170Views8Replies

Author Options:

LED Series Resistor Answered

I just finished soldering a board to flash an IR LED using a 555 timer, and attached it to 9V PP3 battery, and the LED burnt out.

The LED was rated at 1.7V 100mA and I was using a 100ohm series resistor. By my reckoning I should have only needed a 73ohm resistor, (9V - 1.7V) / 0.1A = 73ohms, so 100ohm was more than enough.

Any ideas?

8 Replies

user
Patrik (author)2008-03-05

Yeah - you want to look at the line that says 1.2V @ 20mA, not 1.7V @ 100mA. That would give you a resistor of around 470 Ohm.

Select as Best AnswerUndo Best Answer

user
Firebert010 (author)Patrik2008-03-05

This is correct, just the other day I've used a 470ohm resistor with a 9v battery and red LED.

Select as Best AnswerUndo Best Answer

user
evilad (author)Patrik2008-03-05

Great, I'll have a better look at the catalogue next time.

Select as Best AnswerUndo Best Answer

user
evilad (author)2008-03-05

So if I take into account the supply voltage as 9.6V and the LED is rated at more like 50mA, A resistor of 158ohms plus should be fine. I think I'll put in a voltage regulator as well to make sure. Thanks for all your help.

Select as Best AnswerUndo Best Answer

user
Patrik (author)2008-03-04

Are you absolutely sure the LED was rated for 100mA?

I did a little bit of browsing around at the various online LED suppliers, and I saw a whole bunch of 50mA and 10mA IR LEDs, but no 100mA ones...

Select as Best AnswerUndo Best Answer

user
evilad (author)Patrik2008-03-05

I am absolutely sure, here is a link, to the component specs.

Select as Best AnswerUndo Best Answer

user
Goodhart (author)evilad2008-03-05

As Gmoon says, using the Max setting is not good, for all the reasons he stated and also that the nominal "fresh" storage can be a bit higher then stated on the case....in this instance, possibly: 9.6 V for some.

Select as Best AnswerUndo Best Answer

user
gmoon (author)evilad2008-03-05

The link indicated that, yes. Looking at the PDF, it also lists 100mA as the absolute max.

However, the PDF also lists characteristics (typical operating condition), 4 out of 5 are cited at 20mA, which would be typical for a single LED. So the device was extensively tested; but @ 20mA. Only "LUMINOUS INTENSITY" is 50mA, half of the absolute max.

I would take the max ratings with a bit of skepticism. This is perhaps an example of a manufacturer (Asian-rim) overestimating the capabilities.

Select as Best AnswerUndo Best Answer