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I connected four 3V LEDs in series across a 12V supply from a PC. They lit for a while but soon died. Their amperage is 0.03A. What value of resistor should I connect in parallel with them so they do not get more than 0.03A?
Actually I need four. I read somewhere that LEDs should not be connected in parallel. So this is what I have done, four in series. Now can't I connect a resistor in parallel with all four of them to limit the current?
You can't connect a resistor in parallel with them to limit the current. When components are in parallel the voltage across them is the same, but the current could be anything.
LEDs need a certain current to light, not a certain voltage. Your LEDs are rated at 0.03 A, so that is what you should provide. You will have to supply around 3 V to each one, but it might be a bit more, it might be a bit less. As you found, you will not get the right current if you connect them to a 3 V supply.
The easiest thing to do is to put two LED in series, with a 220 ohm resistor in series with both, and connect that lot to a 12 V supply. If you want more than two LEDs, you can make up as many groups, each with two LEDs and a 220 ohm resistor, as you want. Connect them all to the 12 V supply.
There are a lot of ways to wire LED's to make them work. The issue I have with running them in series is that a single LED failure will take out the whole string. It's pefectly valid to do, as long as you don't have too many, but your voltage requirements climb pretty fast.
When I say to run the LED's in parallel, I mean that you should give each one it's own resistor.... not that you should run a single resistor and run all the LED's in parallel. (that's just asking for trouble).
Out of curiosity, which LED's were you using? 3v sounds a bit odd to be honest. They're usually close to 2v for most colors, with 4v or higher in the case of blues and whites. Was it a pre-wired LED intended for a pc case mod by any chance?
Hey guys! A slightly different approach but it worked!
I took each one to be 2.9V instead of 3V. Now multiplying 2.9V by 4 gives 11.6V. Using the formula R = [ V(S)-V(L) ] / A
R = [ 12 - 11.6 ] / 0.03 = 13.3
I need 13.3 as the current limiting resistor. Now I had only 33 Ohm ones so I connected two in parallel to give 16.5 ohm which is close to 13.3. Now they work perfect.
I don't really care if one of them blows up, my project is really simple.
HHaase, I purchased a pack of 100 LEDs and they are super bright red. Yes 3V was a bit surprising for me too. And the amperage was rated 0.03A.
Nobody makes and nobody sells a 3V LED. LEDs have a range of voltages. Maybe the max voltage for your LEDs is 3V so some might use only 1.8V or 2.2V or 2.5V. Then of course they fail when their current is not limited.
What is the 30mA rating? Is it the absolute maximum current when it is cooled somehow?
Most ordinary 5mm LEDs are rated at 20mA with 30mA as the absolute max when cooled.
In all honesty, I'd probably try and track down a pre-made marine grade unit for that application. The electrical side of engineering that type of layout is easy enough, but the corrosion issues are pretty significant out on the water. Particularly if you're on salt water.
In all honesty, I'd probably try and track down a pre-made marine grade unit for that application. The electrical side of engineering that type of layout is easy enough, but the corrosion issues are pretty significant out on the water. Particularly if you're on salt water.
Driving LEDs just barely above the threshold voltage is quite tricky and not recommended. In this case, even a slight increase in the input voltage will also increase the current very quickly. Just 0.5V more, and the resistor should be 30 ohms to have 0.03A. With the same 13.3 ohm resistor, the current will be almost 0.07A at 12.5V.
Putting just two LEDs in series would be easier. They would be way more above the threshold voltage, and slight change in voltage would not affect the current so dramatically.
R = [12 - 5.8 ] / 0.03 = 206 ohms
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