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  #1  
Old 04-25-2005
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question about wiring leds

http://metku.net/index.html?sect=vie...calc/index_eng
okay so i fount the above link...and basically im trying to see if im doing this correctly...i want to put 20 leds in lets just say my 3rd brake light...they have to be in parrallel right? and the chart it asks for desired current and voltage drop...di i use the right things (see specs)
oh yea these are the specs to the possible leds:
Emitted Colour : WHITE
Size (mm) : 5mm T1 3/4
Lens Colour : Water Clear
Peak Wave Length (nm) : N/A
Forward Voltage (V) : 3.2 ~ 3.8
Reverse Current (uA) : <=30
Luminous Intensity Typ Iv (mcd) : Average in 13000
Life Rating : 100,000 Hours
Viewing Angle : 10
Absolute Maximum Ratings (Ta=25C)

Max Power Dissipation : 80mw
Max Continuous Forward Current : 30mA<--this is my desired current?
Max Peak Forward Current : 75mA
Reverse Voltage : 5~6V <--this is my voltage drop?
Lead Soldering Temperature : 240C (<5Sec)
Operating Temperature Range : -25C ~ +85C
Preservative Temperature Range : -30C ~ +100C

so if i hook up 20 leds in parrallel...the calculator says to use a 12 ohm resistor...is that one? or do i put one on each led?
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Old 04-25-2005
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The forward voltage (~3.5V) is your LED voltage drop and the max forward current is the most that you can safely run though the LED (0.030A).

20 in parallel would be 20 * 0.030 = 0.6 Amps maximum. Voltage drop is about 3.5.

13.8V-3.5V = 10.3V to be dropped by the resistor.

R=V/I = 10.3/.6 = 17 ohms (this is the minimum resistor size and will drive the LEDs to their maximum rated current.)

P=VI = 10.3 x 0.6 = 6.2W

So, use a single resistor rated at 10W and 17 ohms+ and put it in series with the group of LEDS wired in parallel.

Last edited by V8 Level II; 04-25-2005 at 12:29 PM.
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Old 04-25-2005
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There's a complete how-to on figuring out resistor values for LED's singly or in series on my Cardomain site.
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Old 04-25-2005
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yea i know i need to read that again...ha like the 3rd time...so just wire all leds in parallel then atach a 10w 17 ohm resistor to it and all should work right?
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Old 04-25-2005
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Quote:
Originally Posted by birdie2000
yea i know i need to read that again...ha like the 3rd time...so just wire all leds in parallel then atach a 10w 17 ohm resistor to it and all should work right?
I'm no LED expert but that's what I get when I run the numbers. You won't find a 17 ohm but you may find an 18 and you will be able to find a 22.
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Old 04-25-2005
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You cannot connect LED's in parallel reliably. They don't have consistent voltage drops. It's series with one resistor (but there are limits to how many at 12 volts) -- OR -- parallel each with it's own resistor.

Parallel them with a single resistor and they might work some days, then only a couple will light another day, and a different couple another day; all based on temperature. Sorry but it's not that simple. I would have posted that case in my how-to if it was practical.

Do what you just suggested, and you will end up with FRIED LED's.
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Old 04-25-2005
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Quote:
Originally Posted by n3elz
You cannot connect LED's in parallel reliably. They don't have consistent voltage drops. It's series with one resistor (but there are limits to how many at 12 volts) -- OR -- parallel each with it's own resistor.

Parallel them with a single resistor and they might work some days, then only a couple will light another day, and a different couple another day; all based on temperature. Sorry but it's not that simple. I would have posted that case in my how-to if it was practical.

Do what you just suggested, and you will end up with FRIED LED's.
John, would this work instead of using individual resistors for every LED?

Make a series leg consisting of 3 LEDs and a properly sized limiting resistor for those 3. Repeat to make up several of these and wire them in parallel, so they all have 13.8V across them. Like 7 sets of 3 LEDs, for example. Would that work or am I out in the ozone again?
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Old 04-25-2005
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It would work fine. As long as each series string has it's own resistor. You have the basic idea the way it's done in simple LED arrays.

