LED short of forward voltage

Discussion in 'General Electronics Chat' started by Pootworm, Oct 26, 2007.

May 18, 2007
29
0
Hey all,

I'm in a situation where the LED I have wants to drop more voltage than I have to provide.

When calculating the current-limiting resistor to go along with an LED, an assumption is made about the voltage drop across the LED. Am I correct in thinking that if I assume a lesser drop across the LED and fix my resistor to draw lesser current (at a point on the forward current vs forward voltage graph, ensuring that the voltage drop is high enough that the LED will still light), the LED will just be more dim?

So, in tangible format. I picked up a few 3.3v LEDs (http://www.avagotech.com/assets/downloadDocument.do?id=4152
), but my system is only running at 3.3v anyway. If I assume a drop of 3.0v, and pick a resistor to give the LED 8ma (as per figure 2 [page 5/8]), am I going to end up with .5 the intensity of running it at 3.3v@20ma (as per figure 3)?

Thanks a bunch!

2. hgmjr Moderator

Jan 28, 2005
9,030
214
How many LEDs do you plan on using and how were you wanting to hook them up? I think I can provide you a solution.

hgmjr

May 18, 2007
29
0
Well, I'm either going to switch them with a micro+transistor as shown below, or connect them directly to a micro that can source enough amperage (but will make the voltage situation even more pronounced - Voh ~= 2.5 in that case). And probably three or four.

Thanks for taking a look, I appreciate it.

4. hgmjr Moderator

Jan 28, 2005
9,030
214
In that case, I think you will need to use a very simple dc-to-dc switcher. With this switcher you would create a +5V power supply from which to power the LEDs. I have had a lot of luck with the LT1110-5 made by Linear Technology. There is an example circuit on the very first page of the datasheet that would work just fine for your needs.

hgmjr

5. Voltboy Active Member

Jan 10, 2007
197
0
Or if you have an extra cellphone battery use it.. they generally are rated at 3.6V or 3.7V.

Oct 21, 2007
43
0
the few micro's i've used sink current better than sourcing. if it doesn't reach its forward voltage it simply will not light. i'd try what you had in mind with a small resistor and see if the led will light. if it does, connect it straight to the micro, but invert the micro's output so it is sinking rather than sourcing.