Led question

[LOBOREX]

Total and complete moron when it comes to electrics. A friend gave me a set of leds, two led nano lights cool white 20 mA each is what it says on the tube they came in. What i want to know is can i run them both off a aa or aaa battery. Any help would be greatly appreciated.

[Rocketeer]

Probably not off a single AA or AAA battery, and probably not off two batteries, either. A white LED usually takes about 3 volts, which is uncomfortably close to the output voltage of two AA batteries in series. When the batteries become a bit discharged, the output voltage drops, and your LED may or may not light. It'd be better to run it off three batteries in series, which would give you 4.5 volts.

You also need to limit the current through the LED; the usual way to do this is by putting a resistor in series with the LED. To limit the current to 20 milliamps, using three AA or AAA batteries in series, you can figure out the resistance you need by using Ohm's Law: V=I*R, volts = current (in amps) times resistance (in ohms). Rearranged by the miracle of algebra, this means that resistance = volts divided by amps.

Volts is your battery voltage, 4.5, minus the forward voltage of the LED, probably about 3, which gives 1.5 volts. Amps is the rated current of your LED, or maybe something less, if you want it to be a bit dimmer--in your case 20 milliamps, or 0.020 amps. So the resistance you need is 1.5 / 0.020, which equals 75 ohms. (Anywhere within about 15% of that ought to be fine)

If using more than one LED in a project, my advice would be to wire each LED with its own resistor.

This thread over on Starship Modeler covers the subject: http://www.starshipmodeler.net/talk/vie ... D#p1703870

[LOBOREX]

Volts is your battery voltage, 4.5, minus the forward voltage of the LED, probably about 3, which gives 1.5 volts. Amps is the rated current of your LED, or maybe something less, if you want it to be a bit dimmer--in your case 20 milliamps, or 0.020 amps. So the resistance you need is 1.5 / 0.020, which equals 75 ohms. (Anywhere within about 15% of that ought to be fine)

If using more than one LED in a project, my advice would be to wire each LED with its own resistor.

This thread over on Starship Modeler covers the subject: http://www.starshipmodeler.net/talk/vie ... D#p1703870[/quote]
First thanks for taking the time, it does not say on the packaging if they have resistors. I'm trying to look them up going over to woodland scenics website see if they have any info on just plug lighting system.

[Spockr]

If you are using resistors, one dropping resistor per led is the correct method

I agree with the 'by the book' approach to using resistors but in my experience it is somewhat less important when using batteries. Two fresh alkaline AA batteries will work fine for white (3.0-3.4) leds. Unlike a power supply, batteries by their nature have some internal resistance which helps limit current with out an external resistor. I've run white leds on 2 AA batteries for upwards of 60 hours before they begin to dim and I can't recall ever burning one out.

Power supplies are a different animal in that they have plenty of unlimited current available that will burn out the led unless a dropping resistor is used.

Here is a calculator tool that will help design the circuit
http://led.linear1.org/led.wiz

Here is a wealth of info about leds in general
http://led.linear1.org/category/led-basics/

[LOBOREX]

If you are using resistors, one dropping resistor per led is the correct method

I agree with the 'by the book' approach to using resistors but in my experience it is somewhat less important when using batteries. Two fresh alkaline AA batteries will work fine for white (3.0-3.4) leds. Unlike a power supply, batteries by their nature have some internal resistance which helps limit current with out an external resistor. I've run white leds on 2 AA batteries for upwards of 60 hours before they begin to dim and I can't recall ever burning one out.

Power supplies are a different animal in that they have plenty of unlimited current available that will burn out the led unless a dropping resistor is used.

Here is a calculator tool that will help design the circuit
http://led.linear1.org/led.wiz

Here is a wealth of info about leds in general
http://led.linear1.org/category/led-basics/

Thanks for taking the time.

[LOBOREX]

Okay first thank you to everyone for the help. Looked everywhere to find out if these leds have resistors and could not. So said the hell with it and cut off the plugs and attached the leads to a battery pack with two aa batteries worked great left them on for five minutes and no problems. Gonna try it a few more times than straight into the model. Once again thanks everyone.

[Rocketeer]

I'd advise doing a good long test before you install them. I know the irritation that results from having to dismantle a model to extract a failed LED.

[ausf]

I'd advise doing a good long test before you install them. I know the irritation that results from having to dismantle a model to extract a failed LED.

Exactly.

LEDs will last 10,000 hours if properly powered and there are enough calculators online to get the right resistance set.

You can find collections of LEDs (with ratings) and resistors now for a few dollars that'll cover everything you'll need for any model you have or plan to have.

I've had overpowered ones give up the ghost after a few hours and when I put them in a model, it's usually a oneway trip.