PWM Led Driver

Discussion in 'General Electronics Chat' started by Ben Terry, Sep 14, 2014.

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  1. Ben Terry

    Thread Starter New Member

    Sep 14, 2014
    Hi guys, looking for a bit of help with a dimmer switch i have driving leds, it was originally designed for incandescent bulbs lighting the cluster in my car. Anyway I want to modify the output it produces, it creates a duty cycle between 100% down to 25%, however on the trailing edge of each high, the voltage drops as if a capacitor is on the output. I have attached a screenshot of what it is doing. I couldn't identify what chip is being used for the circuit, but it is driving a transistor for the output.
    Im looking to remove the trailing edge of the output, just not sure how to. I have tried removing each of the larger capacitors on the circuit, one of which i was sure was changing the output, but did not produce what i want.
    I dont have a circuit diagram of it, but could possibly make one up if needed.
  2. crutschow


    Mar 14, 2008
    I don't see that the exponential trailing edge is a problem for driving LEDs. It's likely due to the forward voltage drop of the LEDs which appears to be about 7V. Below that voltage the LEDs stop conducting so there is very little conduction to drain the stray circuit capacitance. That has no adverse affect on the PWM operation.
  3. Ben Terry

    Thread Starter New Member

    Sep 14, 2014
    That makes sense actually, thanks for pointing that out.

    Now i just need to figure out how to lower the min duty cycle, my guess would be swapping out a resistor, just gotta figure out which one. Reason i want it to go lower is sometimes the LEDs are still a little too bright at minimum in some situations
    Last edited by a moderator: Sep 14, 2014
  4. MrChips


    Oct 2, 2009
    The owners of All About Circuits have elected not to allow discussions of automotive electrical system modifications/enhancements due to safety concerns, the potential of legal ramifications and the possible circumvention of vehicle regulations at the state and federal level.

    This thread is against the AAC forum rules, Chapter 6, as seen here:

    This can be found in our Terms of Service (ToS)
    Please try to understand why this action is being taken.
    Here is a site that may provide you with some assistance: Electro Tech Online

    Good luck.
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