30 LED array single resistor?

[stluke]

Hi guys, I'm a bit of a noob when it comes to electronics sorry,

Iv'e just spent the last few days building a very simple 12volt parallel array using 3mm 16k white LED's which i assumed were 3.2volt 20ma LED's.

I found an online resistor calc that showed if i wanted to run 30 of the above i would need to run 1 x single 15ohm 8.8 watt Resistor. (10watt) being the closest.

This indeed did cut the voltage to 3.1-3.2 volts!

I have set the array up but have found the Resistor runs pretty hot, not that that is my major concern. From what i now read here and elsewhere it becomes apparent that

i should have run these in smaller bunches of maybe 5 or less? with a smaller resistor for each cluster. To pull the array out no would be a massive headache as its already installed in a headlight. The resistor runs externally.

Is there any safe guard i could add in now to help? ie adding another resistor? lower the voltage again maybe to a safer limit 2.9 volt?

Any advice is very much appreciated.

 

[bshanahan14rulz]

http://led.linear1.org/led.wiz This link helped me greatly when I was making my array for my dome light years ago.

 

[Changchung]

For 12volts I recomend connect three Leds in serie with a 180ohm resistor and all the groups in paralell


SFMI4UT

 

[jason 77]

I would sugest that if you are not willing to take the leds out and redo the wiring, that adding a 12V voltage regulator to supply a constant 12 volts to the circuit would be a good idea.

 

[JohnR66]

Running the LEDs the way you have it means you waste a lot of power in the resistor. 12v - 3.2 = 8.8volts across the resistor. Thirty 20 ma LEDs would need 600ma. 600ma x 8.8 volts = 5.28 watts to dissipate in the resistor. 8.8v / 600ma = 14.6 ohms, so very close to 15.

As stated, you should use series parallel arrays. 10 parallel of 3 LEDs in series. 12-(3.2*3)=2.4 volts and 10*20ma=200ma, so you only have to drop .48 watts using a 12 Ohm resistor. Since the LED array may get hot with all the little LEDs close to each other, use 15 ohms to lower current a bit.

If these are LEDs from HongKong bought of a certain auction site. Pitch them and get quality LEDs. Get a 9.6 volt Bridgelux LED and use a 10 Ohm resistor. Put the LED on a heat sink and your good to go.

 

[stluke]

Thank you so much guys for the great advice and replys,

I will change the arrays to smaller 3x10 clusters, for the sake of doing the job proper.

After a bit more web searching i notice a lot of people put a little Schottky diode on the - wire, i'm not sure of the purpose but is this something i should also consider?


I will try and take some piccys to post up once the lights are back out and open.