Forward voltage vs Current

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scswift:
Hey guys,

I was looking at the datasheet for this LED and I noticed it says the maximum forward voltage is 5.2v, which is more than the 5v I'd planned to supply the leds with.  The typical forward voltage is 4.3, so if I did use 5v, I could assume most of the LEDs I bought would work, but unless I don't understand forward voltage properly, some would not.

http://www.vishay.com/docs/81260/vlww9900.pdf

I did however notice a section of the datasheet where it graphs forward voltage vs current.  And I was wondering if someone could explain to me what that means.

I know that if I have a 5v source, and I use a 15ohm resistor, I'll put 50mA through the LED if the forward voltage is 4.3.  And I know how to calculate that with Ohm's law.  But that's the extent of my knowledge regarding that.

I guess what I'm wondering is... If I do get an LED with a forward voltage of greater than 5v, is that chart telling me how I should adjust the current so it will still function, or am I just screwed at that point and will have to test all my LEDS before I solder them into my circuit if I choose to go with this LED, and a 5v supply?  I'm guessing it's the latter.  But I would still like to understand what that graph is trying to tell me.




madworm:
What ultimately kills the LED if it is overdriven is the heat it can't get rid of. As usual it is (forward voltage * current). As I is a function of Vf and vice versa, it is sufficient to make sure that the current won't exceed the maximum permissible value, in this case 50mA. Here this can be accomplished by using a suitable resistor (ignoring variations of temperature in the LED).

Depending upon your requirements, the variations in the "typical parameters" of this LED may or may not affect you (uniformity in brightness for a display...). Personally I have not had any serious issues with single color LEDs in that respect though, but with RGB matrices and color balance. The worst difference in Vf I had to deal with in a single batch of cheap LEDs was about 0.1V, but I use LED drivers, so it doesn't matter much.

I doubt you can actually buy LEDs that would have values such as: 4.3, 5.0, 4.1 ... in the same batch. These would come from a pretty shitty factory. I don't even know if it's possible to produce LEDs with such a deviation and call it a 'working process'.

scswift:
Quote

What ultimately kills the LED if it is overdriven is the heat it can't get rid of. As usual it is (forward voltage * current). As I is a function of Vf and vice versa, it is sufficient to make sure that the current won't exceed the maximum permissible value, in this case 50mA. Here this can be accomplished by using a suitable resistor (ignoring variations of temperature in the LED).

I understand that heat kills an LED, and that the absolute maximum rating here is 50mA.  But I still don't understand the purpouse of the graph.

If the graph were showing me how much current I could put through the LED if it had different forward voltages, then wouldn't the current be inversely proportional to the forward voltage, rather than proportional to it?

In other words, let's say you had two leds, one with a Fv of 4.3v, and one with a Fv of 4.7v.  4.3*50mA is less than 4.7*50mA.  So one would think the led with the higher forward voltage would heat up faster.  But this graph seems to show the reverse relationship.  That an LED with a Fv of 4.3v is generating more heat than one with a Fv of 4.7v for the same mA.  How can that be?

tomm:
Surely heat is directly related to current, so if you're putting the same current through each it doesn't matter?

The graph just shows how much current would flow through the LED if directly connected to that voltage, if you're using current limiting resistors, then the graph doesn't make much sense.

scswift:
Actually I found this page which I think answers my question:
http://www.maxim-ic.com/app-notes/index.mvp/id/3070

"For a standard LED of 5mm diameter, Figure 1 shows the forward voltage (VF) vs. forward current (IF). Note that the voltage drop across an LED increases with forward current."

So I think what the graph is showing is how the forward voltage changes as I put more current through the LED.

With that in mind, I think that means if I reduce the current I'm trying to put through the LED, the forward voltage will drop.  So if these LEDs can have a max forward voltage of 5.3v at 50mA, then if I reduce that to say 40mA, I will have less of a chance of getting a batch of LEDs that simply won't work at 5v.

Plus I've been told I should stay 80% below absolute maximum ratings anyway, so 40mA would not be unreasonable.

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