Hi guys, was hoping there were some Electronic techs around that could help me out. 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.
If I understand correctly you have all 30 in parallel? In that case, yes, I would advice you create more groups. Just so that you can have less witing running all over the show. In terms of power wasted/heat it pretty much ends up being the same thing regardless of how its wired up. So you have 3 LEDs in each branch, and then you run 10 parallel branches. In each branch you would then place a resistor of suitable size. You could even simply run 4 LEDs in each branch, the volt drop across each LED would get close to the 12V input, and so no resistor is needed. I would not recommend this, I'm just saying its possible But, since you say you've already done it all, just leave it as is. Adding another resistor will only dim the LEDs. Perhaps an in-line 1A fuse if you are really worried, but more than that there isnt much you can do.
The only way to lower the voltage is to add another resistor which will heat up too. You could run two 30ohm resistors in parallel to split the load over the two, this would reduce the heat output on one resistor but the total would remain the same.
Cheers for the replies guys I have decided to rip them apart and start again working with smaller groups of 3, so 10 in each array and adding the smaller 120ohm resister.
I'd suggest groups of four ( 4x 3.2 V ) as the car will generate 14.4 V maximum under normal use. dunx P.S. In groups of 5 you wouldn't need any resistors...
In most modern cars there is a voltage regulator that stops the full charge from the alternator going to the electrics so the alternator only charges the battery at that power.
You mean the voltage regulator in the alternator ? The voltage readout on my HU says 14.2 V on many of my trips to work... dunx
