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Problems with the NTE 3083 (4N32) Optoisolator

Discussion in 'Electronic Basics' started by Jag Man, Nov 19, 2004.

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  1. Jag Man

    Jag Man Guest

    I am using a pair of these as the output stage in my dual pulse generator
    discussed
    in several recent threads. When breadboarded, it worked. If I connected a
    10k
    resistor between +12 and the optoisolator collector, and grounded the
    emitter,
    I could see the optoisolator switch from open to conducting. When open
    the collector was at +12 and when it conducted this voltage droped to 0
    almost
    instantaneously. When it opened again it went back up to +12, albeit a
    little
    slower than I had expected.

    Now, after moving the circuit over to a PCB and soldering it up the
    optoisolator
    doesn't seem to be working. The diode section (between pins 1 and 2)
    is getting switched, as I can see on a scope and by means of an LED I have
    in
    its circuit. However, the Darlington section doesn't switch. It seems to
    be closed all the time. That is, when I power up the board the voltage at
    the collector stays at nearly zero all the time.

    I've checked and double checked my connections and joints and see no
    problem there (famous last words, I know).

    Only thing I can think of is maybe I cooked the Darlington while soldering.
    I was careful to do most of the soldering before inserting the chips in the
    sockets, but
    I put them in to do some preliminary tests and did not remove before
    soldering
    the connections to pins 4 and 5. However, the data sheet says it should be
    able to
    withstand 260C for 10 seconds. I'm no pro put don't think I exceeded that.

    Any other thoughts?

    TIA

    Ed
     
  2. John Fields

    John Fields Guest

     
  3. You could desolder the chip and then test it. If it's ok, you know that
    it is the circuit.

    If the collector of the darlington is always at 0V, then it is always
    on. That could happen with a leakage of current through the diode, or if
    its fried. Do you have a diode across the relay you are driving? If not,
    then you can easily exceed the maximum voltage of the darlington, which
    is 30V. Also, you need to ensure that the current through the thing is
    below the max rating.

    --
    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.
     
  4. peterken

    peterken Guest

    check value of resistor (at least I hope a resistor is still connected to V+
    ?) in collector in output and verify if now a capacitive load is connected
    to the pcb
    if switching frequency is too high the transistor appears to conduct all the
    time if capacitive load and resistor is too high (time constant)
    lowering resistor value or buffering so no more capacitive load exists
    solves things
    and it's normal the voltage drops fast and rises slower : switching on
    transistor givers a fast edge, but desaturation of a transistor (and
    capacitive loads) slows down edges
     
  5. Jag Man

    Jag Man Guest

    Looks like I've cooked the NTE 3083s. I pulled them out of the sockets and
    set them up independently on a breadboard. Connected pin 1 to +12 through a
    pushbutton,
    and connected a 510 ohm resistor between 2 and ground. Thus the
    LED section is activated by the pushbutton. Pin 5 is grounded,
    and pin 4 goes to +12 through a 10k resistor. If it were working I would
    expect
    to see 12 volts at pin 4, droping to close to 0 when the button is pressed.
    Instead, I see about 9 volts at pin 2 regardless of the button.

    The LED section seems to be OK, as I get about 20 mA through the 510
    resistor when the button is pressed.

    Whatever I did wrong I did it consistanlty, as both of my 3083s now behave
    that way.
    The question is, is there a wiring error in my circuit, or were they heat
    damaged while soldering? Guess I better go over the circuit again...

    Ed
     
  6. Jag Man

    Jag Man Guest

    Fortunately, it is socketed so I just poped it out. Good idea though... see
    my other posting today.
    So far, it's only been exposed to resistive load, 10k at 12 volts. But
    thanks for
    mentioning that. I'll put a diode in there so when I hook it to something
    else it
    will be protected.

    Ed
     
  7. Jag Man

    Jag Man Guest

    Oops. My testing setup was in error. Pin 4, the emitter, should be grounded
    rather than pin 5, the collector. When connected correctly the 3083s work.
    They are OK. It cost me $6 to find out, though, as I bought new ones based
    on my erroneous conclusions!

    So, now it's back to the original question, i.e., why they don't work on my
    circuit
    board. Only difference is on the circuit board I have an LED,
    a 400 ohm resistor, and a NTE123AP transistor in the cathode path to
    ground.
    Hmmm.

    Ed
     
  8. I'm unclear about whether it is always on or always off while in the
    circuit. If you put a 10k resistor to 12V on the collector, and tie the
    emitter to ground, the measured voltage at the collector is always at
    ground, right? Also, you aren't connecting to the base pin, are you?

    What kind of LED are you using? Does it light up when the opto is
    supposed to trigger?

    --
    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.
     
  9. Jag Man

    Jag Man Guest

    Robert,

    The emitter of the 3083 (pin 4) is tied to ground in all cases except my
    first
    reported attempt to test the 3083, earlier today. The Collector
    (pin 5) is fed from a 10k resistor that is tied to +12.

