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Yueh	#1 19/10/2008 - 02h57 Top
Class : Apprenti Posts : 5 Registered on : 12/10/2008 Off line	Is it possible to find out what current LEDs take if you have no MSDS info? The Source by Circuit City (Canadian equivalent to radio shack) have very limited supplys of LEDs and they are overpriced. However there are several electronic surplus stores that have buckets of LEDs for $0.15 each, including some IR ones. I have access to a multimeter so i can check the voltage on diodes but no idea how to test current. Any ideas? BTW awesome software. I'm having as much fun building this thing and tearing apart old cameras as I think I ever will playing any of the games. I almost don't want to finish it.
nilsca	#2 20/10/2008 - 04h27 Top
Class : Apprenti Posts : 11 Registered on : 14/10/2008 Off line	Don't know how to check the forward voltage/current that unlabeled LEDs have. I've played with some 'IR' LEDs that needed some crazy current to get active ... If you're in Toronto http://www.creatroninc.com on College has the SHF485(P)s (for $.085 iirc) Cheers Nils
Yueh	#3 20/10/2008 - 06h07 Top
Class : Apprenti Posts : 5 Registered on : 12/10/2008 Off line	Brilliant. Thanks. I'll head down there this week sometime.
tristan68	#4 20/10/2008 - 10h12 Top
Class : Electronicien en chef Posts : 651 Registered on : 10/07/2007 Off line	Hello Yueh I have access to a multimeter so i can check the voltage on diodes but no idea how to test current. Sorry to tell it , but there's no way to "guess" the max admissible current in a led, without destroying it Since the voltage/current curve is device dependant , and has a very steep angle (also device dependant ), you can't know what current to use , based on the voltage. The only thing you can do is to use "failsafe" values : forward voltage : about 1.2v current : about 20 mA (every led accepts 20mA) - create a test setup using these values. Calculate the appropriate resistor - measure the real forward voltage using your multimeter (since the voltage/current curve is that steep, changing the current won't affect the voltage much). - re-create a final setup using 20mA as forward current, and the foward voltage you just measured, calculate again the appropriate resistor using these values. The test setup allows you to test the forward voltage of the led without frying it, by limiting the current to about 20mA. cya
Yueh	#5 26/10/2008 - 01h02 Top
Class : Apprenti Posts : 5 Registered on : 12/10/2008 Off line	Thanks. I wonder what people are using all these unlabeled LEDs for. You'd think someone at these stores would know that you can't use them without the current/voltage specifics. These aren't the kind of stores the general public walks into... only electrical engineer types or hobbyists maybe. Maybe at $0.15 each no one cares if they keep burning out.

FreeTrack Forum > FreeTrack : English Forum > Support : Tracking System > Determining current for LEDs if not on packaging

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