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How to get a pwm output for a 35 V, 0.35 A dc input signal?

  1. May 18, 2016 #1
    I am doing a project where I need to control brightness of a led light. according to presence. the brightness can be controlled using pwm but I cannot find a way to do that for high voltage input signals.
     
  2. jcsd
  3. May 18, 2016 #2

    CWatters

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    Are you asking how to convert 0-35V into something like 0-5V ?

    Google "Potential Divider"
     
  4. May 18, 2016 #3

    berkeman

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    Welcome to the PF.

    Can you link to the datasheet of the LED light fixture that you are wanting to use? What do you have for a power source that you want to use for the PWM circuit? Will you be doing a high-side drive or a low-side drive of the LED light with your PWM signal?
     
  5. May 18, 2016 #4

    jim hardy

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    Good questions Berkeman. A problem well stated is half solved.

    Well you really haven't asked a question for anybody to answer.
    Are you asking for a pointer to instructions or for a parts suggestion ?

    Try a search on "35 volt Buck Converter " and you'll find "plug in and go" modules
    or on "LED Driver Module" ..


    We learn best by doing.
    A good way to learn about PWM is to build one.

    I'd suggest TL494, it's been around for decades and the datasheet is a "How To" tutorial with examples.
    Being so old it still comes in a DIP package that an amateur can solder or plug into his breadboard.
    It's rated for input up to 40 volts.

    http://www.ti.com/lit/ds/symlink/tl494.pdf
    and
    http://www.ti.com/lit/an/slva001e/slva001e.pdf



    If you really need 3.5 amps you'll have to add current drivers to output. See fig 10.2 of datasheet.

    old jim
     
  6. May 19, 2016 #5
    No it is a completely different thing that is ask for.
    Google "Pulse Width Modulation"
     
  7. May 19, 2016 #6

    Baluncore

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    There are many different types of LED lights. We need to know exactly what type(s) you are using.
    We understand PWM.
    What do you mean by “presence”? How is that signal received?
    Where does power for the light come from?
     
  8. May 19, 2016 #7
    The driver circuit of the led light I am currently working on gives a dc output of 35V/0.35A. This goes to light the led's. I want to perform pulse width modulation in between these two steps. So I need some circuitry that can take an input of 35v/0.35A and perform PWM on it which then goes to light the led's at a lower brightness.
     
  9. May 19, 2016 #8
    The led's are simple white led's used in any led lamp.
     
  10. May 19, 2016 #9
    Hi,
    The driver circuit of the led light I am currently working on gives a dc output of 35V/0.35A. This goes to light the led's. I want to perform pulse width modulation in between these two steps. So I need some circuitry that can take an input of 35v/0.35A and perform PWM on it which then goes to light the led's at a lower brightness. I can use another 5V as a power supply but I need pwm signal for the 35V signal. I have been stuck on this step for quite some time now. Can you suggest a way to do this?
     
  11. May 19, 2016 #10

    Nidum

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    You have a 35 V DC supply and you want to use this to drive LED's .

    Just using basic PWM will not solve this problem . Without additional circuitry the LED's could still see 35V peaks and fail rapidly .

    Whatever you use has to drop the output voltage to a safe level for the LED's .

    There are many ready made modules and many published circuits .
     
    Last edited: May 19, 2016
  12. May 19, 2016 #11

    Nidum

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    What is the number and arrangement of the LED's ?
     
  13. May 19, 2016 #12

    jim hardy

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    here's another guess
    http://www.ledsupply.com/led-drivers/dynaohm-dc-resistor-power-module

    When you refuse to give enough information for anybody to help you, nobody can help you .

    Nidum Baluncore and Berkeman all tried to help you formulate your question so it would be answerable .

    There's an old saying, "When the student is ready a teacher will appear. "
    I suggest you come back when you're ready.
     
  14. May 19, 2016 #13

    CWatters

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    We need to know if the existing driver is a constant current source or a constant voltage source?

    You imply constant voltage but are you sure because raw LEDs normally need constant current.
     
  15. May 19, 2016 #14

    berkeman

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    In addition to the other issues that folks are pointing out in their replies, you may not be able to just put a PWM switch between this power supply and your load. If this power supply is a switching power supply, then in general you cannot be switching its load on and off at high frequency. This can cause instabilities in the control loop of the switched mode power supply, which can cause it to go unstable and overvoltage its output.

    If the source power supply is a linear supply, then you would generally be able to put a PWM switch circuit between its output and the load.
     
  16. May 23, 2016 #15
    The driver circuit is a constant current source.
     
  17. May 23, 2016 #16
    My power supply is a 220V AC supply. The driver circuit for the led arrangement gives an output of 35 V and 0.35 A which goes into the led lights. I need to insert a pwm circuitry to the dc output of the driver circuit so that the brightness can be reduced. The led's are already working at maximum brightness at that voltage and current. First the 220V AC is converted into 35V/0.35A dc by the driver circuit(It is a 12W led light). The I need to insert a pwm circuit after the driver to reduce the brightness. All I need to know is how can we do pwm for a higher voltage. I have done the same thing earlier for a smaller arrangement with 5 led's which required only around 5 V using the 555 timer ic. But I cannot find a way to do wpm for a higher voltage.
     
  18. May 23, 2016 #17
    Yes I do get a linear power output from the driver circuit which already exists.
     
  19. May 23, 2016 #18

    Nidum

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    You have an arrangement of a large number of LED's daisy chained so that when your 35V is applied at the ends of the chain the individual LED's turn on to maximum brightness .

    A PWM driver is needed to control the brightness of the LED's .

    So you need to generate timing pulses and use these pulses to turn your 35V LED supply ON and OFF in a controlled way .

    Basically a simple timing circuit and a power switching output stage .

    There are ready to run commercially made controller units available which will do what you want . They are relatively low cost .

    At least one third party supporter of Arduino offers a complete PWM driver system . Again relatively low cost .

    If you want to design and build the nescessary circuits yourself then please get back to us .
     
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