Optocupler behaviour explanation

Discussion in 'Analog & Mixed-Signal Design' started by Domen Ulbl, Jul 4, 2016.

  1. Domen Ulbl

    Thread Starter New Member

    Jul 4, 2016
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    Hello!

    It's my first time here on AAC. I bought some MOC 3061 optocuplers and I am planning to use them as switches to open or close another circuit. For testing purposes I've build the following circuit:

    [​IMG][/url][/IMG][​IMG]

    The 5V source is an Arduino with a button hooked up to act as a switch. The circuit I plan to control is powered by 9V battery and contains two 100Ohm resistors and a green LED1.

    Can somebody explain why the triac driver (is this correct?) continues to conduct after the current stops flowing through the 5V circuit?

    Any help would be greatly appreciated!
     
  2. OBW0549

    Well-Known Member

    Mar 2, 2015
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    Because that's the way triacs work. To turn them on, they only need a brief pulse on the gate; to turn them off, you have to interrupt the flow of current through them. SCRs are the same way.
     
  3. WBahn

    Moderator

    Mar 31, 2012
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    Because that's what triacs do!

    Think of them as a self-latching relay. Once you power it up with a shot of current, the current that starts flowing in the main circuit keeps the device on. To shut it off, you have to momentarily remove the current in the main circuit. After that, it will be off until the next shot of current form the control circuit. This is why triacs are normally used in AC circuits -- the zero crossing is enough to shut them off. It sounds like you want a normal phototransistor output stage.
     
  4. Domen Ulbl

    Thread Starter New Member

    Jul 4, 2016
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    I guess I learn something new everyday....
    So what kind of an optocupler (or other element) should I get to act as a regular DC switch?

    Eventually I plan to control a camera flash with 80V between the terminals.
     
  5. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Your circuit is not complete, how are you switching the triac on, You could sub it for a logic level Mosfet.
    Use transistor output opto.
    Max.
     
  6. WBahn

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    Mar 31, 2012
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    The circuit shown is complete. It's an optically coupled triac -- the light from the LED (D2) produces electron-hole pairs at the triac's gate.

    Though before I commit to that too firmly, let me look up those part numbers. I expected a single part number, not two.
     
  7. Domen Ulbl

    Thread Starter New Member

    Jul 4, 2016
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    The circuit of the MOC is VERY approximate. the actual circuit is like that:
    [​IMG]
    where terminals 1,2 & 4,6 are connected. Substituting this optocupler for a one with a regular mosfet instead of the triac should work then?
     
  8. WBahn

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    Mar 31, 2012
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    Your schematic is confusing -- it show two discrete parts (with discrete part numbers) but then with a red box around them and a different part number. Are you using/simulating two discrete parts or a single optocoupler?

    Also, are you simulating this or describing the behavior of a physical circuit you've constructed?
     
  9. WBahn

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    Mar 31, 2012
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    Thanks for the clarification.
     
  10. Domen Ulbl

    Thread Starter New Member

    Jul 4, 2016
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    I admit this schematic is confusing. I have the physical circuit in front of me, in which I am using the MOC 3061 optocupler. I used discrete elements in the schematic because i could not find an optocupler in the element libraries.
     
  11. EM Fields

    Member

    Jun 8, 2016
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    Your circuit isn't at all representative of an MOC3061 but, no matter how you've contrived to turn the TRIAC ON, the reason you can't turn the TRIAC OFF is because in order to do that you must reduce the current into the load to be close to zero while the gate signal is OFF.
     
    Last edited: Jul 4, 2016
  12. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Are you hoping to achieve it with a triac?
    Max.
     
  13. Domen Ulbl

    Thread Starter New Member

    Jul 4, 2016
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    It would be nice if I could use the 3061 but as others havevpointed out it is not suitable, so i am looking for a cupler with regulat mosfet
     
  14. OBW0549

    Well-Known Member

    Mar 2, 2015
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    If your end use of this circuit is to trigger a photoflash, the MOC3061 might actually be appropriate for the job. Photoflash triggering is normally done by discharging a capacitor into the primary of a small step-up transformer, producing a pulse of several kV which initiates the ionization in the flash tube, which then discharges the main photoflash capacitor, producing light. If I recall correctly, SCRs or triacs are usually used for this purpose, as shown here, so you may be OK.
     
  15. WBahn

    Moderator

    Mar 31, 2012
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    What are the details on this flash unit. Many flash units have a control signal input that is either a logic level signal or activated by pulling a floating pin to common. Or are you digging into a flash unit and trying to jury rig a control signal into a unit that wasn't intended to have one? Or are you trying to build a flash unit from scratch? A bit of detail about what you are actually trying to accomplish might get you information that is more relevant to your needs.
     
  16. Domen Ulbl

    Thread Starter New Member

    Jul 4, 2016
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    What I am trynig to accomplish is to make a circuit that could trigger the flash when voltage is applied to the optocupler by the arduino.
    The flash I use is the Metz 30 bct 4. The voltage measured between the terminals on the foot of the flash is about 74V.
    The actual circuit:
    [​IMG]

    The upper part is the same as original schematic: 5v output from arduino connected over resistors to pins 1&2 on the MOC 3061. The flash is connected to pins 4&6 as is the voltmeter. When voltage is applied to pins 1&2, the voltmeter reading goes to zero immediately but the flash does not fire. The only way to trigger the flash is to (manually) short the pins on the foot.
    I would appreciate some insight as to what might be going on.
     
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