Skip navigation
TechXchange
Digi-Key TechXchange Communities > Projects and Designs > Discussions
Up to Discussions in  Projects and Designs
21880 Views 11 Replies Latest reply: May 9, 2012 10:57 PM by JimHarrison RSS
dukeofdata Novice
dukeofdata
Currently Being Moderated

Apr 25, 2012 9:58 PM

LED - Resistor Circuit Design Question

I have several types/colors of LEDs, both 3mm and 5mm, that specify a range for Forward Voltage (such as 3.2-3.5V) and/or a range for Current (such as 20-25 mA).  I'm using some of the free online LED circuit design programs.  My application is in RC airplanes where I can use a steady 5V or 6V power from the receiver or power from the flight battery which will typically vary from about 12.6V to 11V as the plane is flown.  The number of LEDs in a single circuit will vary from 2 to about 15.  Given this application, should I tend to design for the lower or higher Forward Voltage?  Lower or higher Current?  Any and all design tips will be appreciated.  Thanks.

 

- Rick 

  • M.Emara Novice
    M.Emara
    32 posts since
    Sep 14, 2011
    Currently Being Moderated
    Apr 26, 2012 8:16 AM (in response to dukeofdata)
    LED - Resistor Circuit Design Question

    Hi Rick,

     

    Your question is not clear. What can I understand – correct it to me if I am wrong – that you want to design a remote control airplane. Any LED has some specific specs depending on its color and outer diameter round. These specs are maximum forward current, maximum forward voltage, typical forward voltage and maximum reverse voltage. There is also an important factor you must consider it while using LED which is ‘Luminous Intensity’ which means the brightness of LED at a given current and it is measured by mcd (mille candela). Check the following table it will give you the specs of some LEDs:

     

    Type

    Color

    IF
    max.

    VF
    typ.

    VF
    max.

    VR
    max.

    Luminous
    intensity

    Viewing
    angle

    Wavelength

    Standard

    Red

    30mA

    1.7V

    2.1V

    5V

    5mcd @ 10mA

    60°

    660nm

    Standard

    Bright red

    30mA

    2.0V

    2.5V

    5V

    80mcd @ 10mA

    60°

    625nm

    Standard

    Yellow

    30mA

    2.1V

    2.5V

    5V

    32mcd @ 10mA

    60°

    590nm

    Standard

    Green

    25mA

    2.2V

    2.5V

    5V

    32mcd @ 10mA

    60°

    565nm

    High intensity

    Blue

    30mA

    4.5V

    5.5V

    5V

    60mcd @ 20mA

    50°

    430nm

    Super bright

    Red

    30mA

    1.85V

    2.5V

    5V

    500mcd @ 20mA

    60°

    660nm

    Low current

    Red

    30mA

    1.7V

    2.0V

    5V

    5mcd @ 2mA

    60°

    625nm

     

    As answering to your question ‘should I tend to design for the lower or higher Forward Voltage? Lower or higher Current?’. On your design consideration you must not let a current passing through the LED higher than the specified maximum forward current of the LED. Example, if you are using a red 5mm LED, according to the table, its maximum forward current is 30mA so if you passed a current higher than 30mA through the LED, most probably the LED would burn out; this is why you put limiting current resistor in series with LED, to limit the passing current through the LED.

     

    Don’t hesitate to send any questions please.

     

    Kind Regards

    • dukeofdata Novice
      dukeofdata
      4 posts since
      Apr 25, 2012
      Currently Being Moderated
      Apr 26, 2012 12:43 PM (in response to M.Emara)
      LED - Resistor Circuit Design Question

      Thank you for your help.  I want to design a circuit for LEDs which will be installed on RC airplanes.  Your sample LED specs are better and more understandable than the specs I am given.  For example (3mm LED Red Ultra Bright):

       

      Technical Specifications

       

      • Wavelength: Red
      • Lens Color: Water Clear
      • View Angle: 25 Degrees
      • Forward Voltage: 1.9-2.3v
      • Current: 20mA
      • Size: 3mm
      • Luminosity: 15,000 mcd

       

      I assume that I should design my circuits based on the lower voltage in my example spec.  If I were using your specs, I would design the circuit using the Vf typ. voltage.

