LED and Resistor help

Discussion in 'General Electronics Chat' started by Fuganater, May 11, 2011.

  1. Fuganater

    Thread Starter New Member

    May 11, 2011
    7
    0
    Hey all,

    I'm new here and need help with replacing a LED and the Resistor for it. This LED is for a temp display for my new computer mod. It is blue right now but my theme is red and black so I need the LED to be red.

    Incoming power is 5v.

    Here is the LED I want to replace. It has 3.1v across it.
    [​IMG]

    Here is the board. Between a friend and me we think its the top right resistor that controls the LED. It has 101 written on it.
    [​IMG]


    So can anyone identify what LED and resistor to buy to replace this?

    Thanks in advance for the help and please let me know if more information is needed.
    -Fuga
     
  2. nbw

    Member

    May 8, 2011
    36
    10
    you'll need a small SMD red LED (Farnells, Maplins, Digikey, etc depending on your location). The typical voltage drop is around 1.7 - 1.8V. You have a 101 resistor - make sure it is the one dropping the rest of the voltage from 5V! - which is 100 ohms. This means the LED is getting around 20mA. You'd want around a 150 ohm resistor to get the same current into your red LED. Probably a 180 or 220 ohm to be save - most SMD LEDs will be happy with a few mA in them.
     
  3. Markd77

    Senior Member

    Sep 7, 2009
    2,803
    594
    Can you measure the LED and resistor as accurately as you can (width and length). They come in different sizes.
     
  4. KJ6EAD

    Senior Member

    Apr 30, 2011
    1,425
    363
    Another option: if sufficient space exists in the housing, you could probably just solder two red LEDs together in series and forget about changing the resistor.
     
  5. Fuganater

    Thread Starter New Member

    May 11, 2011
    7
    0
    Good thing I just got a Digital Caliper last week :D

    LED = 3.23mm x 2.03mm
    Resistor = 2.36mm x 1.51mm

    How can I make sure its the right resistor?
     
  6. KJ6EAD

    Senior Member

    Apr 30, 2011
    1,425
    363
    If you remove it and the light goes out...actually you can deduce that it's correct by measuring the input voltage to the board, assumed to be 5V, then subtracting the LED voltage which you've stated is 3.1V, leaving 1.9V for the resistor. If this is the voltage you measure across the resistor, it's likely correct.

    In your fuzzy photo, it's the first one next to the power input leads? It looks like it has one end connected to the + bus and the other has a trace to one of the LED lead wires.
     
  7. nbw

    Member

    May 8, 2011
    36
    10
    Let us know what voltage you see across that resistor when the LED is on :)
     
  8. KJ6EAD

    Senior Member

    Apr 30, 2011
    1,425
    363
    That's what I said, less succinctly. I'm practicing my perspicacity. ;)
     
  9. Markd77

    Senior Member

    Sep 7, 2009
    2,803
    594
    Looks like these are what you have; an 0805 resistor and a 1206 LED
    0805 (2013 metric) : 0.08" × 0.05" (2.0 mm × 1.25 mm) Typical power rating for resistors 1/10 or 1/8 watt
    1206 (3216 metric) : 0.126" × 0.063" (3.2 mm × 1.6 mm) Typical power rating for resistors 1/4 watt
     
  10. Fuganater

    Thread Starter New Member

    May 11, 2011
    7
    0
    1.9v across the resistor
     
  11. Fuganater

    Thread Starter New Member

    May 11, 2011
    7
    0
  12. Fuganater

    Thread Starter New Member

    May 11, 2011
    7
    0
  13. KJ6EAD

    Senior Member

    Apr 30, 2011
    1,425
    363
    Your Digikey links don't work right so I can't see what you had in mind there. One of the Mouser resistors was 10kΩ (wrong value). The original LED was being used to illuminate a frustrated total internal reflection (FTIR) display so a water clear LED (non-diffused) with a lower viewing angle (45°-60°) would work better.
     
    Last edited: May 12, 2011
  14. Fuganater

    Thread Starter New Member

    May 11, 2011
    7
    0
    Here is the fixed mouser 0805.

    180 or 220 ohms seems really high. I was using a voltage calc with the LEDs i have selected so far and got 106 ohms for the 0805 and 133 ohms for the 1206.
     
  15. Markd77

    Senior Member

    Sep 7, 2009
    2,803
    594
    All you need is one 0805 size resistor and the 1206 size LED (although it looks like you could use a smaller LED if you wanted).
    If the LED uses 1.7V then the resistor needs to use 3.3V
    The current with the blue LED is 19mA (1.9V/100 ohms) so you want to use that or less.
    New resistor value = 3.3V/19mA = 173 ohms so as KJ6EAD has said, use 180 ohms or 220 ohms.
     
  16. KJ6EAD

    Senior Member

    Apr 30, 2011
    1,425
    363
    Correction: those values were suggested by nbw, who I was quoting.
     
  17. Markd77

    Senior Member

    Sep 7, 2009
    2,803
    594
    Good point, we're all three in agreement on the value.
     
  18. Fuganater

    Thread Starter New Member

    May 11, 2011
    7
    0
    Thanks for the help everyone. I ordered the necessary items and will give it a go as soon as they arrive.
     
Loading...