What led to use.

[dirtyb15]

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 !

 

[dch222]

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?

 

[eng1]

I wonder if it is a white/blue LED. The voltage drop varies depending on the LED's colour (and current, of course).

 

[dirtyb15]

Yeah, i had it hooked up correctly. Lights fine on a power supply at 3.3 volts.
I wonder if maybe the unit is not really supplying 5ma.....

 

[dch222]

Lights fine on a power supply at 3.3 volts.
So the problem's nothin to do with the LED then

 

[AllVol]

Lights fine on a power supply at 3.3 volts.

Did you have a resistor in series for this test? If not, your LED might cooked now.

Even at low voltage, a red or green or yellow LED likes to see 20 mA or less current.

 

[dirtyb15]

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.

 

[eng1]

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.

Is that line supposed to go high or is it an open collector output??

 

[dirtyb15]

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.

 

[justDIY]

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.

 

[Hero999]

Use a red LED, most will turn on at 1.8V.

 

[dirtyb15]

Turns out the manual was wrong, they changed it mid design. It's supposed to sink, and i can use the trigger in for my source.