MOC3021 BT136 Circuit Questions

Discussion in 'The Projects Forum' started by tracecom, Oct 15, 2014.

  1. tracecom

    Thread Starter AAC Fanatic!

    Apr 16, 2010
    Attached is a schematic drawing of BT136 triac circuit. I have successfully used this circuit before, but I don't really understand the value choice for R6.

    1. How is the 180 ohm value of R6 calculated?
    2. Is the wattage of R6 related to the load?
    3. How is the wattage of R6 calculated?


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  2. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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  3. #12


    Nov 30, 2010
    Nobody ever told me what R6 is about, so here is what I discovered. R6 is about time.
    Assuming the MOC LED is on, the sine shaped power wave has to achieve enough voltage amplitude to send current through R6, the MOC triac, and the larger triac gate circuit. At that time, the larger triac takes over and the MOC is useless until the next half cycle.

    So, why include R6 at all? Why not just let both triacs fire at about 2 volts?
    Because triacs have a minimum "hold" current. A big, fat triac might need 50 ma to keep it conducting, so you have to delay the gate firing until the power line voltage is high enough to sustain the minimum holding current through the larger triac and its load.

    I have played with this on a 'scope and found that the only difference R6 makes is how much time delay happens as the sine wave rises.
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