    The LED is in the Cathode path to ground. I.e., pin 2 -> LED->380 ohm
    resistor->
    transistor->ground. It is a garden variety with about 1.8 volt offset. It
    lights up
    when the 3083's internal LED fires. That firing is caused by the transistor
    in its circuit.
    My circuit is shown in
    http://sowell.ecs.fullerton.edu/jag/fuelingsys/tester111704.jpg

    However, that diagram does not show the load that I put on the
    3083.

    thanks for your inputs.


    Ed
     
  10. Can you list the steps you have taken to determine if the optoisolator
    is working? What were the results, in terms of voltages? What kind of
    test equipment are you using?

    --
    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.
     
  11. Jag Man

    Jag Man Guest

    Robert,

    Let me describe the two test setups I've used.

    1. breadboard with just the 3083

    +12 volts goes through a push-button to pin 1
    pin 2 goes through 510 ohm resistor to ground
    10k resistor between pin 5 and +12
    pin 6 grounded.

    Test: using digital VOM measure voltage at pin 5.
    button not pushed, read 12 volts
    Push the button, read 0 volts.

    2. PCB with socketed ICs and all components shown in the diagram ( See
    http://sowell.ecs.fullerton.edu/jag/fuelingsys/tester111704.jpg)
    soldered in place.

    Since the PCB does not have anything connected across pins 4 & 5 of the
    3083,
    I used a separate breadboard to simulate a load. On this breadboard I have
    a 10k resistor connected
    to +12, the other end of which is connected to pin 5 of the 3083 on the
    PCB with alligator clips &
    test leads. Pin 4 is also jumpered to ground.

    Test: Using an Hitachi 212 oscilloscope monitor various voltages. For
    example,
    I measure the voltage at pin 3 of the 555 IC which feeds to the base of
    the 3904 NPN transistor,
    where I see a positive 12 volt pulse of width (2-10 ms) determined by the
    pot going to pins 6 & 7 of the 555.
    The frequency (6 - 43 Hz) is set by the pot on the 555 at the far left of
    the diagram. What see here is what I would expect.
    Then I measure the voltage at pin 5 of the 3083. What I see here is
    anomalous and variable. It should be normally 12
    volts, dropping down to 0 while the output of the 555 is high. What I see
    instead is sometimes 6 volts, dropping
    down to 0 when the 555 output is high. Yesterday, the normal was only
    about 2 volts. Later today, it was coming back up to 12
    volts, which would be normal. I don't know why it varies, perhaps
    temperature. Also, I notice that the pulse width
    doesn't seem to be stable at the 3083 ouptut, although it is at the 555
    output.

    My current interpretation of all this that the 3904 NPN transistors may
    not be doing a very good job for me. For example,
    perhaps they are letting current through sometimes when the 555 output is
    low that would allow current to flow in
    the 3083's LED section, thus turning on the output when it shouldn't.

    That's about all I can say at this point.

    Ed
     
  12. Ed:

    There are a couple of possibilities I can think of.

    1) The optoisolator isn't recovering fully. Try slowing down the
    oscillator, and see if it gets better.

    2) The 2N3904 isn't turning off all the way. Make sure that 10k resistor
    at it's base is actually grounded, and not attached to Vcc.

    3) You have some issue with the layout, maybe a small solder bridge that
    is leaking current.

    I'm sure the pros can think of some other possibilites.

    --
    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.
     
  13. peterken

    peterken Guest

    There IS another possibility...
    Some opto isolators have rather high leakage current on their output,
    meaning from several hundreds of uA to even 1mA
    using a 10k resistor (rather high value) "takes" all the voltage away due to
    leakage, not due to signal....
     
  14. You may have something here. The datasheet test resistance is about 180
    ohms, giving an Ic of 50mA. I wonder why a darlington would leak so
    much, though?

    Maybe Jag could rerun the test with a 180 ohm resistor and see what happens.

    --
    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.
     
  15. Jag Man

    Jag Man Guest

    Yes, I believe that's it! I put a 510 ohm resistor in there and it seems
    to be behaving as it should. Guess I had the 10k in there simply because
    I had one handy and figured it was big enough to be sure the thing wasn't
    overloaded. Pure sloth on my part... even some mental artihmetic
    should have told me that something in the 100s was more
    appropriate.

    I am still getting some jumping around of the pulse on the scope.
    It's like the width snaps from one value to another, or the triggering point
    changes from one sweep to the next. This thing operates at
    very low frequencies, 6 to 40 Hz, and at the low end of that range
    is where I see that behavior. But this may be a scope triggering issue.
    My skills at using the thing (a Hitachi V-212) are minimal.

    Thanks!

    Ed
    ..
     
