Electronic – Why is LED lighting up despite Supply voltage < Forward voltage

current-limitingledpower supply

I have an LED which has specified Typical forward voltage of 3.5V and Maximum forward voltage 3.9V.

I applied 3.3V across it with a 300 Ohm resistor in series. Why did it light up?

I am wondering if I can pick this LED as a reliable choice for my design (which as noted runs at 3.3V supply across the board).

My thinking:

The LED datasheet has a curve of Forward voltage vs current (I'm also confused why they put Forward current on the Y-axis instead of X, given that current is what one would vary here). Anyway, the curve shows a decrease in the forward voltage at smaller currents; perhaps this is the explanation?

Here is the downloadable PDF datasheet for this LED (it's a tricolor LED and in this question, I was referring to the specs for Blue and Green).

Best Answer

You are correct - the forward voltage depends on the forward current.

The forward voltage you see in the table of typical values is for a current of 20mA, which is too high when all 3 colors are used at the same time (footnote two in the absolute ratings table on page 3 - 15mA is the maximum in that case).

When you look at diagram 2 in the data sheet, you can see the relation between forward voltage and forward current. What you see here is that for a forward voltage of 3.3V, a forward current of 20mA can be expected. With 3V, it would be 8mA. A higher resistor value doesn't make this more reliable, it just makes the LED darker. You want to have the resistor as small as possible.

The resistor should be only large enough to drop the forward voltage to about 3.1V with a current of 15mA - this would mean a value of about 13.3 Ohms (the one for the red LED needs to be larger, though).

Whether this LED is usable for you depends on the brightness you need. If you don't need it to light up fully (or you use a version with higher intensity, see page 4), it would work. If you want to be sure you can use the full intensity, you need to use another one. Olin is right - the variation between batches can also mean that some are brighter than others. To ensure a uniform brightness, you need to control the current flowing through the LEDs.