# Why did the LED burn?

I made the simplest circuit with a 9V battery, a green LED (3.3v and 20ma) and a 550 ohms resistor (green green brown gold)

The LED was brand new and died immediately and the resistor was burning hot.

The same circuit with a red LED worked fine.

Does it matter where the resistor is in the circuit (in this case I put it at the end, between the LED and the - of the battery as I read it doesn't matter)

Does it matter which end of the resistor you connect to the battery? (does it have a + and -?)

According to http://www.hebeiltd.com.cn/?p=zz.led.resistor.calculator a 330 ohm resistor would have been enough for a 3.3v LED in a simple circuit.

• Even if the LED was faulty and was short circuited inside, the 550ohm resistor would have to limit the current to 9V/550Ohms=16.3ma and the power to 147mW on the resistor. For how many watts is the resistor rated? Oct 15, 2013 at 11:26
• I don't have that information. It's a very tiny resistor I found in a small portable radio.
– gyc
Oct 15, 2013 at 11:36
• It must be 1/8W. Either the LED was faulty and had a short circuit, or the resistor you used was not 550ohms but less. Oct 15, 2013 at 11:40
• Maybe the LED was reversed? 9V reverse biased on a led with only 5V reverse bias will cook I think ... It's been a long time since I've done that.... Oct 15, 2013 at 12:17
• @Spoon: As long as the current is limited by the resistor, reverse bias current should not damage an LED. Oct 15, 2013 at 12:24

Maybe you used a 5-band resistor like here: -

Or maybe it was a four band without tolerance?

Green, green, brown, gold would give you 55.1 ohms and this would fry the resistor/LED with about 120mA.

OR maybe it's an inductor!!

• I use this: digikey.com/us/en/mkt/4-band-resistors.html to calculate the resistor and for green green brown gold it says 550 ohms, if I knew it was 55 ohms I wouldn't have used it...
– gyc
Oct 15, 2013 at 11:43
• oh! As you said it might well be a 4 band resistor without tolerance. It's very small so I can't figure if there's a space...
– gyc
Oct 15, 2013 at 11:47
• Do you have a meter to measure it or is it fried to dust and debris? Oct 15, 2013 at 12:05
• Sadly I don't have a meter but that's my next investment... The resistor is in one piece, it doesn't look fried. I just burned my finger a little trying to remove it from the breadboard.
– gyc
Oct 15, 2013 at 12:51
• I think it's important you get it measured. It could of course be an inductor and it will have virtually no dc resistance. Oct 15, 2013 at 12:52