How LED lights are so efficient?

[jakconly]

I want to use LED light in a small lamp project. Glow should be purely esthetical, it doesn't have to lighten enything (like desk or room).

 

I found 0,05W indicator LEDs way too dimm, and went looking for some 1W ones. than I did some calculation. My lamp will be powered by 2xAAA batteries for total of 3,3v and around 800mah capacity. with 1W LED it's something like 2,5H of battery life. It's like nothing for a lamp that should stay on for long evenings.Those LEDs also come with a heatsink.

I've seen many christmass lights, powered by 2xAA, many light sources in a string (like 20/30). And they were quite bright, no heatsinks and working for days.

 

Am I missing something? Are those not LEDs? Does my mind trick me and they are not as bright as i think?

[jakconly]

1 minute ago, Caroline said:

Two AAA batteries won't give you 3.3V if each is 1.5V, quick maffs.

well it's nominally it's 1,5, really it's higher

 

3 minutes ago, Caroline said:

any diode can be an LED if you drive enough power through it /s

haha..

 

4 minutes ago, Caroline said:

multiple light sources = more lumens = brighter than a single source

I meant more like: individual source is brighter than mine and all multiple sources seem to draw less power than my single one

at least I feel like it's that way

 

5 minutes ago, Caroline said:

What kind of leds are you using? conventional? surface mounted? filament like?

well, the most recognised THT led indicators, most common ones, but I feel like it is mistake so I'm searching for a better solution

[jec6613]

Well, two things: 1) AA's have more than triple the capacity as a AAA, and 2) a string is bright, individually they're super dim, like 1/10 of a Watt or even less is normal for those.  My 9 1/2 foot tree with ~800 lights draws around 40W, so 1/20 of a Watt each.

[jakconly]

19 minutes ago, jec6613 said:

Well, two things: 1) AA's have more than triple the capacity as a AAA, and 2) a string is bright, individually they're super dim, like 1/10 of a Watt or even less is normal for those.  My 9 1/2 foot tree with ~800 lights draws around 40W, so 1/20 of a Watt each.

yeah, like I said, i'm not sure about what I see. It's possible that singular light source is super dim and I'm tricked into thinking it is brighter. it is also possible that what i've tested is really enough for me. And also AA's capacity may be important tho I have no room for them anyway.

 

Thanks for reply, I could at least compare my attitude to those lights.

[mariushm]

Different color LEDs have different forward voltages, it depends on chemistry used. 

For example, some reds can be as low as 1.5v .. 1.7v 

Some green and yellow leds will be around 2.0 .. 2.2v 

Blue and white leds will have a forward voltage of around 3.0v .. 3.2v

 

The brightness will depend on the lens and the angle .. a 15 degree angle will seem to be much brighter compared to a 120 degree angle led at the same current, but the 15 degree angle will be more or less like a laser pointer. 

 

Usually the leds used in Christmas lights, are driven at 15-35mA, and they just put them in parallel with a single resistor to limit the current. For example, they have 20 leds x 20mA each = 400 mA for white / blue, 20 yellow/green with 25mA  for 500mA and  10 red leds with 15mA for 150mA  ... so in total you have 50 leds consuming around 1A of current. 

A couple AA batteries can do 1A for a few hours... but they extend the life of the batteries by making the leds blink, so at any point there's fewer leds turned on. 

Sometimes they drive the leds at higher than nominal current, like 50 mA for example, because they take in account that the led blinks, therefore duty cycle is less, so the led has time to cool off after it's turned of for a few ms... and therefore it won't die from overheating.

 

If you want to have something for pure lightning, always on, but you need it powered from a low voltage (ex 2-4 batteries or USB), you would pick efficient leds even if the voltage is higher than your battery voltage, and then you would use a led BOOST / STEP-UP driver, which boosts the voltage until the voltage is at least equal to the forward voltage of the leds (when connected in series, you add the forward voltages). 

 

For example 6v warm white surface mount leds are very common as they're mass produced for monitor backlights and for other things, typically they're 6v at 100mA or something like that, but you'll still get reasonable brightness at lower currents 

 

here's an example : 

 

6.35v forward voltage, 150mA current , neutral white (4000k) :  https://www.digikey.com/en/products/detail/creeled-inc/JK2835BWT-W-H40EB0000-N0000001/10820135

And here's datasheet: https://cree-led.com/media/documents/data-sheet-JSeries-2835.pdf

This gives you around 140-150 lm  at 150mA current  (or 144 lm/W  because 6.35v x 0.15 = ~ 1w ), depending on temperature of the chip.... and at lower currents, like 100mA, you'll still get 75% of the brightness, or around 110lm

 

I'd use a step-up / boost led driver  to boost 3v  up to around 12-18v ... so groups of  2-3 leds and then have these groups in parallel. If my input is 5v, I'd go maybe up to 4 such leds in series. Basically, I'd keep the boost to maximum 5-6x the input voltage, higher boosts would make the process less efficient, so you'd waste more input power.

 

There's also plain near 3v leds that are quite efficient , for example : 

 

2.7v 65mA , neutral white (4000k) : https://www.digikey.com/en/products/detail/creeled-inc/JB5630AWT-P-H40GA0000-NZ000001/10820131

See datasheet here : https://www.cree.com/led-components/media/documents/data-sheet-JSeries-5630.pdf

 

These are more efficient, close to 204 lm/W ... and you could use 3 alkaline/rechargeable batteries in series (3x1v ... 3x1.6v = 3v..5.4v, or a lithium 3.6v...4.2v, or usb 5v with a step-down/buck led driver to get most efficiency by paralleling a bunch of these (ex 10 leds in parallel to get 500-650mA of current)

Could also use a step-up/boost driver and then you're put a bunch of these in series (ex make groups of 8 at a time)

 

Linear led drivers would also work but be less efficient .. for example you could have a 12v wallwart adapter and make groups of 4 leds to get a forward voltage of around 11v, and then use a resistor for each 4 leds to limit the current to 65mA or less.

Or you could have a 16.5v...18v laptop adapter, which means you'd be able to make groups of 6 leds ( 6 x 2.7 = 16.2v) and limit each group of 6 leds using a basic resistor.

 

 

[jakconly]

Okay, this was a really comprehensive response! Thank you mariushm. A part of your info was overkill for my project, as I don't need that much light. It is also meant to be really low budget home made product, so no power circuits are in consideration. Also there is room for no more than 2xAAA. Nevertheless I won't dare to say that your information was useless. I more than surely will be getting back to this post while doing other projects!

 

On 1/3/2022 at 12:44 PM, mariushm said:

but they extend the life of the batteries by making the leds blink

Cheap designs  and ideas really fascinate me. It is also fascinating to learn things like that! And the lights blinking fascinated me since i was little and LED Christmass lights got popular and affordable (at least in Poland, so it was not long ago...). i was used to those old 240v chains of 100 fire and electrocute hazards, that did shin constantly. That made me search for no blinking mode in LED lights, as i liked it most. I changed modes in all lights and after some time found that modes changed - they are constantly cycling. I thought that it's just cheap controller limited in functions. It might have been truth, but it also had a reason for it!

 

Other smart design I've seen was a bread toaster that dropped line voltage to power it's control circuit using it's heating coil!