How to determine LED Vf and If (from Monitor Backlight)

[kolbep]

Hi All,

I have pulled a LED Backlight strip out of a dead laptop.
The LED Strip is approx 3mm to 4mm wide, and has 10 Parallel Sets of 3 Leds in series.

I would like to use the strip in another Industrial HMI that has CCFL's that have failed,
(The HMI has 24vdc available, so I will be stepping that down).

however as the Laptop was not working, I was unable to measure the Voltage and Current for the LED Strips.

As a test, I slowly ran the set up in voltage. It barely lit at just over 3 volts. 8v gave a decent light, but without current limiting, I did not want to risk taking the strip more than that.

What is an easy way to determine the maximum Vf and probably more importantly the If of the strip, As I would like to run them brighter, but do not want to risk it. My plan is to get a CC / CV module, and then hook it up to the backlight.

There is no markings on the Strip or LED's


P

[ledtester]

You can put a resistor in series with the LED strip to prevent sending too much current through it.

For instance, a 1K resistor will allow at most 24 mA from a 24V power supply. I'd use a 1W resistor -- or four 1/4W resistors in parallel.

Once you have 5 to 10mA going through the LED strip you'll get a good idea of what the Vf is.

[bob91343]

Typically a single LED will draw 20 mA.

[wraper]

Typically a single LED will draw 20 mA.

LED will draw as much current as you push through it regardless of its ratings (until magic smoke escapes). LEDs used for backlight and illumination usually are rated for way more than 20mA.

[TimFox]

As with other forward-biased diodes, the voltage V_f across an LED is roughly proportional to the logarithm of the current, and decreases with increasing junction temperature at constant current.
Driving such a device from a constant voltage is therefore dangerous, since the current will increase at a constant voltage as the junction temperature increases, possibly leading to catastrophic failure.