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Custom LED-->Need Help<--

post #1 of 12
Thread Starter 
Hi, For my case mod i was looking to install many LED but all the one in modding shop cost way too much.

I'm looking to buy this

http://cgi.ebay.ca/50-pcs-5mm-Round-...item1c16f56e09

So i need to plug them on 12 or 5 volt, and i was wondering to i need any resistance? i would like to plus them in series and not in paralele to help me with the wirring. can someone explain me how to do it? will be really nice.


Thanks to anyone that can help , even if is just a bit!
post #2 of 12
Quote:
Originally Posted by pelplouffe View Post
Hi, For my case mod i was looking to install many LED but all the one in modding shop cost way too much.

I'm looking to buy this

http://cgi.ebay.ca/50-pcs-5mm-Round-...item1c16f56e09

So i need to plug them on 12 or 5 volt, and i was wondering to i need any resistance? i would like to plus them in series and not in paralele to help me with the wirring. can someone explain me how to do it? will be really nice.


Thanks to anyone that can help , even if is just a bit!
You can't really do that many in series, I mean they run at 3 to 3.4 volts so you could put 4 of them in series at a time and run that off the 12V molex pin (thats the yellow wire to a black wire[ground]), but you wouldn't be able to put more than 4 in a single series.



That way you wouldn't need any resistance. Setting them up in parallel you would need I believe an 800Ohm resistor before each one to run off the 5V rail:

5-3.4/20mA = 800Ohm (I'd like somebody to confirm that I'm not being stupid)
    
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post #3 of 12
Thread Starter 
That mean if i want 20 LED (just an example) i could start all from the same molex and start 5 circuit in paralelle and all of them will have 4 of them in serie. Like that i would be able to install al of them without any extra material (no resistance).


??
post #4 of 12
Quote:
Originally Posted by pelplouffe View Post
That mean if i want 20 LED (just an example) i could start all from the same molex and start 5 circuit in paralelle and all of them will have 4 of them in serie. Like that i would be able to install al of them without any extra material (no resistance).


??
Yup
    
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post #5 of 12
Thread Starter 
Can someone can confir this? i'm going to order them today.
post #6 of 12
I have ordered a kit from Ebay similar to that. was 100pcs Red LEDs and 100pcs 100Ohm resistors, shipped from hongkong for $8. they were rated at 2-2.4v, so i could run 5 LEDs with one 100Ohm resistor on 12v. I used this site to calculate it.
http://ledcalculator.net/
Those look to be much brighter than the ones i got tho.
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post #7 of 12
Thread Starter 
Thanks a lot, and yes the reason i choose them is because they are White and really bright. they will go under my motherboard.
post #8 of 12

This might help. i have 20LEDs running off the same 12v lead, 4 groups of 5 LEDs in series.
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post #9 of 12
Designing series/parallel arrays of LEDs and resistors is not that hard; you just have to know a few things:
  • voltages add up in series
  • currents add in parallel
  • the current through elements in series is constant
  • the voltage across elements in parallel must be constant
  • if the sum of voltages across elements in series is equal to the supply voltage, no resistor is necessary
  • the voltage drop across a resistor is defined by Ohm's Law -- V = I * R
  • the power dissipated by an element -- P = V * I or P = I^2 * R for a resistor
  • if the necessary resistance value is not a standard size resistor, round up to the nearest standard size (or use a resistor combination); tables of standard resistances here and here
  • given the power dissipated by a resistor, round up to the nearest power rating

Other useful notes on equivalent resistances:
  • for resistors in series: Req = R1 + R2 + R3 + ...
    e.g. get 50 Ohm resistance from five 10 Ohm resistors in series
  • for resistors in parallel: 1/Req = 1/R1 + 1/R2 + 1/R3 + ...
    e.g. get 50 Ohm resistance from two 100 Ohm resistors in parallel



Now on to your issue in particular.
  • The red, yellow, and amber LEDs have a forward voltage of 3V, so you could have 4 in series across the 12V supply with no resistor; 3 or fewer LEDs would require a resistor.
  • all the other colors have a forward voltage >3V so with a 12V supply you could have at most 3 LEDs in series and a resistor would be necessary
  • the forward current through those LEDs is the same regardless of color (if = 20 mA = 0.02 A) so you can mix and match colors within a series string, just be mindful of the voltages when calculating the resistance


To make an interesting example, let's say we want a red LED, blue LED, and pink LED in a series string with a 12V supply

VS = 12V, VL1 = 3.0V, VL2 = 3.4V, VL3 = 3.2V, if = 0.02A
So:
voltage drop across the resistor:
VR = VS - VL1 - VL2 - VL3 = 12V - 3.0V - 3.4V - 3.2V = 2.4V
necessary resistance:
Rd = VR / if = 2.4V / 0.02A = 120 Ohm
This is a standard size resistor so:
R = 120 Ohm (if Rd were 123 Ohm, then we would have selected R = 130 Ohm)
The power dissipated by the resistor:
P = if^2 * R = 0.02A^2 * 120 Ohm = 0.048W (note: use actual resistance not the design resistance)

In this example, use a 120 Ohm 1/8 W (or higher power rating) resistor.
Edited by radodrill - 4/20/11 at 10:06am
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post #10 of 12
I give you a Clicky for your viewing pleasure, this might clear up some questions as to how to actualy do it hope this helps, I plan on modding all my high flow fans with this guid
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