I have a string of Christmas lights made up of 10 warm white LEDs. The lights are connected in parallel (not in series). The string works with 3 AA batteries (that is, 4.5 VDC). And all that stands between the power source and the lights is one 200 Ohm resistor (this is a photo of the resistor, in case I read it incorrectly):
Now, that's all I know/understand of the string of lights. What I want to do is change the power source: I would like it to work off the 5 VDC that come from a USB output.
My basic knowledge of electronics tells me that there's basically 2 things I have to worry about here:
1) How do I drop that 0.5 volt I don't need? (Although, playing around with LEDs, I think they'll have no problem working with half a volt over what they were originally working with); and
2) Is the current coming out of a normal USB outlet (such as from a telephone charger, 0.8-1.2 amps) going to be too much and, if so, how can I drop it to the current needed? (If I'm not very much mistaken, the current will be too much because a normal LED takes 15-20mA, right?)
I understand this issue is very basic but so is my knowledge of electronics. I would really appreciate any help you can afford. I think that if I could "fix" this issue, find a way to get it to work, I would be able to understand a little more about how it works. So far, reading theory… well, it's all mixed up in my head; perhaps if I could solve this real-life situation, the little I understand could start falling into place and organising itself.
So thanks a lot in advance!
Best Answer
You did read it incorrectly, Red|Black|Black is 20 Ohm, not 200.
White LEDs (which are actually blue LEDs with white luminophore) drop about 3V (a multimeter would give you an exact value), leaving about 1.5V to the resistor. This amounts to 75mA of current. To have the same current with a 5V supply, you need a resistor which lets 75mA through at 2V voltage drop, which is around 27 Ohm.