Resistors and Accent LEDs

[88fett]

I have a few questions regarding accent LEDs and the resulting resistors I would need for them.

1)I have a few LEDs lying around the house I'd like to use, and while their currents are the same at 20 mA, their forward voltages differ from 1.8 to 3.7. can I use these in a parallel circuit with one resistor or would they have to have separate resistors? I used someones resistor calculator, but I couldn't enter two different voltages.

2) Would my layout from #1 be affected if I added a illuminated switch? How so?

3) Assuming everything has made sense so far, could someone help me find the proper resistor needed. I currently have a four AA battery back I could use, but I can order a different battery back if necessary.

Thanks for the help.

[mikec]

You could use one resistor for both, but both would get the same forward voltage. If each LED has a different Vf, you'd have to select a resistor that either 1) underbiases one led, i.e. draws too much current (risk of burn out or shorter life), or 2) overbiases one (i.e. not as bright due to lower current). Or a combination of the two, meeting in the middle.

If you use LEDs of two different Vf, and want both to draw 20mA current, you'd need two different value resistors. The resistors you choose are determined by figuring out how much voltage you need to drop to supply the Vf value at 20mA, for each one.

When you think of LEDs, don't think of them as 20mA current LEDs. Think of them as having a forward voltage, which is the published voltage they drop when drawing 20mA. Typically you don't want to go beyond 20mA, but some LEDs do fine with a bit more current (some don't). But the point here is that the amount of current they draw is a function of the voltage that you give them. Too much voltage = too much current.

You can use a LED calculator. Just calculate each independently, and that's the resistor you use for each leg of the parallel arrangement back to B-.

[Forgetful Jedi Knight]

Welcome to the Forums.

If you wire LEDs in parallel (which I do recommend) you need to put a resistor on each one. Calculating resistors depends on what kind of battery solution you are using if the 4 AAs you mention are alkalizes, that equals 6 Volts. To calculate resistors, you need to use Ohm's law. you know the Vf of your LEDs and I just gave yoyo the battery voltage, so you should be all set there.

[88fett]

Thanks for all the info! I also did some reading into the matter. Am I correct on saying that from a 6v battery pack I would need a total of THREE resistors: a 220 ohm for my 1.8v LED, 120ohm for my 3.7 v LED, and a 150ohm for my 3 v illuminated switch?

If I do need three perhaps I will simplify things.

[Silver Serpent]

Those values look correct to me.

Hooking up a resistor is no more complicated than hooking up a LED. Less so in fact, since you can't hook a resistor up backwards.

[88fett]

Thanks a bunch!