# How to find voltage & current of Led

#### mrel

Joined Jan 20, 2009
171
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

#### DickCappels

Joined Aug 21, 2008
6,801
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.

#### dl324

Joined Mar 30, 2015
12,255
led are not mark with voltage and current. How do i find out how much voltage and current these led can use?
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.

#### DickCappels

Joined Aug 21, 2008
6,801
That was when LEDs were little more than indicator lights and the emitters in opto couplers.

#### dl324

Joined Mar 30, 2015
12,255
That was when LEDs were little more than indicator lights and the emitters in opto couplers.
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.

#### ScottWang

Joined Aug 23, 2012
6,978