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LED in strings of 4, or use resistors

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Member - Posts: 1
Member spacespace
Joined: February 01, 2006
Location: United States
Posted: February 02, 2006 at 12:00 AM / IP Logged  
I'm looking to illuminate my rear hatch with around 20 blue led's. I belive if you wire 4 (3.4v) led's in series, than it can go right to 12v, no resistor. Is this legit, or do i need a resistor on each. Plannin on putting them in the engine bay too. I know there's 40 million led q's here, but it's my one and only. thanks.
Member - Posts: 21
Member spacespace
Joined: January 17, 2006
Location: United States
Posted: February 02, 2006 at 2:55 PM / IP Logged  
no im pretty sure you will need a resistor, atleast at the beginning if you wire in series
Member - Posts: 22
Member spacespace
Joined: February 17, 2006
Location: United States
Posted: February 17, 2006 at 10:39 AM / IP Logged  

My leds require a restitor at anything over 3V. I put an resistor on each postive coming off of each led. Arn't they all still getting 12V? If i ran a 12V from the front of my car to the back wouldn't i still have 12V at the back even if i have 20 things running off of it? Maybe thats a question to your question. isn't it constant 12V all the way throught the wire. so i think you would need a resistor at each led. But Im only 15 and i havn't learned much about 12V and wiring yet But i know im right about the leds.

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Platinum - Posts: 3,527
Platinum spaceThis member consistently provides reliable informationspace
Joined: April 14, 2005
Location: Ohio, United States
Posted: February 17, 2006 at 3:27 PM / IP Logged  

In a series configuration each device will draw a portion of the voltage, therefore if you have a 12vdc power supply and 4 3vdc leds wired in series each one would see 3vdc.  However, you do need to worry about current.

LEDs are not resistive devices, they are considered discreet components.  If you try to measure the resistance of an LED you'll run in to issues (the same issues you run in to when measuring the resistance of a diode).  Therefore, if you wire 4 LEDs in series with NO resistor you will have nothing to limit the load, and that can be a very bad thing.  It wouldn't take much to destory your chain of LEDs, especially considering that if one goes out the entire chain will go out.

I would recomend calculation exactly how much current you think you'll need, and pick a resistor to add inline with the LEDs.  Use one LED for the entire chain. 

Kevin Pierson
Copper - Posts: 78
Copper spacespace
Joined: July 04, 2005
Location: United States
Posted: February 17, 2006 at 5:25 PM / IP Logged  

this is a neat little site I found, it'll calculate the size of resistor you need

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Silver - Posts: 492
Silver spacespace
Joined: November 13, 2005
Location: United States
Posted: February 18, 2006 at 8:22 PM / IP Logged  
I always run my LEDs in parallel, with a single resistor. I have installed about 50 LEDs in my friends vehicle, from strings of 2 to 10. and about 80 LEDs in my xbox, with one string of 16 LEDs, Only problem for the typical user is you wont have enough resistors on hand. I bought TONS of varios 47-470 on ebay and I always have the correct one. I have never had a burnout, standard LED will use a 470 for a 12v/14.4v system. But whatever you do, definatly check out a site like the one above before you plug in that positive.
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Posted: February 20, 2006 at 5:43 PM / IP Logged  

When wiring groups of LEDS in series, I found that every-other LED was dimmer.  so it looked spotty. 

It's better to get the right value of resistor for each LED.

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