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Could you connect 8, 3.2 volt LED's to a 5 volt power source using parallel? If so, what resistor?


I'm trying to connect 8 3.2 volt Blue LED's to a 5 volt power source for a cube lamp I am making. I've done this a few years back but with red LED's and didn't really know what i was doing. But I just found out that Blue LED's need more power to light up. I have them soldered together in a parallel configuration that i found on instructables

https://www.instructables.com/id/LEDs-for-Beginners/step8/Wiring-up-multiple-LEDs-in-parallel/                          

but i dont know what resistor to use for them. One website says to connect a 100-ohm resistor to each LED and another says to connect a 10-Ohm to one end. I apologize ahead of time because i'm not very knowledgeable in electronics so if its not too much trouble, could you put it into simple terms.

Thank you so much for your time!

Picture of Could you connect 8, 3.2 volt LED's to a 5 volt power source using parallel? If so, what resistor?
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frollard6 years ago
Use a wizard if you don't understand the math just yet:
http://led.linear1.org/led.wiz

5 volt source
3.2 volt leds
20mA each (general number applies to most 5mm leds, check the datasheet to be sure)
8 leds.

Since you can't have 2 leds in series (it would need 3.2x2 = 6.4 volts, and you only have 5) - they all have to be in parallel.
So, that means each led gets its own resistor, and acts completely independently of the other leds

Backstory: Leds have a needed voltage. They're happy at that voltage and technically don't need a resistor. Trouble is, you go 1 millivolt over that number (3.2 in your case) and you get burnt out leds. Since internal equivalent resistence in an led is so small, tiny over-voltages cause current spikes that destroy the led.

The answer: a resistor that drops the incoming voltage to the correct voltage, and does so with a relatively large resistance such that it doesn't allow too much current to flow:

Ohms law states yadda yadda V=IR. Volts = Amps times Resistance.
You always have 2 knowns and 1 unknown. In this case, you know the volts, and the amps, but need to know the resistance.
Your circuit simplified is 1 led and 1 resistor on 5 volts.
The led NEEDS 3.2 volts, and remember the sum of the whole circuit has to be 5 volts, or the led will eat that extra voltage and asplode. THUS, the resistor has to absorb 5-3.2 = 1.8 volts.
The amps required by the led are 20mA (20/1000 of one amp, or .020Amps)
Calculation time!
V = I*R
rearrange
V / I = R
1.8 / .02 = 90 ohms
Closest real resistor is 100 ohms that 11% difference is also a good safety factor. It makes the led SLIGHTLY less bright, but will elongate its lifespan considerably.

Test by connecting one led to one resistor in series, and hook that up to 5v. It should light with proper polarity. Now make 7 more, and hook them up in parallel, + to +, - to -. In your picture you have them in series, which is no good for this application. (that's + to -, end to end)

Play with the wizard on the site I linked and see what happens if you change your source voltage to around 7 volts, or 12 volts. See how it offers different options based on the available solutions?
wolfsheim_lvr1 (author)  frollard6 years ago
Thank you so much for your information. I put one led and one resistor together in a series and it worked. I tried putting two together based on the diagram from the website you suggested and it did not work. Is it because i have to put them all together?

Another question I have is how would I connect them together? Just to each other or a wire in between? I tried both when trying to put two leds together and it did not work.

I attached the diagram I'm looking at and the resistors should all be connected on one side and the positive leds on the other. How would I be able to produce this? I hope I am being as clear as possible.

Thanks again for all your help!
led.pdf30 KB
wolfsheim_lvr1 (author)  wolfsheim_lvr16 years ago
I also forgot, I decided that two or three would be enough for the purpose I need. Eight might be a slight overkill since I'm using super bright LEDs. Its still the same exact configuration as above, using 100-Ohm resistors.

So how exactly would I connect them all together?

Thanks!
Just as an example, tell the calculator you have 2 leds at 3.5 volts each, 20ma

Then try both 5 volt and 8 volt supply.

5 volt will show parallel connection (the only possible option)
8 volt will show a series connection because its possible and more efficient, and a parallel option.
wolfsheim_lvr1 (author)  frollard6 years ago
Frollard,

Thanks for all your help! I got three working together with an 100-ohm resistor a piece and connected them to the power source. It worked perfectly and I was able to finish my project. I connected the resistors all to one side and the leds to the other and got them working. I might have been connecting them wrong to the power source because the wires are not labeled.

Thank you again!
Lamp.jpg
All the positive leads from the leds go to the + side. All the - leads individually go to an individual resistor, then all the other leads of the resistors go together.

If you put them end to end, it won't work because they need 3.2 volts each, and if you put them end-to-end (Series) there isn't enough voltage to light them.

The diagram is a bit misleading because it doesn't easily show the - side connection. On that picture left side is +, and right side somewhere around the middle you can see connects to -.