Problem with fading leds over time

Other Types of Lights LED Light Bulbs
1

Hi,

Some time ago I made a Fallout nuka cola quantum bottle:

From a simple 3 led push light you can get at a dollar store:

Wired it to a protected Panasonic 18650, so not to waste AAA’s .

Problem is that every 4 weeks I must resolder new leds on the board because they put out less light.

Now these leds are from similar salvaged lights and I think they are not ment to stay on for 8 hours a day.
Maybe that is why they fade away so fast? Oh also these are white light leds, it’s the liquid that is blue (printer ink and some kitchen soda, so the light particles have something to bounch off of)

Anyway is there a simple solution for this problem?
Here is also a close up shot of the resistor

2

Those crappy lights which use 3×AAA only work because of one thing: the internal resistance of each cell limits current to those 7/9/11-LED flashlights. Use AAs instead, and they’ll overcook the LEDs. Use a Li cell whose resistance is in milliohms, and the poor LEDs will be screaming for mercy.

Frankly, I’m surprised they didn’t pop outright, or at least go all blinky.

Those teeny little LEDs, you absolutely can’t go DD, even with a small resistor like that.

What is that? brown/black/black? That’s only 10Ω!

—

Thinking… damned hearty LEDs to withstand Li cells like that for so long!

3

Brown black black gold

Edit: What does DD mean?

Digital Doom? oh i’d like it if it would be Digital Doom. Is it? is it Digital Doom?

4

10Ω current limiting resistor, three 20mA emitters in parallel. That resistor drops a volt when 100mA of current go through it (10Ω = 10V/A, ten volts per amp), so you are overdriving them a bit (and, mind you, these aren't mounted on copper DTP boards).

Change the resistor for at least a 20/30Ω one, and I believe your problem will be fixed. You may also notice the actual light output isn't that much lower.

Cheers ^:)

P.D.: DD stands for Direct Drive, or the act of driving a led with minimal to no current limiting devices besides the emitter and the battery.

5

those are cheap LEDs
they might be rated only 20mA

if they run 3V and your 18650 has 4V you have 1V to be eat up at the resistor

so you get 100mA/3 per LED 33.3mA
60% overdriven might not be really healthy, when the battery is 4.2V its even worse
then you got 40mA per LED

change the resistor to 20 Ohm or solder 3 parralel leds and you are fine

those lights are often designed to last not many battery charges for max brightness

6

Yeah the LEDs are low quality, tiny die type that can’t handle the drive current. While I agree that doubling or tripling the resistor value would fix it, I think that might be too dim for the effect you want.

Instead I would consider what materials and components you have on hand. If you have a spare scrap of perfboard or other PCB material, or (lol) even cardboard, you could abandon that circuit board and wire up one resistor per LED, using around 100ohm/LED, or one resistor for each set of two in parallel with 47 ohm /2 LEDs.

If you just want to do it the cheap quick way, you could just drill holes for the leads and add three more LEDs to that circuit board, scratching away copper traces where needed and wire-wrap connecting them in parallel to those already there, then you have half the drive current for each LED and about the same (slightly higher) brightness. However I hate to see so many LEDs in parallel as their minor forward voltage difference will mean some die earlier than others then cascade failure of each weakest one in turn, which is why I initially suggested that each have its own resistor.

How long do you want it to last? Whatever # of LEDs and drive current and # of resistors, try to aim for 10mA at most to get good LONG life out of them. 20mA just isn’t a realistic spec for many generic LEDs I’ve tried, even with ample copper on the PCB they still degrade from constant use.

Today higher power LEDs are cheap, so I’d as soon under-drive higher powered LEDs. You’re using white so I’d pick up a 5 pack of XT-E from FastTech for $3.87:
https://www.fasttech.com/product/2058900-cree-xt-e-450lm-4500-5000k-led-emitter-5-pack

Drive ONE of those XT-E with that 10 ohm resistor in series and it’s going to be about 130mA peak drive current off a fresh charged battery, a little under half a watt so you don’t need any heatsink, just mount the star directly to the area where the existing PCB is/was. Plus, then you have 4 other XT-E to play with… they make a sickly greenish-white tinted light, but it’s not terrible if you don’t have something else to compare against, and they are a very cheap way to light up things where you just want up to 100 lumens or so with the bare minimum addt’l parts possible.

However, you may not need that many lumens, so if you happened to have a 22ohm, 1/4W resistor lying around, that’s what I’d use for this project. Lol, that’s close to what Barkuti already suggested to keep using the same LEDs, but I think it is pushing it to expect only 3 of those 5mm single-die LEDs to provide enough light with good lifespan.

7

A cree XPE at 50-70mA will be brighter

At that low current a 20mm star is more than enough heatsink

8

At least 30Ω. Those small LEDs aren’t meant to push more than 20mA through them.

I made arrays of N×3 LEDs for 12V systems, and getting cute with currents, pushing even 30mA instead of 20mA would end up with blinkies. Random flickering. I even see that on some walk/don’t-walk signs around here.

When I’d keep them to 18mA or so, not a single blinky, even running 24/7.

9

Not much heat sinking in those traces. We over drive power LEDs quite a bit more now that DTP boards are available so increasing the mass/area of the traces might possibly increase longevity. Since they already survive for a fairly long time that’s not an unrealistic hope. Changing to an SMD led would certainly help since the distance from the die to trace is so much shorter for SMD LEDs than for through hole LEDs. Adding more with the same total current would divvy up the power and heat would be another way to go.

10

Thanks for the help.

I went through some scrap electronics but couldn’t find and 20 or 30 ohm resistors.
Went on Ebay and found this
Gonna buy a set of the lowest values, which has some 20 and 33 ohm resistors in it.
I’m a total noob when it comes to resistors though, so it would give something to practice with.

Let me know what you think

11

The existing LEDs are all in parallel so it may be possible to simply replace them with any of the two pad (no heat pad) 1.6mm Cree LEDs that are capable of the higher current if adding resistance makes these too dim.

12

Can someone tell me what the difference is between blue coated and brown coated resistors?
I can buy 1500pcs 75 values from 1ohm to 10m ohm blue coated resistors for $5.50
Or the same amount and values for $7.50 brown coated.

13

^ Dealing with generic sources it’s harder to say if they stick to standards or play games with colors, but usually blue are metal film and (but not necessarily) higher precision (stated in number % like 1% or 5) while brown are carbon. For your purposes of current limiting within a few deviation it doesn’t matter but if they are near the same price for same wattage, I’d rather have blue, higher precision metal film, which are also better for audio signal path… if you’d ever have that use for them.

If I doubted I’d use them for anything other than LEDs, I’d buy from whoever offered the faster free shipping…

Edit: I should mention that the carbon are usually more hardy when ran near their wattage rating. Metal film will more easily delaminate off and fail if they get too hot, but you’re nowhere near the 1/4W of the resistors you linked in a later post. and personally, I never run resistors higher than 50% of their wattage rating. They just aren’t expensive enough to skimp on unless it’s just a prototype for proof of concept that’ll be built better later.

14

Thanks for that info! yes both have 1/4W, so I bought the blue ones.
I got them HERE in case anyone else needs them

Edit: I found an awesome app for reading the colors on resistors and much more and it is free HERE

15

Get 3 10 ohm, solder in series, viola 30 ohm

orange black black gold will be the colors

actually putting higher value (within reason) is fine as well…a few mA will not really dim it much, so a 40-50 ohm will work just as well

16

See this right here is how green I am :smiley: Cheers!

17

the leds that come in that type of item are garbage even if run under spec.
try these