Simple LED circuit burning out

[PhantomWang]

I am working on a small project in which I need to embed an LED into hole drilled on a wooden box. The first time I completed the circuit I began to line the bulb up in the hole and it immediately began to burn out and die. I chalked it up to perhaps a short in my wiring. The second time, I completed the circuit and then covered all the open points in electrical tape. Yet, as soon as I touched the LED bulb to the wood it burned out and died again. Can anyone help me figure out what is going on and how I can remedy this? I'm using this LED:

https://www.sparkfun.com/products/8861

In circuit with a 9v battery and a 1 watt 82 ohm resistor. Any help is greatly appreciated. Thank you.

 

[Audioguru]

Simple arithmetic:
1) The huge LED has a maximum allowed current of 80mA but 70mA is recommended.
2) Its minimum forward voltage is 3.0V.
3) The 82 ohm resistor might be a little low at 78 ohms.
4) The current is (9V - 3V)/78 ohms= 77mA. It should survive but the wood is blocking its cooling airflow.

Did you use that huge powerful green LED or did you use a smaller lower power red LED?

 

[PhantomWang]

Thanks for the reply. I'm using the super bright green LED. Should I try using a stronger resistor? The point of the project is that I have a figurine showcased in the box and I want the LED in the back to light it up. I thought I could simply drill a hole the same size as the bulb and then tape it in place, but this doesn't seem to be possible. I'm ordering replacement bulbs now, but would like to pull this off without burning more lol.

The bulb was barely running for a few seconds, and burned out as soon as it touched the wood. Would placing a rubber O ring around the hole help?

 

[Audioguru]

The huge LED in the article has a maximum allowed current of 80mA. Your "super bright" LED has a datasheet that states its maximum allowed current that might be only 30mA. Most ordinary LEDs are spec'd at 20mA. Of course it will burn out if it has a current of 80mA.

20mA will not cause the LED to overheat when it is mounted in wood. (9V - 3V)/20mA= 300 ohms for the resistor. It dissipates 6V x 20mA= 0.12W of heat so use a 1/4W resistor.

 

[(*steve*)]

so use a 1/4W resistor

1/4 watt or greater.

If it is possible that your resistor shorted out after you mounted it, that too could have caused the LED to fail.

Heatshrink tubing over exposed wires is a good idea.

 

[PhantomWang]

Okay thanks for your assistance. I was going off of this datasheet so I thought 80mA would be okay to use for the forward current calculation. I will order some 1/4W 330 ohm resistors and that should help play it safe. Thanks again.

 

[(*steve*)]

That datasheet is fine for that LED, but not for others.

 

[PhantomWang]

Well, that's the LED I was using so I'm still not sure why it burned out so easily lol unless the datasheet is wrong. Regardless, I placed an order for some different LEDs rated at 20mA forward current and some 1/2 watt 330ohm resistors, so I should be good to go. Thanks.

 

[Audioguru]

The product is Chinese so maybe they over-rate their stuff.
Maybe the seller shipped replacements that have less power rating.
Maybe your wood is blocking cooling by the "Ta=25 degrees C".

 

[(*steve*)]

Apart from electrical tape being a crap product to insulate wires (and that's mostly in the long term) I can't see why the LED fails when it goes into the wood.

 

[Audioguru]

I can't see why the LED fails when it goes into the wood.

You would also burn out if you are Chinese, you are operating at your maximum exaggerated power and are insulated by being stuck in a tight hole in wood.