led's in parallel

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jokerzwyld

Member - Posts: 4

Joined: November 19, 2005

Posted: December 14, 2005 at 9:45 PM / IP Logged

hello

ok i have heard that it is bad to run led's in parallel with one resistor. i understand this, but the project i am trying kinda needs it. anyway i am lookin for help with the calculations. this is for a vehicle. i used 13.5v for the supply current. now 6 leds @ 2v 20mA each in parallel makes 2v 120mA according to the math and what i have read. so the resistor would be 13.5 - 2 / .120 = R. is this correct? if so then i need to figure the power rating. using the V of the Led in ohms law, then i get .24w, but if I use V of the vehicle - led it comes out to be 1.38w, and if using the vehicle x .120 i get 1.62. so which is it? the V*I, V-Vled*I, or Vled*I? This has been buggin me for a couple days. any help is appreciated. and is my origional calculations correct. ie: 6 led with 2v 20ma, you add the C but not the V because they are all the same and it would come out to be 2 anyway. and it equals .120mA?

thanx

jokerzwyld

Member - Posts: 4

Joined: November 19, 2005

Posted: December 14, 2005 at 10:10 PM / IP Logged

i forgot the R values i got was 95.8 (100) using the V - Vled / .120mA

ff-mike

Copper - Posts: 199

Joined: November 15, 2004

Location: United States

Posted: December 15, 2005 at 2:50 PM / IP Logged

You are correct with the calculations

If you stay with parallel, be aware that after 1 LED goes, the rest are going to die off at a quicker rate.

A series solution would use less current and a smaller resistor (75 ohms), and/but the string would go out if 1 LED failed

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led's in parallel

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