  16. Rich Grise

    Rich Grise Guest

    For my money, you've got a short or open somewhere in your test setup. If
    the LED lights, it's pretty safe to assume that the LED in the opto is
    lighting. If so, the transistor should conduct. Now, did you say it's
    acting short, or acting open? Where are pins 3 and 6 going to?

    One thing you could do is measure the voltage between pins 1 and 2 of the
    opto, just to make sure your LED is ledding, but since your schematic
    doesn't show anything at all at the optos' transistors, it's impossible to
    tell what your test setup is doing.

    Could you please post the rest of the circuit?

    Thanks,
    Rich
     
  17. Jag Man

    Jag Man Guest

    Hi Rich,

    Here is the test configuration. All I do is connect the collector through a
    pullup resistor
    to +12 and ground the emitter.

    http://sowell.ecs.fullerton.edu/jag/fuelingsys/testerTester.jpg

    The resistor is nor 510 ohms, so I get better results. But it really does
    seem that
    there is a parallel path to ground somewhere, because it doesn't rise up to
    12 volts
    when the isolator switches off. I have inspected by board with a magnifiying
    glass
    and see no shorts. Only thing I can thing of is perhaps some solder ran
    under the
    socket somehow. Guess I'll try to unsolder them and look.

    Any suggestions welcome.

    Ed
     
  18. peterken

    peterken Guest

    as far as i see your explanations i can only imagine the transistors 2N3904
    don't get to the off-state completely
    try measuring the collector voltage on them, see if it rises to V+

    other thing is of course again leakage currents of the opto's, if it's say
    250uA you get a voltage drop of 125mV on the output....
     
  19. Jag Man

    Jag Man Guest

    That's what it seems to me.
    That's what I AM measuring, am I not? The scope is on the wire going from
    the
    pull-up resistor to the collector. It drops very close to zero, but does not
    rise back up to +12... at least not all the time. I hedge here because When
    I had a 10k pullup resistor it stayed so close to 0 that I though nothing
    was happening at all. Then when I put in a 510 ohm it seemed to work...
    I pronounced the problem solved. Then while finalizing things I ruined my
    510
    resistors so put 620 ohm ones in (happened to have ) and now it's not
    working again. Comes
    up to maybe 5 volts. When I parallel two 610 it comes up to perhaps 9 or 10
    volts,
    but not 12.
    I have 4 NTE 3083 and I keep trading them around. It does make a difference,
    but when it's not working I can't find one that does.

    What I need explanined, however, is how are these things SUPPOSED to work?
    IOW, what am I doing that is so different from what others do with the NTE
    3083?
    Can I do something to deal with the leakage?

    TIA

    Ed
     
  20. peterken

    peterken Guest

    How they are SUPPOSED to work is simple :
    led on = output on, led off = output off
    (some opto's have also a linear part, so might be used linear)

    Dealing with leakage is simple :
    Design as if ALWAYS the max leakage exists (datasheet, and add some to be
    safe)
    This means, calculate the output voltage of the opto in the off-state : Vo
    = V+ - (load resistor * max leakage current)
    thus meaning "using" the output may *not* rely on an exact V+, but rather on
    a level calculated above to be very safe

    One other thing I noticed in your schematic :
    All your digitals are fed at 12V, yet the outputs are "only" divided by 2
    (10k-10k) towards base of transistors....
    Bad practice, I'd rather use say 10k towards base and 1k5 towards ground.
    This gives a division of say 1:10 (or about 1.55V thereby ignoring the
    junction of the transistor) as "control voltage" towards base of
    transistors, which still is far over the junction voltage to "open" the
    transistors.
    This avoids the effect of say a "non-fully-zero-low" of the digitals causing
    still opening the transistors a tiny bit and fouling the operation of the
    circuit completely.
    Tthis way the "low-level" of the digitals may be even upto say 3.5V *before*
    any "opening" of the transistors arises.
    In your setup a "low level" of say 1V already causes some opening the
    transistors.

    Also, the led of the opto's is directly connected (through the series
    resistor) to the collector of the transistors.
    ALSO the transistors have some leakage current, thereby *maybe* causing a
    "continuous glow" of the leds of the opto, thus causing the output not to
    behave correctly.
    did you verify the leakage of the transistors ? Maybe a parallel resistor
    to the leds can remedy this (say 4k7 to be safe to "catch" any leakage)
    (tip : short or disconnect the led input of the opto's, this verifies if the
    leds in the opto do glow continuously caused by leakage currents)

    One other thing :
    (must say, I didn't go check it so sorry if I'm wrong here)
    Doesn't the 555 need a pullup at it's output ? (assuming it's an open
    collector output here)

    Tip learned in a 24 years carreer in R&D :
    As a designer *never* assume anything works perfectly, *always* ask yourself
    "what if..."
    This is called "worst case design", and as a result the circuits are more
    reliable.
     
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