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PWM channel to control LED's

Discussion in 'LEDs and Optoelectronics' started by sebyte, Oct 13, 2011.

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  1. sebyte

    sebyte

    4
    0
    Oct 13, 2011
    Hi,

    This is my first post and I hope that the question is OK for this board!

    I have Reef Angel Reef Tank controler, it has two PWM channels with 12v 30mA output. I want to control 8 LED's that draw 80mA, which is to much for one channel. I could split them across the two ports but that would be a waste of one port.

    What I need is a circuit to bridge the gap! Sorry but I am not sure what this type of circuit is called. i.e. to alow the primery side to proportional control the secondery side. That is to say that secondary would have a variable output of 0 to 12v 80mA in line with the PMW output.

    I hope that makes sense :rolleyes:

    Any suggestions on the type of circuit or a design would be welcome.

    Regards

    Steve
     
  2. OLIVE2222

    OLIVE2222

    690
    25
    Oct 2, 2011
    Hi, and welcome to this forum.

    To convert the PWM signal to a DC voltage you need a low pass filter.
    A simple resistor followed by a cap from the R to the ground should be OK.
    Then you can use a unity gain buffer to get your 80mA. garden variety op amp can't do that. So I suggest to use a TS922 high current opamp.

    Olivier
     
  3. sebyte

    sebyte

    4
    0
    Oct 13, 2011
    Hi Olivier

    Thanks for the suggestion, I think I follow what you said. I will look at the TS922 spec sheet and see how it functions. I have been looking on the net for DIY LED dimmer circuits and they mostly use a variable resistor to change the input value to the Op-amp. If I understand you correctly I think that I could substitute the input value from the variable resistor for the PWM output signal through a low pass filter as you suggested.

    Thanks again.

    Steve
     
  4. OLIVE2222

    OLIVE2222

    690
    25
    Oct 2, 2011
    Steve,
    If you use a voltage controlled dimmer just test my suggestion, what you can also try is to forget the filter and drive the leds from the op amp directly, the PWM duty should dim the leds.
    Olivier
     
  5. BobK

    BobK

    7,682
    1,688
    Jan 5, 2010
    You do not need a low pass filter or an opamp to do what you want. All you need is a transistor switch which would increase the current available while still using the PWM controller. A mosfet would be best.

    What are the specs of the LED's? Given that I could tell you how to wire it.

    Bob
     
  6. OLIVE2222

    OLIVE2222

    690
    25
    Oct 2, 2011
    Sure, the op amp suggestion is coming from the PWM to DC conversion idea.
    If we stay in PWM a FET will be nice!
     
  7. sebyte

    sebyte

    4
    0
    Oct 13, 2011
    Reply to Bob

    The problem I have is that the LED's are already built into the Hood of my Reef tank, and to get at them is a PITB! I know that the eight LED's are wired in Parallel with a resistor in series with each LED. At the moment I don't know the spec for the LED's.The LED's are 4mm Royal Blue with a supply voltage to the circuit of 12DC. I seem to remember that they drew around 80mA, but I cannot for the life of me find that reference again.

    This is all part of a larger project using the Reef Angel controller to provide an output on the PWM Chanel to simulate the phases of the moon, i.e. to dim or make brighter the "Moon Lights" in line with the monthly cycle of the moons brightness. As well as handle pumps and other lighting systems, as well as monitoring certain critical parameters on the system.

    Reef Angel have a three LED Moonlight array that can be connected direct to one of the PWM Chanel s, which is OK as it draws less than 30mA. As I mentioned before I could split the circuit across two channels, but that seems a waste of a usable Chanel, and make the programming more complicated..

    If you have any circuit suggestion I would appreciate it.

    Steve
     
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,496
    2,837
    Jan 21, 2010
    It sounds to me like you could use an N channel mosfet as a low side switch. The gate would be driven by your pwm output (via a small resistor (47 ohms)) and the gate would be tied to ground via a 100k resistor.

    I would hazard a guess that almost any small power mosfet would work :)

    Use google to search for "mosfet low side switch schematic" (and then look at the image results) and you should find plenty of examples.
     
  9. sebyte

    sebyte

    4
    0
    Oct 13, 2011
    Thank you Steve for your suggestion.

    Attached is a circuit that I think might work for me. R in would be 47 ohms, and R gs 100k, as you suggested.

    Can you please confirm if this is the correct configuration.

    All I need to do then is find a suitable MOSFET.

    Thanks

    Steve
     

    Attached Files:

  10. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,496
    2,837
    Jan 21, 2010
    Looks fine.

    Try to find a logic level mosfet rated at 20 to 50 volts, capable of switching maybe an amp to a couple of amps in a suitable package (i.e. probably nothing surface mount)
     
  11. BobK

    BobK

    7,682
    1,688
    Jan 5, 2010
    The circuit is fine. But it is a very inefficient way to drive the LEDs from 12V. If you were to put 3 LEDs in series, it would draw the same current as one LED is drawing now. In fact, if the current arrangement is drawing 80ma that is 10 ma per LED. If you made 3 strings of 3, 3 and 2 LED's each drawing 10ma you would be within the spec of your PWM controller without an additional MOSFET!

    You would have to rewire all the LEDs and compute two new resistors, (one for the strings of 3 and one for the string of 2).

    Bob
     
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