What size Resistor to use with LEDs??

1. Matty_VTR Saxperience Forum Bum Joined: Sep 2007 Darlington/Newcastle Posts: 2609 Saxo VTS / Twingo RS133

   #1 30th April 2009, 18:44

   Basically putting some LEDs in my mark 1 dials (will look good when done trust me)
   
   anyway Im wiring the LEDs into the dash illumination pin in behind the dash, then running each LED into a 330ohm resistor? Is this the right resistor?, I couldn't find anything straight forward on the net and would like to know this just incase I blowup all my LEDs.
   
   Thanks Matty.
2. Matty_VTR Saxperience Forum Bum Joined: Sep 2007 Darlington/Newcastle Posts: 2609 Saxo VTS / Twingo RS133

   #2 30th April 2009, 21:29

   anyone know this please?
3. NICK2506 Saxperience Post Whore Joined: Apr 2008 Surrey Posts: 6424 Black '03 Mk2 VTS

   #3 30th April 2009, 23:25

   im doing alevel electronics. i should know. but i dont. i swear with 9v we use 120 dont take my word for it.
   
   EDIT: just spoke to my mate....whos like the biggest computer geek ever. i just asked what resistor to use and he said 330. so you were right i wasnt...im gunna fail alevel electronics

   1 user thanked this post: Matty_VTR
4. Matty_VTR Saxperience Forum Bum Joined: Sep 2007 Darlington/Newcastle Posts: 2609 Saxo VTS / Twingo RS133

   #4 30th April 2009, 23:37

   Thanks mate! yea i did GCSE Electronics and I have 330 stuck in my head but wasn't sure if thats what I used on a 12V circuit.
5. NICK2506 Saxperience Post Whore Joined: Apr 2008 Surrey Posts: 6424 Black '03 Mk2 VTS

   #5 30th April 2009, 23:40

   No probs. I think its 120 on 9v. hope you get it all working
6. Jack_VTR_MKII L-plater Joined: Mar 2009 Posts: 55 Citroen Saxo VTR MK2 X reg (90bhp model)

   #6 1st May 2009, 00:15

   I'm a trainee electronics engineer, for a normal red LED you want a maximum of 30 mA (normal operation @ 20 mA, However most reach maximum brightness @ 10 mA), therefore you need a current limiting resistor, depending on your voltage.
   
   For 9V you want a 330 - 500 Ohm resistor. I'd suggest using 330 if the voltage used is 9V.
   
   330 Ohm resistor: Voltage supply take away LED voltage drop = Voltage over the resistor, which can then be used to give current by using ohms law.
   
   Thus the calculation: 9 - 1.92 = 7.08 / 330 = 21.46 mA
   
   If 12 Volts are being used, you want a 500 ohm resistor: 12 - 1.92 = 10.08 / 500 = 20.17 mA.

   1 user thanked this post: Matty_VTR
7. Jack_VTR_MKII L-plater Joined: Mar 2009 Posts: 55 Citroen Saxo VTR MK2 X reg (90bhp model)

   #7 1st May 2009, 00:17

   If you use 120 Ohms on 9V, you will blow the LED component, certainly.
8. user_15516 Saxperience Post Whore Joined: Apr 2007 Posts: 6640 Integra DC5R & VTS

   #8 1st May 2009, 00:20

   Is this for 12V? I'm pretty sure I've used 480/ 560 ohm resistors in the past.
9. NICK2506 Saxperience Post Whore Joined: Apr 2008 Surrey Posts: 6424 Black '03 Mk2 VTS

   #9 1st May 2009, 07:31

   aah maybe its 180 or 220 i use on 9V
10. Jack_VTR_MKII L-plater Joined: Mar 2009 Posts: 55 Citroen Saxo VTR MK2 X reg (90bhp model)

    #10 1st May 2009, 09:52

    The maximum current rating is 30 mA as I said...
    
    9 - 1.92 = 7.08 / 180 = ~40 mA, 10 mA higher then maximum current rating.
    
    9 - 1.92 = 7.08 / 220 = 32~ mA, still higher then the maximum current rating.
    
    And by exceeding the current rating the component will blow.
    
    I assume the the LEDs will be running 12V anyway, so my advice to matt: use the 500 ohm resistor as said, but make sure you put your LEDs and resistors in parralel from the voltage supply:
    
    
    
    This is important as if connected as the wrong method shows, then your current will divide between your LEDs, so if you had three LEDs the current would drop bellow maximum brightness @ 10 mA, and become very dim as you added more.
    
    Whereas the correct method allows them to run the same current; therefore you can use as many as you want really and still have the same brightness level.
    
    Also, make sure you connect your LEDs the right way: Anode (positive) to the resistor end, Cathode (negative) to ground (0V), the cathode is identifiable by the flat edge on the LED plastic casing.
    
    Hope this is of help mate.

    2 users thanked this post: Matty_VTR, turbomadvtr
11. turbomadvtr Trouble Maker Joined: Oct 2008 NEWCASTLE Posts: 438 Saxo VTR MK1

    #11 1st May 2009, 10:42

    Quote:

    Originally Posted by Jack_VTR_MKII The maximum current rating is 30 mA as I said... 9 - 1.92 = 7.08 / 180 = ~40 mA, 10 mA higher then maximum current rating. 9 - 1.92 = 7.08 / 220 = 32~ mA, still higher then the maximum current rating. And by exceeding the current rating the component will blow. I assume the the LEDs will be running 12V anyway, so my advice to matt: use the 500 ohm resistor as said, but make sure you put your LEDs and resistors in parralel from the voltage supply: This is important as if connected as the wrong method shows, then your current will divide between your LEDs, so if you had three LEDs the current would drop bellow maximum brightness @ 10 mA, and become very dim as you added more. Whereas the correct method allows them to run the same current; therefore you can use as many as you want really and still have the same brightness level. Also, make sure you connect your LEDs the right way: Anode (positive) to the resistor end, Cathode (negative) to ground (0V), the cathode is identifiable by the flat edge on the LED plastic casing. Hope this is of help mate.

    And i thought i was good at electronics
12. NICK2506 Saxperience Post Whore Joined: Apr 2008 Surrey Posts: 6424 Black '03 Mk2 VTS

    #12 1st May 2009, 10:56

    Quote:

    Originally Posted by Jack_VTR_MKII

    Croc physics ftw!
13. turbomadvtr Trouble Maker Joined: Oct 2008 NEWCASTLE Posts: 438 Saxo VTR MK1

    #13 1st May 2009, 11:47

    LOL
14. Jack_VTR_MKII L-plater Joined: Mar 2009 Posts: 55 Citroen Saxo VTR MK2 X reg (90bhp model)

    #14 1st May 2009, 12:04

    The program consists of epic fail if you ask me, nothing better then working circuits out on paper with some formulas ...but it's alright for graphical representation; hence use.