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

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M.Emara

30 posts since
14-Sep-2011

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26-Apr-2012 3:16 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	I_F max.	V_F typ.	V_F max.	V_R 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

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dukeofdata
4 posts since
25-Apr-2012

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26-Apr-2012 7:43 AM (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.

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- M.Emara
  30 posts since
  14-Sep-2011
  
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  26-Apr-2012 8:45 AM (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
  
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  - dukeofdata
    4 posts since
    25-Apr-2012
    
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    26-Apr-2012 1:11 PM (in response to M.Emara)
    Re: LED - Resistor Circuit Design Question
    
    This is the LED Circuit Design program I've been using: http://ledcalculator.net/default.aspx I check the results with this program: http://ledcalc.com/#calc I like the program you linked because it gives several different options for multiple LED circuits. Thank you for the good information.
    
    I will also start buying my LEDs (actually all electronics) from Digi-Key. Digi-Key has much better specifications and much better prices with honest shipping rates!
    
    - Rick
    
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    - M.Emara
      30 posts since
      14-Sep-2011
      
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      29-Apr-2012 2:03 AM (in response to dukeofdata)
      Re: LED - Resistor Circuit Design Question
      
      Digikey presents a pretty wide stock and many different verities. I always depend on Digikey when I want to purchase electronic components. In case you didn’t find what you are looking for in Digikey –which is never happened before with me– you can check this supplier “RS Components” I depend on them usually, also a wide stock with specs details:
      
      http://uk.rs-online.com/web/
      
      Wishing you all the best
      
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JimHarrison
70 posts since
9-Sep-2011

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1-May-2012 12:39 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

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Azure
1 posts since
5-May-2012

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5-May-2012 5:15 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 .

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JimHarrison
70 posts since
9-Sep-2011

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8-May-2012 9:07 AM (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

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- dukeofdata
  4 posts since
  25-Apr-2012
  
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  8-May-2012 1:02 PM (in response to JimHarrison)
  LED - Resistor Circuit Design Question
  
  Thanks, Jim. I downloaded the brochure and it looks very useful. For the time being, I've settled on these for my RC airplane LED lights: http://www.hansenhobbies.com/products/lighting/electronics/led_cl_20/ They are about as simple to use as can be and the cost is in the same ballpark as the chips by themselves.
  
  - Rick
  
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  - Ben
    34 posts since
    21-Mar-2011
    
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    9-May-2012 12:46 PM (in response to dukeofdata)
    LED - Resistor Circuit Design Question
    
    Take a look at the LEDdynamics "constant current resistor" 788-1091-ND . The resistor term may be only in spirit, but it is through hole and cheaper than the hobby store above. There's also some surface mount stuff from MCC in the this link http://search.digikey.com/us/en/cat/discrete-semiconductor-products/diodes-rectifiers-single/1376383?k=current%20limit%20diode
    
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    - JimHarrison
      70 posts since
      9-Sep-2011
      
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      9-May-2012 5:57 PM (in response to Ben)
      LED - Resistor Circuit Design Question
      
      A constant current diode – which only work up to a few milliamps - can be boosted with a transistor as shown below. Perhaps that what this is what the “DynaOhm” is about. Current diodes are not real accurate +_ 15% usually and some of them are not very temperature stable. The DynaOhm must have been compensated as they spec 10% over -40-+100°C. Pretty good. And they come in 20, 25, or 30 mA versions.
      
      It is still better to use a pulse-width-modulator. If you use a resistor or this diode your loosing 20 mA x perhaps 5 V drop or 100 mW. A low-cost PWM circuit should have just a few mW loss in the drive circuit.
      
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25-Apr-2012 4:58 PM

LED - Resistor Circuit Design Question

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