why won't white leds work at less than 3 volts?

danno

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why won\'t white leds work at less than 3 volts?

been monitoring this forum for a while and I'm finally ready to ask a stupid question. Why won't a white LED work with less than 3 volts? As I understand it, there is a 1.7 v drop across the led and the rest of the voltage drop is usually taken up by a resistor. Given that the resistor is in series with the LED, doesn't the LED actually "see" less than 3 volts? If so, why won't the led work at 1.7 volts with no resistor?
The reason I ask is that I'm working on converting a solar/crank powered radio/flashlight that has an internal 2.4v ni-cad pack for power. I was going to replace the 2.4v pack with a 3.6 volt pack and put a resistor in series with the pack to drop the voltage to the radio. It occured to me that it was stupid to raise the voltage of the battery pack only to put a resistor in place to lower it again.
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Re: why won\'t white leds work at less than 3 volts?

White led's need 3.6V across the LED itself. If you have 3.6V source, then there would be no need for a resistor. I'm not sure where the 1.7V number you referenced came from. Good luck with your project.
 
Re: why won\'t white leds work at less than 3 volts?

Perhaps that 1.7V was in reference to older non-white LEDs. The type used for indicator lights (eg. power on/off lights). I think that if you connect the Nichia LED in the reverse manner, you might find it dropping 1.7V but it won't light up.

If I recall my electronics lessons correctly, a diode passes current easily in one direction but greatly resists the current when assembled in reverse. This is because of the way that the semi-conductor materials are doped, so that one side has a surfeit of electrons (free electrons) and the other has a dearth of electrons (holes). When assembled to pass current, (ie +ve lead to +ve), the electrons are supplied to the semi-conductor side that has the surfeit of electrons. When a critical point is reached, the free electrons cross over the "barrier" and fill the holes, thus allowing current to pass. The small voltage drop you will observe across a diode is the result of this "barrier" or critical point. ie some resistance or power loss is felt in the diode to set it up to allow current to pass.

In the same way, if you connect the diode in reverse, the holes are presented to the incoming electrons. The critical point is much harder to reach because there are so many holes to fill before the diode can start passing current. However, with sufficient voltage, the holes can be filled and current can start passing. This is known as the breakdown voltage. In most cases, this will ruin your diode.

Now... if you're still with me and I haven't made a complete fool of myself by explaining the concept all wrong:

In special diodes, the semi-conductor material is made specifically to break down at a certain voltage, without suffering any damage. These are typically known as zener diodes and are used to regulate voltages. LEDs do this as well. But at the breakdown voltage, the electrons of the semi-conductor material are excited at the correctly energy levels to move between certain energy states. The amount of energy required is very specific for each type of material and it is known as a quanta. The electrons cannot make the leap unless they have sufficient energy. However, this excited state is not stable and the electrons will give up that energy, again in that specific quanta. The energy is released as photons, which is just another way to say that it is released as light. (You will see this material again.)

But that is a simplification, as photons of different energy levels actually produce light of different colours. Since the electrons release their energy at very specific quanta, so too the photons have very specific energy levels and the light produced is of a certain frequency/colour/energy. Thus you get your red, green, blue and yellow LEDs.

Don't falter now... we're getting to the crux of the question.

But white light is actually a composition of your 3 basic colours. So how can LEDs, which produce very specific colours, give white light? The old way was by running an LED which was really 3-in-1. 3 dies produced the 3 basic colours and these were mixed to produce white light.

The newer method only became achievable when some bloke figured out how to make blue light (ie which semi-conductor to use, how much doping of what material, etc.). Some tweaking and they were right at the blue/ultraviolet boundary. The final part of the puzzle was fitted in to place with the use of fluorescing material. The fluorescing material (similar to the stuff used to make fluorescent tubes) could produce some of the other colours needed and a little bit of blue was bled through. The resultant mix was white, and required only one die (and 2 leads), rather than 3 dies (and a minimum of 4 leads) of different colours.

Finally....

At present, the semi-conductor material used will only break down at 3.6V (nominal). It cannot work at a lesser voltage because the quanta energy level required is quite specific (not quite true because some glow is evident at slightly lower voltages, but that has to do with energy probability concepts). They can work at slightly elevated voltages, but the excess energy is dumped as heat. Too much voltage and the excess heat will fry the LED.

And that is why they don't work at less than 3V.
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I hope I didn't bore anyone
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or make any flagrant mistakes in the explaination. If anyone does spot a mistake in the explaination, please let me know.
 
Re: why won\'t white leds work at less than 3 volts?

Thanks for the replies. I was looking at a reference for older (i.e., red) LEDs that catagorically stated that the voltage drop was 1.7 volts across any LED. Thanks for setting me straight.

I guess the simple solution to my conversion is to rig up 3 ni-mh batteries in series and not use a resistor. Sounds easy enough.
 
Re: why won\'t white leds work at less than 3 volts?

Yep, just 3 NiMH or NiCd batteries will give the appropriate voltage, and the internal resistance of the batteries will help ensure that there will be no thermal run away.

But mind that you make sure your generator and solar modules supply sufficient voltage to charge the battery. This should be around 1V to 2V more than the rated voltage of the battery. So in this case, 4V to about 5.6V.

Good luck with the modifications.
 
Re: why won\'t white leds work at less than 3 volts?

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by danno:
Thanks for the replies. I was looking at a reference for older (i.e., red) LEDs that catagorically stated that the voltage drop was 1.7 volts across any LED<HR></BLOCKQUOTE>

Sounds like whenever that particular reference was written, the only commonly available visible LED that existed was red. So to that author, all visible LEDs would have had a 1.7v drop.
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