How do I properly size the power requirements?

[jkiker]

Hi all,

Another in my continuing series of questions, trying to be specific rather than general.

If you have read my previous posts, you will recall that I want to light a Y-wing project. I'm going to use two 5mm white LED's for the engines rated at 8000 mcd, plus two likely 3mm white LED's to provide lights in the dual seat cockpit. Note that I'm still trying to figure out how bright the cockpit LED's ought to be (not that I want bright lights in there, just enough that they can be seen in order to stay somewhat in scale).

Today's question revolves around correctly sizing the power supplies, both battery and AC-adapted. The two engine lights need about 3.5 volts. I think the two smaller, less bright cockpit lights will also need something similar; being white, the ones I've found on-line seem to need a bit more power. The point is, do I need to total up the individual LED power requirements in order to correctly size the total power required?

It seems to me that since the two engine lights need a total of 7 volts, I will need more than 9 volts to run all four lights, assuming the two cockpit lights (I have yet to buy these) will need 2-3 volts each. If I am correct, I think I need 12 volts of power, with an appropriate resistor for each LED. And remember, I plan on using just the resistors but no circuit board on this project.

Am I right, or where have I gone wrong?

Thanks, Jim

[Scott Hasty]

Are you wiring in series or parallel?

[jkiker]

Hi Scott,

Well, a bit of both, based on diagrams I've seen. The resistors and the LED's need to go in series, and the sets of LED's/resistors then get wired in parallel. Is that correct?

And based on further research, if that is correct, then to size the resistors I would take the total voltage available and subtract the nominal voltage of each LED to run the "voltage equation." Correct? If that is so, then I don't need 12 volts; 9 should do fine, I think.

Thanks, Jim

[torgg]

I found both of these articles to be very helpful. They are simple to follow and understand and can get you over the leaning curve very quickly and I think answer your questions

http://www.terragenesis.co.uk/infopages/page446.html
http://www.terragenesis.co.uk/infopages/page450.html

Good Luck,
David

[Rocketeer]

Hi Scott,

Well, a bit of both, based on diagrams I've seen. The resistors and the LED's need to go in series, and the sets of LED's/resistors then get wired in parallel. Is that correct?

And based on further research, if that is correct, then to size the resistors I would take the total voltage available and subtract the nominal voltage of each LED to run the "voltage equation." Correct? If that is so, then I don't need 12 volts; 9 should do fine, I think.

Thanks, Jim

Yes, that's right. E=IR ==> R = E/I, where E = power supply voltage minus the diode's voltage drop and I = something like 20 milliamps (0.020 amps).

9 volts is entirely adequate. I've run LED setups just fine on 9V batteries.