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LED bulb dimmer

Discussion in 'LEDs and Optoelectronics' started by t21200634, Aug 18, 2013.

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  1. t21200634


    Aug 18, 2013

    I used the following circuit to dim LED bulbs and it works fine.
    However, when I replace the potentiometer (R2) with a digital potentiometer (X9C103P), LEDs cannot be dimmed to the maximum level (no light emitted) like it did with the analog potentiometer,
    Voltage across the LEDs (V1) can be dropped only to half of the supply (V+). For example, if V+ is 12V, V1 is around 6V,


    Can anyone help me with this?:)
    Thanks in advance
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    I would guess that the 555 circuit would have to operate from 5V for this to operate (and it may not work at 5V) because otherwise the mosfets inside the digital pot can't switch.

    There's probably something in the spec telling you that the wiper (and possibly the other terminals) can't exceed the supply rails.

    No time for me to read more at the moment, but you can... here.
  3. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    The concise summary of the limitation is:

    Note that Vcc is 5V, so +/- 5V is allowed on the potentiometer pins. You will exceed that with your circuit.
  4. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    Hi and welcome to the Electronics Point forums :)

    There are various ways to do what you want.

    First, if you're controlling the digital potentiometer from a microcontroller, PWM generation can be done in firmware. Many microcontrollers include PWM generators on-board; for those that don't, there are ways to implement PWM generation using a timer. In this case, tell us what microcontroller you're using, which internal peripherals you're using for what, and what other functions the micro needs to perform.

    Second, there are many types of oscillators that can run at 5V. In fact the 555 can run at 5V although it's probably not the best option. You can switch a higher voltage using an NPN or N-channel MOSFET in common emitter/source configuration, with the LEDs (and current setting resistor) between the collector/drain and the high positive voltage rail.

    An oscillator such as the single-gate Schmitt trigger oscillator (using a 74HC14, CD40106, etc) can easily be configured as a (roughly) constant-frequency, variable-duty-cycle oscillator. Another option is to generate a fixed frequency and feed it through a monostable with an adjustable pulse width.

    If you describe your project in more detail, we will be able to suggest the most appropriate solution.
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