LED Dimmer fires when connecting load

Discussion in 'General Electronics Chat' started by kkawula, Jun 28, 2016.

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  1. kkawula

    Thread Starter New Member

    May 27, 2013
    I'm trying to design light dimmer for driving the LED bulbs.
    Initially design was based on standard triac circuit with optocoupler. Unfortunately that solution wasn't especially good for driving low power LED bulbs at small PWM values.
    I have searched internet and I found how to make alternative circuit with galvanic isolation provided by transformer. Circuit from the attachment has been built and tested. Every thing works ok, starting from very low PWM values to 100%.
    Unfortunately now I have to face up different problem. From time to time, during connecting the load, I can observe that triac occasionally fires and blink could be observed.
    I have tried triac which are not EMI sensitive like BTA312-800C, but it does not change anything.
    Any ideas what can I improve and why triac fires?

    Best regards
  2. #12


    Nov 30, 2010
    From time to time, during connecting the load, you connect the AC power when the voltage is at its peak value. The dv/dt at the triac approaches infinity in the form of 170Volts peak/0 seconds = infinity. Adding a bit of inductance in series with the power line is the usual cure.

    I suspect your R73 is a bit high. Less resistance would allow C33 to bypass some of the peak voltage, thus reducing the stimulus across the triac.
    Hypatia's Protege likes this.
  3. AlbertHall

    Distinguished Member

    Jun 4, 2014
    Might it also be the stray capacitance across the transformer - depends on the transformer construction?
  4. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    I would remove D2, and make R73 100 ohms.

    Most triac loads are in the Mt2 side, and pulses fired between the G and Mt1..
    Last edited: Jun 29, 2016
  5. DickCappels


    Aug 21, 2008
    This looks like an interesting discussion but we are lot allowed to discuss LEDs powered from the mains on allaboutcircuits.com, according to the User Agreement.

    6. Restricted Topics the following topics are regularly raised however are considered "off-topic" at all times and will result in Your thread being closed without question:
    • Any kind of over-unity devices and systems
    • Devices designed to electrocute or shock another person
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    • Rail guns and high-energy projectile devices
    • Transformer-less power supplies

    If You come across a thread or post discussing any of the above topics please report the thread so We can close, and if necessary remove, the offending thread/posts.
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