LED Lesson

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tray262

Member - Posts: 39

Joined: November 18, 2003

Location: United States

Posted: March 09, 2004 at 3:15 PM / IP Logged

Ok, I did a search but the anwsers I found wern't easy enough for me. So here's what's happening. I want to make some led automotive bulbs. I'm looking at these http://www.partsexpress.com/pe/showdetl.cfm?&User_ID=18811645&St=1066&St2=-39583967&St3=-78106785&DS_ID=3&Product_ID=15768&DID=7 Here are the spec incase you don't wanna look. "very bright RED at only 20 mA. 45° viewing angle. Typical forward voltage range 1.7 VDC-2.2 VDC." I want to use these to replace my 3rd breaklight. I also found this calc. http://wolfstone.halloweenhost.com/TechBase/litled_LEDCalc.html So what I'm trying yo figure out is. How many LED and what resiter do I need sorry for such a stupid question. Thanks again guys.

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xetmes

Silver - Posts: 586

Joined: May 18, 2003

Posted: March 09, 2004 at 3:56 PM / IP Logged

Well you want to do banks of series LEDs then hook them in parallel. I would assume the forward voltage 2V for calculations. If you string 5 together you will drop about 10V, use a resistor to get the 20mA. Since you will drop about 2V on the resistor it would have to be 100 ohms (2/.02). then just hook each bank of 5 in parallel to get as many as you want, you then need 1 resistor for each 5 leds... good luck

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tray262

Member - Posts: 39

Joined: November 18, 2003

Location: United States

Posted: March 09, 2004 at 9:22 PM / IP Logged

Ok major confusion "Well you want to do banks of series LEDs then hook them in parallel. I would assume the forward voltage 2V for calculations. If you string 5 together you will drop about 10V " I understand up till that spot.

"use a resistor to get the 20mA. Since you will drop about 2V on the resistor it would have to be 100 ohms (2/.02). " That's were the problem comes in.

"then just hook each bank of 5 in parallel to get as many as you want, you then need 1 resistor for each 5 leds... good luck" Also No problems there.

Sorry for not being too good at this. Also what formula's are you using ? Thanks so much.

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xetmes

Silver - Posts: 586

Joined: May 18, 2003

Posted: March 10, 2004 at 6:11 AM / IP Logged

for the current calculation just used ohms law.

V = I * R

so the voltage across the resistor is 2V, we want 20mA (.02A)

so R = V / I = 2 / .02 = 100 ohms

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tray262

Member - Posts: 39

Joined: November 18, 2003

Location: United States

Posted: March 10, 2004 at 6:06 PM / IP Logged

Thank you so much, and sorry again for the dumb questions. Hope everything works out and thanks for the luck I'll need it.

Tray

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Mad Scientists

Silver - Posts: 380

Joined: February 07, 2004

Location: United States

Posted: March 12, 2004 at 6:10 AM / IP Logged

http://www.semiconductor.agilent.com/cgi-bin/morpheus/home/home.jsp

On the right hand side select LED..

Tonnes of information on LEDs.. look for application papers.

Regards,

Jim

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LED Lesson

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