driving 36 leds from 1 pwm output

Discussion in 'General Electronics Chat' started by Dimpi, Jan 14, 2014.

  1. Dimpi

    Thread Starter Member

    Sep 6, 2013
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    helo all

    I need to drive 36 red leds from 1 pwm output of micro...


    similarly 36 green leds from 2nd pwm output,36 blue leds from 3rd pwm output and 36 white leds from 4th pwm output...
    can u please suggest me the reference circuit for it??
    do i need to use 4 led drivers or i can acheive it using transistors??
    the constant current to be provided to led is 1.5A and voltage at which it will be driven is 24 V...

    any help would be highly appreciated..

    tnx in advance
     
    Last edited: Jan 14, 2014
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    What do you prefer to use. Bipolar Transistors or FET
    U will need a Transistor on each separate string.

    Logic level MOSFET is the way to go with micro and that much current.

    Do you prefer SMD types or thru hole types.
     
  3. Dimpi

    Thread Starter Member

    Sep 6, 2013
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    thank u sir for quick rply..

    I want to use mosfet..because it gives more current compared to bjt(correct me if m wrong)...
    but how its is possible to drive 36 leds from 1 mosfet.do all the Leds needed to be connected in series??..can u help me with any reference design to it??

    how the specifications of the FET to be selected??
    guide me as i am new to the domain

    I prefer through hole types..
     
  4. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    To determine the FET rating we should first determine the LOAD.

    U want white, green, red and blue LEDs.
    So u need 4 PWM from micro.
    At 24V u cannot use 36 LED's in series. U have to use series parallel configuration which will increase current demand.

    First we would need the LED data. or u have to find it.
    An assumption is 30mA single LED current for adequate brightness.

    Problem is the forward voltage drop is different for LEDs with different colors.

    For example.
    Green LEDs are like 1.5V forward voltage. So 36 LEDs at 24V u can use 3 series LED string each containg 12 LED's. So 12 LEDs in series connected in parallel with a current limiting R on each string.

    RED LEDs take around 2.2V forward drop. This will limit series no of LEDs and increas parallel strings for RED

    Blue and white even take high forward drop like 3V to 3.3V.

    In order to make this work, we need to know the LED types u have. Or the data. If u dunno how to get data, show us a picture of the 4 color LEDs u have
     
  5. Dimpi

    Thread Starter Member

    Sep 6, 2013
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    sir i am sorry for 1 thing that i have not mentioned previously that is
    the card i am making doesnt consist of leds rather the card just contains the four led driver(driven by four pwm outputs )..the signals from led driver(mosfet )is fed to the connector...n then the connector is connected to the connector of another card which consists of LED's(used in disco lights)...so i am having a model ..i dont know how to decide specifications of Leds and how to decide mosfet's specifications??
    please guide...and i wil post you the pics of the model tomorrow positively..till then co-operate please..
     
  6. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    If that is the case.
    U will need to post good pictures. Attach them to the forum., You can do this from "Go Advance " button next to "Post Quick Reply" button.

    The pictures should be good quality.
    Or u have to identify the part no.s we ask for after looking at the pictures.
     
  7. Dimpi

    Thread Starter Member

    Sep 6, 2013
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    ok sir the leds are 1W rated and as it is driven by 24v supply...the current needed to drive 36 leds is 1.5A.(36/24=1.5A)
    so now what should be the mosfet specifications??

    please guide sir
     
  8. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Leds are current driven elements.
    What are the specifications of the leds?
    You state that they are 1 Watt leds, so I think they need a current of 350 mA each.

    Bertus
     
  9. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    You cannot drive 36 LEDs in series with a 24V supply.
    It can be only driven by parallel connection but in this case we are talking about serious power loss from the current limiting resistor at 24V.

    The info in not complete.
    I cannot guide without looking at what you have.
    Sorry. There is not just enough data to make a suggestion

    The LED data is needed to tell u what to do.
     
  10. Dimpi

    Thread Starter Member

    Sep 6, 2013
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    sir,
    after doing some research work now m using led driver(CS8902) which would be giving constant output current of around 1Amp..the mosfet is used to switch on and off purpose(correct me if i am wrong anywhere)..
    after calculation the total load current i require from each led driver is around 0.5 Amps..

    now i am having some fundamental probs like
    the attached pdf shows the n channel mosfet..how the specifications are to be decided...

    I have connected the LD pin of the IC t0 the I2c pin of microcontroller for giving the control signal or linear dimming input to the led driver ic..is it correct..

    i am using 4 different led drivers to drive 36 leds OF 1W EACH of red,green,blue,and white respectively(as arranged in the attached pdf)..so whether i need to use different control signals from micro to different led drivers(linear dimming input) or only 1 control signal would be sufficient from micro to all 4 led drivers..wat more m i missing.please suggest...
    thanks in advance..
     
    Last edited: Jan 17, 2014
  11. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Ur diagram is wrong.

    Compare the CS8902 PDF's diagram and ur diagram.
    The LEDs need to be between 24VDC and Drain of MOSFET

    I need the data or part no. of the LEDS. Without tht it's impossible to determine the LED connection.

    U simply cannot drive 36 LEDs in series with a 24VDC. Not even 1V LEDs. And even the least of LEDs (Green) take more than 1V to light
     
  12. Dimpi

    Thread Starter Member

    Sep 6, 2013
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    0
    sir i dont have exact part number of leds...only i know is the Leds are of 1WATT......n approximate voltage drop of leds is taken in consideration like red leds have 2.1V...(similarly for green,white and blue leds)

    YES sir my mistake ,about the connection...the +ve part will be connected to 24v supply n negative to the drain...

    sir i have that much info only..using this info only please help me out..
    moreover should i make the circuit as it is in CS8902 datasheet??..
     
  13. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    If u need help, post pictures of LED.
    That is final

    I cannot help u without knowing what have. That is why I am asking for pictures.
    Atleast then I can figure out any trial and error methods.

    Do you have the IC's that u said in post #10

    Tell me can u desolder one LED from the board to test the forward voltage drop
     
  14. Gibson486

    Member

    Jul 20, 2012
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    Linear Technology LT3476

    Edit...

    Holy crap...wait...36 LEDs? Each LED usually has AT LEAST a 1V drop, but they are usually more than that..... You need to something like a power supply that goes beyond 40V to even get the LEDs to be a little lit.

    You will have to loosen your design...like have several LEDs of one color be controlled by different PWMs from one micro.
     
    Last edited: Jan 17, 2014
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