Im trying to pick the correct led for an application, and not really sure what i should go with. I will simply need to run an led with 3.3 volts and 5ma. I tried a led i had laying around, (no idea on specs) with a 300 ohm resistor and it wouldnt even light. Anybody point me to a suitable led? Thanks in advance !
It should have lit and drawn about 5mA unless it was one of the newer blue or white types in which case you would need a much lower resistor.
Sure you had it connected the right way around?
yes, i used a resistor . I just assumed this LED had a drop of 1.8 volts, so with a 3.3V supply i figured i needed a 300 ohm resistor to limit the current to 5ma. I used a 280 ohm and the power supply, and i was pulling about 5.2 ma and the led was pretty bright. So i can only assume that the device i am using is not giving me 5ma as the specs say. (i did measure 3.3volts, so that is not the problem.) BTW, it is a trigger output line for a portable data aquistion system, so it is designed to light an led so you know when the DAS is being triggered. Time to contact the manufacturer i guess.
I dont have any schematics so i dont know for sure, but if it was an open collector output, i should have a positive voltage line correct? (there is not one.) The manual says the trigger output supplys 3.3 volts and up to 5 ma's, so i figured it would go high. Maybe i should go check it out.
it has been my experience that blue leds light brightly with less current than the other colors. I have some blue leds that I've soldered a 1k resistor to as one of the legs, which I use on breadboards. even at 5v the led is pretty bright.