Hi guys,
I'm looking at the spec sheet for an RGB LED and the blue LED has a forward voltage of 3V. Here's the spec sheet. Looking at the current vs voltage curve for the blue LED (also attached below), it almost seems like the LED won't even light up at 2.8V. Is that correct? However, from my experience, usually slightly lower than Vf is OK and it will still light up, just drawing a lot less current and being less bright. I just want to get some second opinions whether this LED will actually light up reasonably well at 2.8V.
Thanks!
It's very unusual (afaik, anyway) to try power an led with the exact voltage. Isn't it usual to give it say 5V and drop the surplus over a resistor, thus taking care of the current?
Yea I understand that, but please just accept that for my particular application I want to use 2.8V, not 3.3V or 5V. I just need to know if this particular LED will light up at 2.8V.
The curve doesn't quite go down to 2.8V. If you extrapolate, it looks like you'd get about 1mA (which is 2.8mW). And, the luminous intensity curve looks you'll get some light output around 1mA.
Of course, those curves are very-approximate and there are part-to-part tolerances, especially when operating out of the normal current/voltage range.
But, there's not much point in using a 3W LED at around 3mW.
For comparison, the blue LED on ESP-12 modules use a 470 Ohm resistor, and assuming a Vf of about 3V (typical for blue LED's), that means the current through the LED is about 0.64mA when powered by 3.3V, and it still is pretty bright.
The LED I am looking at draws 10mA at 3V for the blue LED, which means it's a 30mW LED, not 3W.
It takes a certain voltage for photons of each color to be released. Blue takes more energy than green takes more than red.
It's just the photoelectric effect in reverse.
You might bias the input a few 10ths of a volt to lift your 2.8V over 3V but get that circuit from a hardware guru.
The LED I am looking at draws 10mA at 3V for the blue LED, which means it's a 30mW LED, not 3W.
You're right! Sorry, I think I misread "3-in1" when I quickly scanned the datasheet.
So you think it will be super dim at 2.8V? Like not even operable?
androidfanboy:
So you think it will be super dim at 2.8V? Like not even operable?
Until you get to the forward voltage there will be NO current flow through the led, NO light from that junction.
But once you reach the forward voltage, all current through makes photons.
And as the led heats up, the forward voltage drops so leave some room for that.
A regular diode needs .7V to get any current through as do BJT's (bipolar transistors).
To do a ballpark test, I grabbed this RGB LED that I remembered I have lying around and tested the blue LED, which has Vf = 3.1V, even higher than the 3V I was looking for. I ran it at 2.8V with a good power supply and it lit up just fine! The threshold for it to light up satisfactorily was actually around 2.4-2.5V. So I think I'll take my chances with the 3V LED at 2.8V.
So now my question is, why doesn't the spec sheet tell you the actual Vf? It makes sense that the LED would turn on at a lower voltage than Vf just by experience, because when you select a resistor you always select one such that the current going through the LED will be less than it's rated for. For example, if you're powering a 3V LED with 5V and it draws 20mA, you need a (5V - 3V) / 0.02A = 100 Ohm resistor but you might choose 150 Ohms instead. This means the voltage drop across the LED is actually less than the calculated 3V, and by experience we know this always works, unless your resistance is really large. So this indicates that the actual turn-on voltage of the LED isn't like the forward voltage of a normal diode, like a Silicon's 0.7V. Can anyone give me a proper explanation of this? Thanks!
And you only lit the blue junction and saw only blue light?
Look up the photoelectric effect some time.
BTW, how do you know what the Vf is supposed to be 3.1V? It is the least voltage for current to go through the junction. If 2.5V can push through then Vf is <= 2.5V. Unless of course there was an error in measuring V.
Yes, I saw only the blue LED on. I got the forward voltage directly from the spec sheet. It says minimum Vf = 2.9V for blue. But it still works at 2.5V. But like I said, it's not only this LED, it's basically all LED's because of the reason I gave of picking a higher resistor than calculated. Also, I am using a very good quality power supply with built-in voltage measurement and I also confirmed it with a voltmeter.
We use the resistors to limit the current going through the led mainly to keep from burning up Arduino pins.
the led may take 50mA but the pin won't even take 40mA for long. We use 220 ohm resistors with many leds because 5V through 220 ohms allows 22mA to flow which is pin-safe to ground, something hardware amateurs like me can appreciate.
In general we try to err on the side of safety and not burning parts up so always expect some margin and look for what guarantees the datasheet makes to see what the margins protect.
For example, if you're powering a 3V LED with 5V and it draws 20mA, you need a (5V - 3V) / 0.02A = 100 Ohm resistor but you might choose 150 Ohms instead. This means the voltage drop across the LED is actually less than the calculated 3V,
Or maybe 20mA is not flowing through the led. You can find the current by measuring voltage across the resistor and applying Ohm's Law.
If you look at the datasheet for a led that has graphs showing the full characteristics then you would know that any forward voltage is dependent on temperature, and gets lower with increased temperature. That's why higher power leds and laser diodes need constant-current circuits.
Why should a Light Emitting DIODE not have a forward voltage? It takes a certain voltage (energy level) to make a blue photon. That's physics. Look up the photoelectric effect. If it took less you might start designing a perpetual motion machine.
The fact is, a voltage lower than the stated Vf turns the LED on, and the photoelectric effect occurs at 2.5V rather than 3V. That's what's bugging me the most.
There is more than 1 frequency of blue and they are all different energy. Blue is a lot wider than green but red looks widest to me.
Collect your data. Try a led that's only blue. Check the current flowing through the circuit, check your resistor with a meter since the tolerance band isn't there just for looks and see how the numbers check out with tighter data and normal operation.
androidfanboy:
please just accept that for my particular application I want to use 2.8V, not 3.3V or 5V.
Maybe it's time for you to spill the beans?
