How to find voltage & current of Led

Discussion in 'General Electronics Chat' started by mrel, Oct 27, 2015.

1. mrel Thread Starter Active Member

Jan 20, 2009
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Hello
I purchase a bag led at local swapmeet ,since led are not mark with voltage and current.
How do i find out how much voltage and current these led can use?
mrel

2. DickCappels Moderator

Aug 21, 2008
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You might want to find datsheets for LEDs with similar packaging to determine how much power in watts that your LEDs are designed to dissipate. After that run a few milliamps through it (note 1 below) to see what color it is and from this you can estimate the voltage (note 2 below). From that just divide the watts by the volts to determine the amps (miliamps most likely).

Note 1: I often use a 9 volt transistor radio battery with a 1k resistor in series to judge the color of unknown LEDs or to verify the claimed color.

Note 2: The chart below provides some guidance on voltage as a function of color. You can also measure the voltage across the reistor while checking the color but as you can see, the voltage varies considerably as a function of current with some LEDs.

3. dl324 Distinguished Member

Mar 30, 2015
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You can build a constant current source/sink of 10 or 20mA and separate the LEDs by forward voltage. I wouldn't go higher than 20mA because most LEDs aren't spec'ed for higher constant current; since your LEDs are of unknown age (date of manufacture), 10mA is the safest current.

Determining maximum current will be a guessing game. If you want to multiplex them, pick a sane peak current.

I scanned an old (1979) HP Optoelectronics Manual and package size didn't seem to correlate to max power dissipation.

4. DickCappels Moderator

Aug 21, 2008
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That was when LEDs were little more than indicator lights and the emitters in opto couplers.

5. dl324 Distinguished Member

Mar 30, 2015
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619
Guess I'm giving away my age...

LEDs operate at higher forward voltages and intensity these days, but power dissipation and peak currents haven't changed much. A T1 LED in the late 70's had a maximum power dissipation of around 100mW; about the same as a modern T1 LED.

Aug 23, 2012
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