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    What kind of triac should I use with this light dimmer circuit?

    Hi all

    I made this circuit to be a light dimmer but it did not work. I use BT136 for the SCR and MOC3022 for optocoupler.
    Can you tell me what is the problem? may be I should use another SCR.

    •   Alt26th January 2007, 18:12

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    logic gate triac schematic

    How do you think it can work ? By PWM ? No.
    Triac can only be opened externally once in half-period, it closes only in zero crossing. To regulate you need to synchronize to zero crossing.


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    kind of triac

    I want to know BT136 is ok or I have to change it?
    If the gate was 1 forever triac is on forever or it go off when cross zero and never be on?



    •   Alt26th January 2007, 20:45

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    pc123 optocoupler

    Maybe this helps you :
    http://www.freescale.com/files/micro...ote/AN2839.pdf

    Added after 2 minutes:

    I want to know BT136 is ok or I have to change it?
    It depends on what power you want to control.
    If the gate was 1 forever triac is on forever or it go off when cross zero and never be on?
    Yes, it will be opened all period, closes for short period in zero cross, then opens again


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    bt136 schematic

    Most likely you don't have enough current through opto's LED. You should drive it with 10mA. On 5V supply, you should use current limiting resistor of 360Ω and make sure your microcontroller or logic gate output has capability of sourcing 10-15mA.
    Optocoupler is needed only when you need electrical isolation between MCU and mains. Most MCU dimmers people make, are not isolated and it is not necessary to drive triac through optocoupler.
    There are two ways of driving TRIAC's without isolation depending on triac used. One is using quadrants II and III and is most common as most of the triacs are not capable running in quadrant IV, and second is using quadrants I and IV.

    Quadrant I and IV are when gate terminal is positive in respect to MT1 and II and III are when gate is negative to MT1.

    So, in case that you are driving some standard triac that does not work in quadrant IV, you could use schematic like this:


    In case for BT136, which is capable of running in quadrant IV as well, you could drive gate with positive current and schematic could look something like this:

    If your controller or gate can source 4-5mA, you could drive gate directly through 1K resistor.
    Gate current can last only few microseconds, and after that triac will remain open untill it passes through 0 voltage. This is significant, since if you have resistor capacitor network for reducing voltage for 5V for MCU, reduced consumption would lead to smaller components. Once you trigger triac you can turn off gate current, and average consumption would depend on your MCU.



    •   Alt26th January 2007, 21:19

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    bt136 schematics trigger

    Urghh, I see that you don't have much idea how triac works. You should punch triac in google, and read carefully few tutorials for triacs that pop up at very top. Once you understand triac and parameters on it's data sheet, you can work with them. For now, forget my schematics, and for starters keep that opto in and your fingers away from mains.


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    triac quadrants

    Quote Originally Posted by NTFS
    Hi all

    I made this circuit to be a light dimmer but it did not work. I use BT136 for the SCR and MOC3022 for optocoupler.
    Can you tell me what is the problem? may be I should use another SCR.
    Hi NTFS, I do not understand what is "it did not work", it did not switch on the lamp, or it switched on the lamp permanently?

    It is easy to check the triac: with no driving voltage/current at pins 1, 2, just simply shortcircuit the pins 6 and 4 of MOC and see, the lamp must be turned on, and removing the shortcircuit, the lamp must be turned off, then the triac is good. Note that the circuit you use is for inductive load application, you should use the simpler circuit for resistive load like lamps that is also picked out from MOC datasheet.
    Be careful of shock risk.

    Then check the driving and MOC circuit:
    If it did not switch the lamp, so you have to know that, the current goes through Rin, means current goes through LED, must be typical 5mA and maximum 10 mA. If it is OK, you have to check your driving LED cricuit which you do not draw here.
    If it lights the lamp all time, and you cannot dim, check your driving LED circuit either.

    nguyennam



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    triac quadrant tutorial

    That circuit not dimmer, it can only switch on/off
    To work as dimmer it need feedback about voltage zero-crossing (it requires one more optocoupler like PC123,PC817 etc and maybe transistor) and circuitry or program peace (if you use microcontroller) to regulate time of triac opening (or "firing angle"), PWM with zero-crossing syncronization



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