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Electronics Help needed with Triac ciruit

Discussion in 'Modding' started by g0th, 11 Dec 2005.

  1. g0th

    g0th What's a Dremel?

    12 Apr 2005
    Likes Received:
    I'm wanting to set up a Triac, in my case i have a BT137-600E, to switch a mains 240VAC circuit - specifically a water pump, so it's a reactive load.

    The switch will be controlled by 5V DC via a MOC3021 optocoupled triac driver.



    You can also see the layout of the PCB.

    Clockwise from top left, these are the pin assignments on the main terminal blocks:

    N in
    E in
    A in

    A out (always on)
    E out
    N out

    N switched out
    E switched out
    A switched out

    N switched out
    E switched out
    A switched out.

    My problem, is that the output stays switched on all the time, even with no input, even with the MOC3021 removed.

    The circuit is lifted straight from the Motorola MOC302x datasheet, so i'm surprised that it doesn't work, i can't figure out where the problem is with my construction. There is nothing obvious such as solder bridges.

    The caps are 250V X2 mains rated polypropylene.

    The power on indicator is a standard little Neon discharge bulb, with 270K series resistor.

    The resistors are all standard 0.25W 5% carbon film.

    Don't bother telling me that the collector, not the base, of the optocoupler needs to be connected to the circuit. As you can see, the optocoupler model used in the schematic is not a MOC302x, but the pinout used is correct.

    Thanks for any help.
  2. cpemma

    cpemma Ecky thump

    27 Nov 2001
    Likes Received:
    Coincidence time, I was looking at a Maplin's circuit only yesterday.


    One difference is the pull-downish resistor R5 on the triac gate. But note triacs need a fair-sized minimum load to turn off, a 270k resistor (R19) & neon strikes me as far too small a load to test on. Or if you've tried with the pump, check the triac spec.

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