run green LEDs at 2.4 volts?

Hi

I have this project where I blink my 5x 5mm clear red LEDs using 2 AAA batteries. Now I want to change replace the LEDs with clear super bright 5mm green ones.

The problem is that the red ones need only 2.4volts whereas the green ones need 3.2 volts. I have checked them with my AAA nimh batteries and the green ones do work but are not as bright as when they are powered with a cr2032.

Is there any simple solution to get them to full brightness using only 2 AAA nimh batteries? The project box is super tiny.

None of my green leds need 3.2 volts, but they do need a bit more than red ones.

mich2016: None of my green leds need 3.2 volts, but they do need a bit more than red ones.

mine works at 2.4 volts, but it does need 3 volts to achieve full brightness.

First off, LEDs should NOT be driven from a constant voltage. They are non-linear current-operated devices (like all diodes). Below the operating voltage (called "breakdown" voltage for a regular diode) very little current flows and the LED will be dim. Above the operating voltage, excess current flows and the LED dies.

With the correct current flowing through the LED, the voltage automatically 'falls into place'.

Do you have a [u]current limiting resistor[/u]? That will make your LED dimmer, but that's the most common way to control the current through a regular LED, and with the correct voltage and the correct current it can operate at maximum brightness.)

Sometimes you can "get by" relying on the internal battery resistance to limit the current, but it's bad engineering practice.

Regular alkaline batteries are nominally 1.5V. NiMh batteries are 1.2v. Two NiMh batteries make 2.4V which is not enough to properly operate the green 3.2V LED.

Green LEDs, due to physics, have a higher voltage drop (specifically, first law of thermo - conservation of energy; the voltage drop must be higher than the per-photon energy in eV). Greens just start to turn on around 2.3-2.4v (hey, look what the energy per green photon is) - so even if you have something that works when the batteries are at full charge, it won't work as they start to discharge.

There isn't really any way around that other than using a boost converter to kick the input voltage up, so you're not trying to run a LED on a lower voltage than it needs.

The exact voltage depends on the material and structure of the heterojunction device (all high brightness LEDs are heterojunction, many layers, very complicated), the wavelength of light sets the minimum voltage you can get away with, but actual devices are typically more.

GaN blue/green/white LEDs are usually the same voltage, for instance, about 3 to 3.2,

my project box doesnt have space for a boost converter.

So my next option is to change the power source.

Alkalines give about 1.5v but that drops to a lower voltage very fast under load. + I intend to leave the batteries in the box so I don't like leaky Alkalines.

NiZn??? I heard they give 1.6 to 1.7 volt per cell, but I couldn't find them anywhere.

Kentli?? too pricey

10440 li-ion or li-fe-po? not available in my country.

energizer ultimate lithium seems like a good choice to me. What do you guys think?

any suggestions?

my project box doesnt have space for a boost converter. So my next option is to change the power source.

Are you sure about that, they are very small you know:-

https://www.pololu.com/product/799

It is tiny I agree, but the only way I can fit one in my project box is if I ditch one of my battery, which will halve my watt hours, so I guess no.

The project box is super tiny, basically it is an old bicycle tail light case that has only enough space to fit 2 batteries, an LED array and a reflector. There is little bit of space beneath the array but not big enough for that boost module.

my project box doesnt have space for a boost converter.

Also known as a "planning failure".

How about a charge-pump? |500x252

So long as the LED doesn't come on through the diode with fresh batteries (add a second 1N4148 in series if so).

Then analogWrite (pin, 128) ought to give a decent brightness.

Increase the size of you project box the buy a converter.

.

pwillard: Also known as a "planning failure".

if that's what you call size constraints.

LarryD: Increase the size of you project box the buy a converter.

.

If I had that option I would fit another battery instead. :)

MarkT: How about a charge-pump? |500x252

So long as the LED doesn't come on through the diode with fresh batteries (add a second 1N4148 in series if so).

Then analogWrite (pin, 128) ought to give a decent brightness.

lemme see if I can fit all those components in that tiny space or not. Nice suggestion btw. :)

You must already have the resistor and LED, the only extra component is a signal diode...