Led wiring help!

[propmainiac]

Hi all
Working on adding leds to my 3d printed Blade runner blaster. Bought a multi pack of leds from Amazon. Theres 3 mm and 5 mm red and green. Perfect I thought. Well the red 3mm keep burning out. Im using 3 volts to power them. Using AA batteries dont have any others at the moment. Not an expert at this but need advice. I tried using 1 AA battery but it didnt power up the led. Should I be using resistors? I will go to a coin cell or a really small battery to power this. Odd thing is the blue lights and white lights work great and dont burn out. Im going to have 7 leds going. Any advice is apreciated.

 

[propmaster2000]

Hi propmainiac,

I guess one question I have is what battery do you plan to use for the prop?
With the correct resistance (resistor in series with each led) you can use just the one battery to drive then all.
Many times I have used between 220ohm to 470ohm resistor in series with the led. It limits the amount of current to the LED.
If you plan to use a 9v, the resistor(s) could approach 1000ohm (1K) in series.
The 3mm only need @ 2volts and about 20ma and a Blue requires close to 4v.
The LED is very current dependent, so you will need some value of resistor to be safe.
Not all LEDs are created equal. There are the type that are diffused and not as bright as the clear ones of the same color.
.

 

[nomuse]

Yup.

Do a google for "led ballast resistor calculator" or words similar to that and you will come up with multiple java calculators. Feed in the color and the voltage of the battery and it will spit out the correct resistance.

You can get away with quite a bit of leeway, especially if you don't care too much about making them all the same brightness.

And like propmaster2000 says, ideally each LED gets it's own resistor.



Um...two more little notes. Each color of LED has an absolute minimum voltage. Rule of thumb is that you can light red with one cell, light most colors with two cells but the blue ones may be a little dark, three cells does everything.

Coin cells have a high internal resistance. They basically act like they have a small resistor already connected to them. That's why you can get away with lighting some LEDs off a coin cell without additional resistor.

Some LEDs are packaged with an internal resistor or blink circuit or even their own constant-current driver. The typical LED strips and various Automotive LEDs are sold wired together in sets of three with resistors already included to make them happy with 12 volts. These will work "okay" with 9 volts -- again, the blue will be dim (or may not light at all).

In any case, given what you've described, I'd go for a double or triple pack AAA, wire a resistor directly to each LED. 220-330 ohms for the red and yellow, 180 or so for the blue or green. Each LED Is A Complete Circuit; don't wire them in series.

 

[JPH]

Okay, so I have purchased two different sets of LED from amazon, the red ones with resistors.
Edit amazon red leds with resitorS ... [URL] edit amazon blue led with battery box

This one is embedded in resin with blue, glow in the dark dye so it glows even without batteries, if left in the sun for a while.

EDIT Added Arc pic

 

[nomuse]

Another amusing factoid:

Typical capacity of a 9v "transistor" battery is 595 mAh. By comparison, a single AAA cell can deliver 1,200 mAh. So a pair of AAA, similar footprint to that 9V, will run (all things being equal) 4x longer. The AA battery is 2,700 mAh in standard alkaline.

All things are not equal. Batteries reach that theoretical discharge rarely (the faster you draw on them, the less they can give overall). But more importantly for LEDs, most of your voltage drops will be under 3 volts. Even the greens and blues, it is unusual to be over 4. Which means if 4 volts is dropping over the LED, 5 volts is dropping over the ballast resistor. For a red LED, with a drop that's rarely over 2 volts, while that LED is burning off 20 mA the ballast resistor is burning off (literally) 60 or more mA.

In short, expect 8x or more lifetime with the AAA's. And it gets better. 9v are a couple bucks each. And also...although the peak voltage is lower, rechargeable AA's and AAA's are very nice these days. I use Sanyo Eneloops in everything I own and have never regretted it.

Geeky math stuff here. mAh is 'milli-amp-hours.' mA is "milli-amps." Typical single LED these days 20 mA. So a 20 mAh battery, if used totally efficiently, would light that LED for one hour. Given ballast resistor/constant current driver losses, etc., etc., figure on 2/3 of that in the right circuit. I built some stage markers for a ballet company and they'd run all week on a pair of AAA's.

 

[Robiwon]

Use the this, the LED Calculator to figure out what resistors are needed with your intended power source. It will even design the wiring schematic for you.

http://led.linear1.org/led.wiz

or

http://led.linear1.org/1led.wiz

 

[penabled]

yup as Propmaster and nomuse have stated already, the voltage required by the red LEDs is far under what you're supplying to them. As semi-conductors they also have a voltage threshold that when exceeded will result in the emitter chip basically melting into a useless mess...and if the voltage is sufficiently over that...dramatically so. In my own experiments as a kid I literally blew apart several 5mm LEDs from the rapid heating and gas bubble that resulted from high voltages. All red ones. Most 3mm red LEDs don't need more than 10mA of current to light 'em and 12-15mA may melt them.

More interesting info for those unaware: The common 9volt alkaline battery is truly a battery, if you cut open the case you will find six AAAA cells inside wired in series. This is also why they have such a low capacity.

 

[nomuse]

Heheh. I rewired a Clear-Com (theatrical intercom system) base station a couple years ago with LED indicators. Weird-as-hell analog circuitry inside. I think the nominal loop voltage is 24 volts. So I needed a honking bit resistor -- 1 watt size -- to handle it. Long story short, because of the weird wiring when I pressed the "call" button on one of the remote headsets my amber LED turned green. Wow! Useful and cool-looking and I didn't think I'd used bicolor LEDs. Well, actually, I hadn't. One puff of magic smoke later and I had a black LED instead.

 

[xl97]

as mentioned REDS colored leds run at 1.7-2.1v Most other colors run at 3.0-3.3v (typically)


so each led should have its own current limiting resistor to ensure its not getting more current than required.


the pre-wired leds you bought were probably rated for 12v.. and hence sucked everything out of the 9v battery as it was the correct voltage.