How to Test an Optocoupler

Discussion in 'General Electronics Chat' started by quant, Oct 27, 2009.

  1. quant

    Thread Starter Active Member

    May 21, 2009
    114
    0
    How on earth can I check a PC817 Optocoupler???!

    As a lame beginner, I'm thinking if connecting it in series with a 330 Ohm Res. and supplying it 5v then the relay part should contact and one should be to check it with continuity or not?

    There is also the below explanation from another forum that I cannot quite understand neither!

    any help appreciated.


    from another forum:

    "I'll tell you my method

    1. Find the input if you barely don't know which one is input.
    2. Use avometer as ohmmeter to check which one is short for each pins.
    3. Ohmmeter is work as current source so when you find short one then it is the pair of input pin.
    4. Power up the input with series resistor.
    5. Measure the output which one is short so you now know how the current flow.
    6. When you power off input that output pins become open.

    Your optocoupler is work.
    "
     
  2. quant

    Thread Starter Active Member

    May 21, 2009
    114
    0
    here...[​IMG]
     
  3. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    The relay part? Do you have the data sheet? In general, some current passes through the emitter (IRLED). 20 -50 milliamps is pretty usual. The phototransistor is placed into conduction by the IR emission. The transistor does not have much drive capability, so use about 47 - 68K to the Vcc supply.

    You will need another transistor driven off the collector of the optocoupler to do useful switching. Google the term - in the images that come up is a circuit application to control a motor.
     
  4. BMorse

    Senior Member

    Sep 26, 2009
    2,675
    234

    The way you are thinking should work, except for you wont get a continuity reading from the transistor output (there is no relay in this particular opto coupler, unless you are referring to one connected to it in a circuit??)..... here is a simple opto coupler testing circuit that should work for you. When R1 is connected to +5 the LED should light up, when disconnected, the LED should turn off, this would indicate the opto coupler is indeed working.

    My .02
     
  5. quant

    Thread Starter Active Member

    May 21, 2009
    114
    0
    There is also this test procedure which is from the data sheet. Can any of these be used to test the Optocouplers? If so which one and how?

    For example can I supply 5v to Input, 330 Ohms R then pin 1 Anode and connect the Cathode to negative of the source?
    Which of the 3 or 4 pins should go to Vcc/Output passing through a Resistor(BTW which type of R?)?

    [​IMG]
     
  6. BMorse

    Senior Member

    Sep 26, 2009
    2,675
    234

    Did you see the post before this one????
     
  7. quant

    Thread Starter Active Member

    May 21, 2009
    114
    0
    So sorry, our posts must have crossed, only now I'm reading the 2 prior posts...
     
  8. quant

    Thread Starter Active Member

    May 21, 2009
    114
    0
    Thank you beenthere for the explanation.

    BMorse, great circuit, exactly what I was looking for. Will go work on it and report back.
    Thanks a lot.
     
  9. quant

    Thread Starter Active Member

    May 21, 2009
    114
    0
    [​IMG] BMorse,

    It worked! Thank you sooooooooo much.
    Your simple solution is the best Optocoupler testing tool/method in all of the net.

    cheers

    Courtousy of BMorse:
    [​IMG]
     
  10. quant

    Thread Starter Active Member

    May 21, 2009
    114
    0
    Here's another question, just for curiosity...

    After blinking it for like 40 times, it stops and then after a short while works again!

    What could that be caused by?

    thanks
     
  11. BMorse

    Senior Member

    Sep 26, 2009
    2,675
    234

    You are only powering it with 5 volts dc right? Is it still connected in a circuit while you are testing it??
     
  12. quant

    Thread Starter Active Member

    May 21, 2009
    114
    0
    Yes, powering it with 5V DC, although not being sure how clean the power is? Meter shows it at about 4.5v. After it stops working, the longer one waits then the more cycles it'll work but if waiting just a couple of seconds you'd get only a couple of more blinks out of it. It's like something heating up which needs cooling down before becoming operable again, kind of strange isn't it?!

    thanks

    p.s.
    yes, still connected in the circuit.
     
  13. ScorpFire

    Member

    Jun 4, 2009
    13
    0
    How about a 6pin optocoupler 4N2? Any ideas on how to use it?
     
  14. BMorse

    Senior Member

    Sep 26, 2009
    2,675
    234

    If the optocoupler does not get warm to the touch, I believe something in your circuit is causing the problem, it almost seems as if a capacitor or something is being charged when pulsing the optocoupler, and it takes time for the charge to dissipate... usually when testing components it is best to remove them from the circuit to get accurate results..... I would probably stop the "in circuit" testing, just so you wouldnt damage anything in the circuit.


    My. 02
     
  15. BMorse

    Senior Member

    Sep 26, 2009
    2,675
    234
    4N2x type opto couplers have a base lead which can be used to switch the output without activating LED.

    here is a typical circuit......

    (Vcc is +5 VDC)
     
  16. quant

    Thread Starter Active Member

    May 21, 2009
    114
    0
    ScorpFire,
    In fact no, I don't know how to use it but, cost will be very important since I'll be using thousands of the Optos in every machine and hence looking for Min. cost and most simplicity.

    BMorse,
    In fact I miss quoted myself, I'm not using it in another circuit other than the one you gave me. You are right, it is behaving exactly like the capacitor being charged! But I am not using any capacitors. Next I'll check the heat to the touch and report back. The Rs. can't overheat now can they? Other than that we only have the Opto and the LED!
    This problem won't cause me a big hassle per since every coupler will be used like once every 30 minutes and only for a second or two at most, but the curiosity is killing the cat! :)

    thanks again

    the 4N2X opto circuit looks very promising, I'll also get it and check it out tomorrow too.
     
    Last edited: Oct 28, 2009
  17. BMorse

    Senior Member

    Sep 26, 2009
    2,675
    234
    Last edited: Oct 28, 2009
  18. quant

    Thread Starter Active Member

    May 21, 2009
    114
    0
    Well, the below quote seems to answer the phenomena?

    All opto-isolators will only work up to a certain frequency. Some are much faster than others. Make sure that the opto-isolator you use is fast enough for the signals you are putting through it - more details in section 4. The reason the rise time is slower than the fall time of the output waveform is that the rising edge is due to the 4k7 pull-up resistor, which has to discharge the capacitance in the opto transistor. If this needs to be speeded up, the 4k7 resistor value can be reduced, at the expense of using more current when the output is low.

    http://homepages.which.net/~paul.hills/SpeedControl/Optos.html
     
  19. quant

    Thread Starter Active Member

    May 21, 2009
    114
    0
    BMrose,
    Thanks for the lead. I see LTV-816Shas 4 pins only whihch is just fine. Will look nto ot's cost tomorrow.

    BTW, do the 6 pin ones have any major advantage over 4?
     
  20. BMorse

    Senior Member

    Sep 26, 2009
    2,675
    234
    I have never used optocouplers in any High speed switching applications, so I have never run into this problem..... but that does explain the "capacitor charge" effect the op was getting.
     
Loading...