      • M.Emara Novice
        M.Emara
        32 posts since
        Sep 14, 2011
        Currently Being Moderated
        Apr 26, 2012 1:45 PM (in response to dukeofdata)
        LED - Resistor Circuit Design Question

        The table I sent you shows typical technical data for some 5mm diameter round LEDs. You must work on the specs of your LED. As example, if you are using 3mm Red LED you must check its specs before working. According to the specs you post it, you must be sure that the passing current through LED does not exceed 20mA or the LED would burn out. In your case let’s say that the forward voltage of LED is 2V.

         

        The luminosity intensity of LED is directly proportional with forward current. This means as more current passing through the LED as more luminosity intensity.

         

        Example: If the supply voltage VS = 9V, and you have a red LED (VL = 2V), requiring a current I = 20mA = 0.020A. Solution: R = (9V - 2V) / 0.02A = 350 , so choose 390 (the nearest standard value which is greater).

         

        A link to calculate value of LED limiting current resistor:

        http://led.linear1.org/1led.wiz

         

        A helpful website dedicate for LEDs and lighting systems:

        http://led.linear1.org/

         

        Helpful website covering many electronic components:

        http://www.kpsec.freeuk.com/

         

        Don't hesitate to contact me

         

        Kind Regards

  • JimHarrison Apprentice
    JimHarrison
    73 posts since
    Sep 9, 2011
    Currently Being Moderated
    May 1, 2012 5:39 PM (in response to dukeofdata)
    Re: LED - Resistor Circuit Design Question

    Rick:

     

    Neither the LED forward votage or current are important. 

     

    Run the lamp(s) with an LED driver for constant current at what ever brighness you desire.  Use the Digi-Key search engine to find the constant current controller (under Power Managment or PMIC).  A Linear Tech LT1932ES6 is an example that works over a 1 to 10 V input voltage range.  Then use the search to find the most efficient LED lamp you can get (MCD or Millicandela per milliamp) in the package you want. Some newer lamps are very efficient.

     

    Jim

  • Azure Novice
    Azure
    1 posts since
    May 5, 2012
    Currently Being Moderated
    May 5, 2012 10:15 AM (in response to dukeofdata)
    Re: LED - Resistor Circuit Design Question

    t

    3V2 Zener with Q1 creates a constant current circuit.

     

    I got 9 but 7 led's was better circuit .

     

    Screenshot_2012-05-05-05-29-14.png

  • JimHarrison Apprentice
    JimHarrison
    73 posts since
    Sep 9, 2011
    Currently Being Moderated
    May 8, 2012 2:07 PM (in response to dukeofdata)
    This new brochure might help

    This new brochure might help:

    ON Semiconductor Releases LED Lighting Solutions Brochure

     

    ON Semiconductor has announced the release of the latest edition of its LED Lighting Solutions Brochure. This publication covers the portfolio of LED driving, LED control, power management, ballast, sensing and communication devices produced by the company to serve the increasingly important solid state illumination sector.

     

    The brochure provides basic details of LED driver products based on different topologies (linear, boost, buck, switching and charge pump), as well as details of constant current regulators, switching regulators, switching controllers, buck controllers, voltage references, PLC modems, KNX transceivers, proximity sensors, ambient light sensors and PIR detectors. A variety of useful reference designs are also described.

     

    This brochure can be downloaded from

     

    http://www.onsemi.com/pub_link/Collateral/BRD8034-D.PDF

More Like This

  • Retrieving data ...

Bookmarked By (0)

Legend

  • Correct Answers - 4 points
  • Helpful Answers - 3 points