LED - What resistor?

[motopsycho]

My power supply is a Nokia cell phone charger. 3.7 VDC & 0.35A

Specs on my LED's are as follows..

Wavelength:470 nm
Luminous Intensity-MCD: Min: 3000 mcd Max: 4000 mcd
Reverse Voltage:5.0 V
DC Forward Voltage: Typical: 3.2 V Max: 3.6V
DC Forward Current:20mA
Viewing Angle:20 degree

Due to the power supply I think I have to have them in parallel correct? What size of resistor do I require. Radio Shack hasn't been much help.

Thanks
Chris

 

[areze]

this for a moonlight?

honestly I dont think thats going to be a good choice for a power source. LEDs always vary in their voltage needs, and if its 1 to a series some of the LEDs wont light and some will be over driven as the current finds the path of least resistance. when they are put 3 or more to a series ideally the differences will average out.

for that reason you really want atleast 11 or 12 volts of source voltage.

but to answer your question, 24 ohm resister.

this calculator is pretty handy for such calculations
http://ourworld.compuserve.com/homepages/Bill_Bowden/led.htm

 

[Highlander]

http://led.linear1.org/led.wiz

 

[motopsycho]

Okay if I wanted to have six leds (for moonlights) and they are 3.2-3.6V why wouldn't the nokia ps do? Okay lets forget that questions, If I run 6 leds in parallel occording to the site provided by Highlander, the resistors should be 5.6 ohms (1/4 watt)? Am I reading that correct?

thanks
Chris

 

[Highlander]

looks like 27 ohms to me

 

[areze]

<a href=showthread.php?s=&postid=6502677#post6502677 target=_blank>Originally posted</a> by motopsycho
Okay if I wanted to have six leds (for moonlights) and they are 3.2-3.6V why wouldn't the nokia ps do? Okay lets forget that questions, If I run 6 leds in parallel occording to the site provided by Highlander, the resistors should be 5.6 ohms (1/4 watt)? Am I reading that correct?

thanks
Chris

its not that it wont do... its that it might lead to varying intensities between the LEDs. as the specs say, 3.2 is typical, but they can go as high as 3.6v. if you had 1 LED that was 3.6 that one would not light up as bright, it would be easier for the current to go through another LED that has less resistance, thus burning that one brighter and the 3.6v less.

for LEDs you always want to keep them in series where possible, when in series their voltage requirements add together, and hopefully when you take 2+ and add them their variations add up and average out. leading to consistant brightness across all of them.

you may wire it as you propose and be totally fine. or you may not. not much harm in trying. just saying, if you find the outputs uneven, youll know why.

 

[Highlander]

motopsycho,
I have mine in parallel using a cell phone supply.
It works fine.
You won't see any perceivable brightness variation from one led to the next. The 3.2 to 3.6V is an operating range, not a physical property of any individual LED. They will all light identically according to the voltage you supply, the resistor is there to drop your supply voltage into the operating range, and the applied voltage will be the same for each one, assuming the same resisitor value is used.

At least that's how it worked when I graduated as an electrical engineer, 25 years ago .

 

[areze]

if I am wrong, I have a few setups that curiously do not work correctly, where putting them in series did. though those had far more than 6 LEDs. 250 actually, every single one in its own series.

about 1/4th of them lit, and 1/2 of them varied in brightness, the last 1/4th were dark. and they were all wired correctly.

 

[stugray]

Here's another way to look at it:

the spec says 3.2 - 3.6 V... lets target the middle = 3.4 V.

Since the power supply puts out 3.7 V, we need to 'drop' ( 3.7 - 3.4 = .3 V ). But the best current to run is 20 mA ( which is .020 A ).

Since V=IR, R= V/I, and R=0.3V/.020A=15. This means that a 15 Ohm resistor on each leg of the parallel LEDs will be close to what you are looking for.

If the LEDs you are using are cheap, then you could try putting them all in parallel with no resistor.

If you load down the power supply, it may self regulate ( you need ~.35A / .020A = 17.5 or 18 LEDs! )

Stu

 

[miatawnt2b]

I had an issue with that exact same Nokia supply. Put a multimeter on the supply and you'll find the supply outputs 12ish volts. It is an unregulated supply. I burned a few LED's with it before I actually checked it. Buy a voltage regulator at Radio Scrap and feed your LED's a good constant voltage.

-J

 

[chipmaker]

I had a few of those Coralife Moon lights, all with individual powersupplies hardwired to each light. I figured since each power supoply was 12 VDC and each light itself had the resistors built in, why ot wire them up and power off one single larger powersupply. I joined all power leads toghether, plugged it in and was dissapointed in light output. One was particularly bright theother 2 were pretty dim...I then wired them up in series, and all appeared ot burn with same intensity. I then switched ythe power supply to the original smaller 12VDC supply and they also burned just fine.....Once I changed the wiring from paralell to series, it was a world of difference in output.

 

[Highlander]

The best configuratiion all depends on the voltage and current capability of the supply you are using. The calculator I supplied above will, in fact offer the best solution of parallel or series for the supply parameters input.
The problem with a series connection is that one LED failing will kill the whole string, like cheep Christmas lights. In parallel, only the defective LED will not light.