Make a 3.3V Led work in 2.2V system

[jabez]

Is there a way to make a 3.3V Led light work in a 2.2V system. I know how to work it the other way but i was wondering if there was something or some way to reduce the voltage on the LED so it would shine good & bright & work in a lower voltage system.

[RickL]

The short answer is use a smaller value current limiting resistor. I think you may be confused about how LEDs work though. Normal LEDs have a voltage drop, that is they need a certain amount of voltage before they will conduct current. Typically this is around 1.4 volts but some colours need more.

To get the LED to shine you need a certain amount of current through it, usually somewhere between 10 and 20 mA, the more current the brighter it shines. To calculate the resistor value to limit the current to what you want subtract the voltage drop of the LED from the supply voltage then divide that number by the current you want to use.

Example: 9 volt supply, 1.5 volt drop, desired current 10 mA.

9 - 1.5 = 7.5 volts. 7.5/10 = 0.75. In this case since the current is in mA the resistance will be in kohms. So 0.75 kohms or 750 ohms.

If the voltage drop of the LED is actually 3.3 volts and you want to use a 2.2 volt supply I don't think there is any way you can get it to work. More likely I'm misunderstanding what you mean by a 3.3V LED. Can you supply a little more information?

[jabez]

Well the specs on the LED are 3.3V & 30ma. The way I calculate & always find it to work perfectly when using a LED in a 9V or what ever circuit is to use this formula. Say in a 9V system: 9V minus the LED voltage devided by amps. Or in this case: 9V-3.3V = 5.7V / .03 = 190 ohm resistor to use. I would use something just a little lower.  I have never had a problem with doing it this way & the LED's last long time & always shines good & bright with out burning out or anything. Every LED has a Voltage rating & a amp rating. I have seen 2V LED's with a 20ma rating and a 30ma rating & all sorts of different ways. I got this formula from here: http://www.stewmac.com/cgi-bin/hazel.cgi/hzpi/u/HzSt01140m151716140m160U0n0n0n0m15110n17170m/hazel.cgi?action=SERVE&sku=I-1561&item=freeinfo/print.html

I might be misuderstanding you or not understanding the technical aspect of this but this works is all I know.

But this is not what I wanted to know. A friend of mine needed a LED to work in some 2.2V PC board & the LED's I had & he has will hardly shine in the 2.2V system. I didn't really know if there was a way to make them work or not. I quess he will just have find a lower rated LED.

[Nasse]

I think it is possible with special circuit. Pocket keychain torches use sometimes special technology for that. I don´t have a link or circuit just now but maybe something can be found later. There exist special chips for that purpose, and some circuits with them or with discrete easy available components have been published  in hobby magazines

Such circuits use some kind of oscillator, and a coil or inductor and caps and such to kick that voltage higher as needed

when typed these words (led+torch+circuit) to google search engine I got over 100 000 hits and first was this: http://www.satcure-focus.com/tutor/page12.htm

[jabez]

Yeah that's interesting. But I think it will be a easier to just find a lower rated LED. thanks. Well actually I have a red LED that works but he wants a Blue LED and the blue ones I have are a higher Voltage & don't hardly shine in his gaget. I don't even know what he's doing really. I haven't even seen it.

[gez]

Nasse is right, you need additional circuitry.  There have been a whole stack of circuits that do this in EPE magazine over the years.

[Nasse]

Or you can take the power to the led elsewhre in the computer, and just take the control off that lo voltage systemo