Calculating and Finding VF and iF of a LED

[zmint]

Hi Everybody,
My obsession with LED have brought me to this crucial task of finding actual VF and IF rating of a LED.
I decided to go for it because a article I read states
"Most white LEDs are typically listed as VF = 3.2-3.6V, IF = 20 mA; their luminous intensities can range from 3,000 to 30,000 mcd with associated ascending prices. Also keep in mind that not all LEDs are created equal. Even if you buy the same batch of LEDs from one source, each LED may have slightly different forward current under the same forward voltage; hence, different brightness. For this reason, it’s better to measure each LED’s VF and IF before using them. Herea is a simple regulated power supply circuit to do so.


It uses the LM317 regulated IC. Its output voltage VO (volts) can be approximately calculated by the following formula: EQUATION Vo = 1.25 (1 + R1/R2)

For instance, using R1 = 500Ω, trimmer potentiometer and R2 = 100Ω, the highest output voltage should be VO = 1.25*6 = 7.5V. Therefore, you can reduce the trim-mer resistor value to get any regu-lated voltage between 3.0V to 4.5V at the output for your experiment. "

I think it is simple enough(geek wise) to accomplish my goal, but How in do I proceed practically. In layman terms, to say, I have got LM317, 9V Dc 500 mA power source, the 500 ohm pot. and 100 ohm r2. Now where do I connect the led, what to measure and at what point etc. etc. in order to get the Vf and IF of a given LED.

Or there is some other better way out also ?

Thanks

 

[QuietMan]

A good way to measure Vf is to feed a constant current into the LED, and measure the voltage drop. Constant current is good way to turn LEDs on anyhow, and using the LM317 as a current source is even simpler than using it for a voltage regulator.

One other note, the LM317 as a voltage regulator really needs a capacitor on the output to keep it from oscillating, something around 0.1µF would do it.

Generally the If is specified on the data sheet. There is no way I'm aware of of finding the best current for an LED, though someone else might know of one.

 

[Boncuk]

To find out the forward voltage of an LED at least the forward current must be known. There are LEDs with a forward voltage of 10V and a forward current of 1A.

How would you find out those values using a variable voltage source?

Driving an LED with an adjustable current source it might easier to find out the forward voltage, since it may vary between two LEDs of the same lot.

Having found the forward voltage says nothing about the allowed forward current.

Boncuk

 

[QuietMan]

As a general rule of thumb (there will be exceptions) 10ma is safe, it was the old standard for older technologies. 20ma seems to be a current standard, and I've run into a few regular types that were rated for 30ma. If the LED goes off color you're definitely overdriving it.

 

[Reloadron]

Also keep in mind that not all LEDs are created equal. Even if you buy the same batch of LEDs from one source, each LED may have slightly different forward current under the same forward voltage; hence, different brightness. For this reason, it’s better to measure each LED’s VF and IF before using them. Herea is a simple regulated power supply circuit to do so.

I believe what they are getting at here is that no two LEDs (even of the same type manufactured in the same lot) will have identical forward current for a given voltage placed across them. For example if we took several LEDs that were identical and used a LM317 as a current source and applied let's say 20mA that the voltage drop across each would vary slightly. Likewise if we used a LM317 as a voltage regulated source and applied a set voltage across each LED we would see they would not draw identical current for a fixed given voltage.

The same can be said for transistors in that if we want a matched pair we can buy them as a matched pair or take a pile of identical transistors and run them on a curve tracer to get a matched pair. No two transistors from a given lot of the same part number will share absolutely the same conduction characteristics.

Even before transistors the same applied to vacuum tubes.... and so the list goes on.

Ron

 

[zmint]

Thanks for all that help. It will add to the knowledge bank of my electronic head.

 

[audioguru]

I have 3 bags of name-brand LEDs and the forward voltage for them in each bag is extremely close.
I also have cheap Chinese and Indian LEDs and their forward voltages are all over the place.