More sophisticated high current LED's (like white illumination LED's) use actual active driver circuits that act as current regulators. In many cases they are modified switching regulators which instead of driving an inductor drive a series string of LED's. The LED's have a very low value "sense resistor" in series so the regulator can detect the current and limit the pulses accordingly.

Such a circuit is great because it doesn't care what the Vf of the LED is. It simply maintains a steady current through the devices regardless of temperature. If the LED's get too hot and begin to go into "runaway", the current will not increase -- the regulator will throttle back to keep it from increasing.

That's more expensive though and only practical for when you use LED's to replace high intensity illumination sources like halogen bulbs. New auto headlights based on LED's that are coming out is a good example.
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Old 04-26-2005
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okay so if i put 3 in series with a resistor then parrallel the sets together i could get it to work? basically...my plan...i am starting small on something i can mess with...i have the xlt 3rd brake light...so three sections....so i want to put a set of like 20 (or if the above is true 7 sets of 3) in each section... maybe the number will be less but im not sure yet...and if i can get this to work im gonna attempt to do my taillights...i have seen it done before but i just want to make sure that i am doing it right.
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  #10  
Old 04-26-2005
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A) 20 high intensity LEDs is gonna be pricey, you know this right?
B) I'm not sure that would qualify as legal for taillights and the sort.

If I'm not mistaken, current limiters like John described are becoming really common in portable applications too. The backlight for your cell phone probably uses a current limiter.

This stuff is pretty hot right now. Maxim makes an IC to do the sensing and everything. There's even a 'design idea' called "High-side current-sensing switched-mode regulator provieds constant-current LED drive" in the April 14 copy of EDN sitting here on my desk. The idea uses a 9VDC source to drive something like 8 LEDs in series. Pretty wild.. ..Probably beyond the scope of us mere hobiests though..
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Old 04-26-2005
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Ha ha! I saw that same app note!

The key to using that switching regulator topology is the current sensing. One problem with switch mode regulators is when they are switching short high current spikes to regulate, they sometimes exceed device limitations. The current sensing type actually senses the current TO the inductor from the switch, not the current to the load, to keep the device in limits. Pretty cool gadget. In this case, it can watch the current pulse through the LED.

Other switched regulators are being used for that also, as well as some analog circuits. If you look around the internet you'll find lots of guys designing their own drivers for their projects.
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Old 04-26-2005
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i can tune a triple carb setup, but im lost in here
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  #13  
Old 04-26-2005
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Sweet! Finally EDN comes through for something more than just idle throne reading..
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  #14  
Old 04-26-2005
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Quote:
Originally Posted by SoundPer4mance
i can tune a triple carb setup, but im lost in here
Little battery plugs into little black micro chip thing.. Pretty lights turn on.. Pretty lights stay bright no matter how many pretty lights you plug in, little battery just doesn't last as long..

Then again, what's a triple carb?? Is that like having pizza, a beer, AND a candy bar for desert or something?
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  #15  
Old 04-26-2005
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And what's even funnier: that app note is probably actually an ADVERTISEMENT. Only in engineering would you find fully designed applications data sheets being used as an advertisement for the company. I like those app notes from the manufacturers in both EDN and Electronic Design. I've actually used some of the circuits from time to time.
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  #16  
Old 04-26-2005
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with the 3rd brake light...im only gonna use leds with about 13000 mcd...i know this might not work but i figured it might not be that big of a project to tackle...i can always take them out if it does not work...and how would it not be legal? cadillacs have leds in there tails and you can buy premade led bulbs
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  #17  
Old 04-26-2005
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What makes them legal or illegal is the specification for the spread of the light from the lamp. There are specs that say it must be visible over a certain angle, and not be less than a certain brightness, or more than a certain brightness.

When a manufacturer makes a lamp or assembly, it tests it and submits documentation to the DOT that proves compliance.

Since your homemade setup can be too bright or too dim, or not visible enough at certain angles, it could be dangerous. For sure, it's not legal because you have not submitted test data to prove it meets the requirements.

The bulbs you cite have been specifically designed to work within DOT specs.
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