the red ones in the photo are like 3mm and the rest yellow and green seem to be a tiny 1mm?
so i'm wondering what the volt requirements are to calculate the resistors needed.
i know some leds are as low as 1.8v so i dont want to cook them.
i will be using single LEDs in parellel with a resistorfor each the par. array formula for a 13.4v supply with different volt drops on the LED at a safe 17mA.
R = (13.4 - 1.85) / .017 = 679 R = (13.4 - 2.00) / .017 = 670 R = (13.4 - 2.40) / .017 = 647 R = (13.4 - 2.80) / .017 = 623
so what volts do 1mm LEDs from a couple years ago use? these are from a surplus store and may be several years old. and am i doing the math right?
LED voltage depends primarily on the LED colour, not on the age or package size. Red LEDs are around 1.8 volts, yellow about 2 volts, and green a little higher, if I recall correctly. Blue and white LEDs are 3 - 3.6 volts, or so.
Your math looks fine - the nearest standard resistor values are 680 or
620 ohms. I'd go with 680 ohms or more - I find most LEDs are bright enough for my use at 10 mA or less, which would require 1000 - 1300 ohms.
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If you are talking about multiple LEDs of the same color, then put them in series, not parallel. No reason to waste all that excess power.
So, for instance, 5 red LEDs in series = 1.8V * 5 = 9.0V; with a 13.4V supply, you'd need to drop 13.4V - 9.0V = 4.4V, and if you want that at
17mA, that's 4.4V / .017A = 259R.
By the way, don't sweat the precision past about 10%. In this application,
259 ohms is not significantly different than 270 ohms, or 240 ohms, both of which are common values for the resistors.
There is one disadvantage of putting a bunch of LEDs in series: if your supply voltage has some variation, it will cause more current variation and thus more brightness variation, for a smaller dropping resistor. For instance, if your 13.4V supply and you are powering a single 1.8V red LED at
17mA, that means R = 682R; now if the supply actually varies from 13.0V to
13.8V, the current will vary from 16mA to 18mA. But if you were powering seven red LEDs in series for a total of 12.6V, R = 47R, so supply variation from 13.0V to 13.8V means current from 9mA to 26mA. How much this matters, and how much supply variation you have, only you can know...
You can put different-colored LEDs in series too, of course; the math works the same way (e.g., two green LEDs at 2.4V and two red LEDs at 1.8V add up to 8.4V). But you might find that the subjective brightness is different, for a red LED and a green LED with the same amount of current flowing through them.
Do you have a voltmeter? You could run them at a low current, say between 3 to 10 mA, get your voltage measurement, and then you'd know what resistor to use for the current you want.
Or, as Walter Harley suggested, run a bunch of them in series and use a single resistor for each series string. If you target
8 to 10 volts for each LED string, you could get 4 or 5 reds, or
3 greens in series.
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