LED driver

Discussion in 'The Projects Forum' started by kaushizcute, Feb 21, 2012.

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  1. Audioguru


    Dec 20, 2007
    You cannot regulate voltage and current at the same time. You can regulate the current for LEDs.

    You must understand that LEDs are not light bulbs, instead they are diodes.
    Light bulbs are driven with voltage and they take their rated current.
    But LEDs are driven with current and they set their own voltage.

    LEDs have a RANGE of voltage so your "3.6V" LEDs might be 3.3V or 3.9V or anywhere in between. If you have three LEDs in series and are powered with 11.0V then they instantly burn out if they are 3.3V or they will not light if they are 3.9V.

    So you connect only two LEDs in series so that their required voltage is from 6.6V to 7.8V and then their current is limited with a series circuit or resistor.
  2. kaushizcute

    Thread Starter New Member

    Feb 16, 2012
    Ok so i should regulate the voltage to 5V and then add a current limiting circuit for the LED
    As of now i want it for 1 LED only.
    My LED's maximum range is 4.1V.
    So how to regulate this voltage? input is 9v-16v output needed 5v
  3. bedfordwheelnuts

    New Member

    Apr 19, 2011
    use an On-Semi device like the 20mA constant current driver NSI45020W. Small Cheap, and does exactly what you want to do. Because it's a constant current device, any variation in voltage, as you say from a car pwer supply, will have no effect on the brightness of your LED. (Easily used with a PWM dimming circuit too.)
    Rgds Jeff
  4. kaushizcute

    Thread Starter New Member

    Feb 16, 2012
    but i need 140mA and i have to attach only 1 led.
  5. Wendy


    Mar 24, 2008
    A properly wired LM317 will work, you have a wiring error somewhere.

    You need one of the other, not both, either a constant current circuit per chain, or a constant voltage circuit.

    I notice you have a duplicate thread. This is not allowed here, so I am closing this thread so you can carry on the conversation on the other thread. Two threads are confusing for you and everyone one else that is trying to help.

    LED driver Circuit

    One other thing, per out Terms of Service, which is linked to on the bottom of the page, automotive circuits are not allowed if they are modifications to the car. Generally if it is on the output of the cigarette lighter it may be allowed.
    kaushizcute likes this.
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