1
\$\begingroup\$

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.

\$\endgroup\$
  • 1
    \$\begingroup\$ 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? \$\endgroup\$ – 1p2r3k4t Oct 15 '13 at 11:26
  • \$\begingroup\$ I don't have that information. It's a very tiny resistor I found in a small portable radio. \$\endgroup\$ – gyc Oct 15 '13 at 11:36
  • 1
    \$\begingroup\$ 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. \$\endgroup\$ – 1p2r3k4t Oct 15 '13 at 11:40
  • \$\begingroup\$ 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.... \$\endgroup\$ – Spoon Oct 15 '13 at 12:17
  • \$\begingroup\$ @Spoon: As long as the current is limited by the resistor, reverse bias current should not damage an LED. \$\endgroup\$ – Dave Tweed Oct 15 '13 at 12:24
5
\$\begingroup\$

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

enter image description 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!!

\$\endgroup\$
  • 1
    \$\begingroup\$ 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... \$\endgroup\$ – gyc Oct 15 '13 at 11:43
  • 1
    \$\begingroup\$ 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... \$\endgroup\$ – gyc Oct 15 '13 at 11:47
  • 1
    \$\begingroup\$ Do you have a meter to measure it or is it fried to dust and debris? \$\endgroup\$ – Andy aka Oct 15 '13 at 12:05
  • \$\begingroup\$ 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. \$\endgroup\$ – gyc Oct 15 '13 at 12:51
  • 1
    \$\begingroup\$ I think it's important you get it measured. It could of course be an inductor and it will have virtually no dc resistance. \$\endgroup\$ – Andy aka Oct 15 '13 at 12:52

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.