Power LED Circuit

Discussion in 'The Projects Forum' started by RodneyB, Jan 7, 2015.

  1. RodneyB

    Thread Starter Active Member

    Apr 28, 2012
    633
    13
    I came across a constant current LED circuit at

    http://electronics-diy.com/power-led-driver-circuit.php

    It looks nice and easy to build. However the Calculations for the resistors and the power make no sense to me.

    I want to change the N channel MOSFET for a IRF520N and the transistor for a BC547 as I have these items in my stocks.

    In the diagram unless I am missing something the Mosfet looks like a P channel yet in the write up refers to an N channel and even the specified part is N channel.

    I really want to work out the resistor, for the current limiting.

    The LED's I have are have the following ratings

    Voltage 3.2 - 3.4 Volts
    Current 350mA

    I want to run them from a 12 Volt supply in different strings from 1 to 4 LED's
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
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    The circuit requires an NFET. It will work with at most three of your LEDs if starting from 12V. If running only 1 LED from 12V, the NFET will have to be on a BIG heatsink.

    R3 in the posted circuit is 0.65/Iled, so to get 300mA, R3 = 0.65/0.3 = 2.2 Ohms.

    Power wasted in the NFET with one LED = E*I= (12-3.3-0.65)*0.3 = 2.5W, so without a suitable heat sink would burn your hand...
     
    Last edited: Jan 7, 2015
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  3. RodneyB

    Thread Starter Active Member

    Apr 28, 2012
    633
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    Thank you very much for clarifying that for me. Would I work out the Resistor R3 the normal way to limit the current to 320mA

    Am not sure how to calculate R1
     
  4. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    I gave you the approach. Just change the current and push buttons on a calculator...
    R1 is non-critical. The 100K given in the original article is fine...
     
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  5. RodneyB

    Thread Starter Active Member

    Apr 28, 2012
    633
    13
    thanks very much for your help
     
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