problem with led

ENT2151 · Post by **ENT2151** » Fri Dec 29, 2006 7:58 pm

i am working on lighting a ETA-2 interceptor and i have 2 5mm blue leds for the engines. i was testing one out connected to a 9 volt battery with a 330 ohm resistor, after about 30 seconds the light turned green, then yellow, and started to smoke. then it died. i used the led calculator and it said that the resistor was the right one. what did i do wrong? or was there someting wrong with the led it self?

Kylwell · Post by **Kylwell** » Fri Dec 29, 2006 8:50 pm

I'd double check and make sure your voltage is correct and if that's so then it was probably a bad LED.

Sparky · Post by **Sparky** » Fri Dec 29, 2006 9:59 pm

Without the specs on the LED we can only guess what the votlage and current ratings on the LED are. These are used to calculate the resistor, also the resistor absolutley dose not protect against variance in the supply voltage. If you're using a battery you're safe they won't put out more than they're rated.

If the LED wants 3.3 volts and uses 20 mA of current the 330 ohm resistor is more than enough resistance, you'll be under driving it a little even with a fresh 9 volts. If the LEd really wanted the current to be 10 mA you will be over driving the LED pretty hard.

Also check you're wiring, do you have a resistor on each LED. and they are hooked in parallel to the battery (positives all come together in one bundle, negatives also all in one bundle)? If you accidently bypassed the resistor when test hooking up the LED it certainly will die a quick death.

Check the resistor with an ohm meter, to make sure you got a 330 ohm resistor and not a 33 ohm resistor, or a 3.3 ohm resistor. The first two colors on all those ratings will be the same, the second to last one (the multiplier will be different). I believe you need it to be orange, orange, brown. (black=0, brown=1, red=2, orange=3, yellow=4. . .) 33x 10^1 is 330 ohms
http://www.leonaudio.com.au/res_code.htm

jwrjr · Post by **jwrjr** » Fri Dec 29, 2006 11:24 pm

Sparky is correct about the resistor color code -

orange, orange, brown = 330 ohms

orange, orange, black = 33 ohms

orange, orange, silver = 3.3 ohms

If that was a 9 v. transistor radio battery, then you stressed it pretty good in burning out that led. You will probably sooner rather than later want to replace it.

Thrusterhead Jones · Post by **Thrusterhead Jones** » Thu Mar 05, 2009 5:06 pm

Before I solder in a resistor I ALWAYS check its actual resistance with a VOM. The resistor's color code's may be faded, or the value may be closer to the upper or lower limits of their tolerances.

Plus it seems that the ol' OEM photon detectors don't work as well as they use to.

jwrjr · Post by **jwrjr** » Thu Mar 05, 2009 6:19 pm

Power supply voltages are not usually reliable, either. For AC adapters that are not regulated the rated voltage will not be accurate unless there is a load on it. (Figure at least 10% of the rated current.) Here is what I do - assume that the power supply is a bit over the rated voltage and that the resistor will be at the low end of its value (usually 95% of the listed value). Calculate the resistor value from that and don't exceed the max current for the led. Then, you won't get in trouble if the assumptions aren't correct.

photoguy · Post by **photoguy** » Thu Mar 05, 2009 6:29 pm

I don't want to offend anyone, or start an argument, but I do have a question.

I have seen several people posting that they want to run model lights off of 9V batteries - and then they're wondering why the LED's are burning out after very short amounts of time.

Now admittedly, I'm not an electrical engineering genius or anything, BUT every model lighting project I've done to date has been based on voltages around 3 1/2V, or two AA batteries. They light plenty brightly, and can be left on pretty much indefinitely.

What's the big draw toward the 9V batteries? Is there something I am un-aware of?

Kylwell · Post by **Kylwell** » Thu Mar 05, 2009 6:36 pm

Ease of connectors I'd guess. No need for a battery bracket.