towards
    something more complex in the circuit. On this assumption, I took the
    liberty of looking at Sharp's web site. I previously assumed you'd
    done that and didn't find anything helpful, so I didn't want to waste
    my time, too. But in this case, I took a chance.

    Try this:
    http://sharp-world.com/products/device/lineup/data/pdf/datasheet/gp1a073lcs_e.pdf

    Look on page 2 at the schematic. This is a three-pin device with an
    OPIC output. What do you think of this possibility?

    Here is a general page for photointerruptors from Sharp:
    http://www.sharpsma.com/Page.aspx/a...00e-4400-b23a-ee90c054389a/Photointerrupters/

    Jon
     
  9. Eeyore

    Eeyore Guest

    It's not a simple phototransistor output device in that case. Their output
    conducts when the light path is unobstructed.

    Graham
     
  10. Guest

    I used mild steel plate about 0.5mm thick. It must be
    effective as a barrier because it turns the output on
    and off when the LED current is above 6mA.
    I didn't have anything handy by way of a photodetector.
    But I shone a TV remote control on the phototransistor.
    It showed the expected pulsed waveform on a CRO.

    [snip]
    Thanks for the link. I did look for a datasheet, but I must
    have looked in the wrong places. It *was* about 3:00 am. The
    device shown looks very much like the one I have. The
    simplified internal schematic accounts for its behaviour,
    especially the way they connect the LED cathode in series
    with the receiver instead of to the return terminal.

    I thought of using it in a project and would have made a
    mistake if not for that datasheet. The 7V max rating implies
    that it's meant to operate at TTL level. The project is an
    analog-digital hybrid using a 12V supply throughout.

    Now I have the options of changing the digital P.S. to 5V
    and adapting the signal levels at the analog-digital
    interface points, OR to provide a 5V supply just for the
    optocoupler and let its output drive a transistor operating
    from a 12V supply rail.
     
  11. Rich Grise

    Rich Grise Guest

    It sounds like there's more to the circuit than you can see by
    looking at 3 leads, like the PHT is getting some kind of bias
    from the current supply. Is it acting like a threshold thing?

    But anyway, if you intend to use it, I'd go ahead and supply
    it with whatever current makes it work, and just go for it -
    you intend to use it as an interruptor, not a linear coupler,
    right?

    Good Luck!
    Rich
     
  12. Baron

    Baron Guest

    The data sheet explains the hysteresis and the apparent strange
    behaviour.
     
  13. Guest

    That's right.
    Thanks.
     
  14. Guest

    Yup.
    Regarding pulse level conversion, I think I'll go for the
    option of providing +5V supply for the device and use the
    circuit to convert the signal level. It's simpler than
    using a dedicated level converter IC. There will be phase
    reversal of the pulse, but that's not important for the
    intended application. Can you see anything wrong with it ?

    +5V +12V

    | |
    | |
    \ \
    47k / / 10k
    \ \
    / / ----- to f-v converter
    \ \ |
    | |-----|
    | | |
    From | |/ ----- to counter
    photo-tr --------| NPN
    collector |\
    |
    |
    _|_ Ground

    At first, I thought of inserting a diode in series with the
    transistor base to raise the threshold level. But the
    low-level output of the phototransistor is specced as 0.35V
    max, so inserting the diode seems to be rather superfluous.

    Resistor values are uncritical. Dark current is not specified
    for the photo-tr, but I think 47k would be a reasonable
    starting point. Just sharing my thoughts. Comments welcome.
     
  15. Baron

    Baron Guest

    Looks about right ! The 47K could be a bit high though.
     
  16. The output transistor of that schematic is NOT a phototransistor.
    I'd expect that anything slower than about 30kHz would look okay. Your
    counter is probably okay, but I don't know what you are using for an
    f->v. If the f->v is a simple low pass filter, you might not get what
    you want as it isn't even close to a balanced output.

    The approach you provide is fine. It will treat the 47k as a rough
    current source (you could also use the +12V to improve that a little,
    but only if the output transistor can handle the voltage impressed on
    its collector -- you need to check the spec on it.) Anyway, this
    means a vaguely constant current that is switched one way or another.
    Looks fine.

    Jon
     
  17. Guest

    Oops. I do keep referring to it as a phototransistor,
    don't I ? :) Of course, the output transistor is simply the
    output device after amplifying and conditioning the signal
    from the photosensitive element.
    Frequencies are well below 1 kHz. The counter is a 4060
    with Schmitt trigger input, so no transition time
    limitation. The F-V converter is an LM2907 that can
    operate even with ground-referenced sine waves and has a
    switching threshold well below +/- 100mV. I intend to use
    capacitor coupling to make it swing above and below ground,
    and then drop the +/- 6V pulse (duty cycle is approx 50%)
    with a 50:1 resistive voltage divider.
    I wish I could power the output directly from +12V to
    obviate the need for a level converter stage - I like to
    keep my circuits as simple as possible as long as they
    work reliably. But the spec sheet gives the max supply as
    7V. The output transistor alone can probably withstand 12V
    or more, but I don't want to risk it.
     
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