Author Topic: Can LEDs dim?  (Read 1074 times)

Can LEDs dim?
« on: March 01, 2020, 11:41:40 am »
Apologies if this is a stupid question. I thought my rear light (inexpensive LED, runs on AA batteries, no flashing modes - just on/off) died and needed new batteries so I put some in and it still didn’t work but then I noticed it was working but very dim which could only been seen in total darkness. I thought that was a bit odd as LEDs are digital devices that either work or don’t work but this was very analog behaviour. I’m not going to try to repair it, not worth it I wondered if someone could explain what is going on.

Thanks.
I am often asked, what does YOAV stand for? It stands for Yoav On A Velo

Kim

  • Timelord
Re: Can LEDs dim?
« Reply #1 on: March 01, 2020, 12:01:43 pm »
Yes, if you reduce the current through an LED, it gets dimmer.  The quantum nature of semiconductor junctions means the wavelength stays the same[1], unlike tungsten.

However, since LEDs are very non-linear in their brightness to current curve, LED lamps that provide a dimming function tend to do so by flashing on and off at high frequency, varying the ratio between on time and off time.  This is more power efficient, and can be done by simple digital circuitry.

My bugbear is that this PWM dimming is nearly always done at too low a frequency.  100-1000Hz is typical, which means you can see the flashing as soon as there's any motion (eg, car taillights become strings of dots), it tends to cause eyestrain when you try to read by it, and it makes life hell for people with photosensitive migraine, autism spectrum conditions, and some kinds of visual impairment.  Ideally, the PWM frequency should be well above the audio band, so as not to cause problems with whistling inductors.


[1] At least for a pure monochromatic LED.  White and some other colours are produced by phosphors, and are a bit more complicated.
Careful, Kim. Your sarcasm's showing...

Re: Can LEDs dim?
« Reply #2 on: March 01, 2020, 12:35:04 pm »
Thank you Kim. I won’t pretend I understand every word of your answer but as I put in new batteries, I can only assume that the circuitry is buggered.
I am often asked, what does YOAV stand for? It stands for Yoav On A Velo

Kim

  • Timelord
Re: Can LEDs dim?
« Reply #3 on: March 01, 2020, 12:38:35 pm »
Yeah, probably a high-resistance connection somewhere.  Dodgy solder joint, or something corroded or whatnot.  Occupational hazard with cheap bike lights.
Careful, Kim. Your sarcasm's showing...

Re: Can LEDs dim?
« Reply #4 on: March 01, 2020, 01:41:15 pm »
The LED itself can also be damaged, usually by overcurrent and/or overheating. They glow dimly rather than normal brightness. It’s hard to see how that can happen with a simple battery powered light, but maybe the resistor was the wrong size, or maybe you just got a dud.

Re: Can LEDs dim?
« Reply #5 on: March 01, 2020, 03:45:05 pm »
I'd suspect a bad connection somewhere too. Battery contacts should be cleaned with a mild abrasive (eg scotchbrite) before the light is condemned to the bin.

However (with apologies if this is obvious) I'd also check that the new batteries were actually OK.... ;)

cheers

Re: Can LEDs dim?
« Reply #6 on: March 02, 2020, 07:05:30 am »
Interesting - I have an old power bank which has a torch light on it.  The light has gone dim, but the power bank still works.  I had no idea what was going on.

Re: Can LEDs dim?
« Reply #7 on: March 02, 2020, 10:53:28 am »
FWIW I have seen some rear lights which were all made with the wrong resistor in series with the LED; obviously they were impressively bright to start with but all failed PDQ.  I'd guess that they used the resistor value which would have been more appropriate for a front light of some kind instead.

 Anyway the net result was that there was around 100mA going through a single 5mm bodied LED, which is probably  about x3 or x4 more than is good for it.  I didn't check these failed units to see if there was a vestigial glow or not; there might have been.

cheers

Re: Can LEDs dim?
« Reply #8 on: March 02, 2020, 10:58:30 am »
Yes I’ve had rear LED lights go dim and then back to normal brightness with fresh batteries.  Because they last so long the dimming can often be missed till someone riding behind mentions it.
If you don’t make time for exercise now, sooner or later you’ll need to make time for ill health.

Kim

  • Timelord
Re: Can LEDs dim?
« Reply #9 on: March 02, 2020, 12:16:58 pm »
Yes I’ve had rear LED lights go dim and then back to normal brightness with fresh batteries.  Because they last so long the dimming can often be missed till someone riding behind mentions it.

That's a battery thing.  Typically the battery will deliver a decent amount of power when you first switch the light on, so you think it's fine, then the voltage drops and it becomes dim over a couple of minutes as you ride along.  Exacerbated if you've just brought it out into the cold from a warm environment.

Some lights will switch off when the voltage drops below some threshold.  You notice it's off, think "oh, I must have forgotten to switch my light on", switch it on, observe decent brightness and set off again...  If it happens a few times, you might suspect a dodgy switch or bad connection.
Careful, Kim. Your sarcasm's showing...

Re: Can LEDs dim?
« Reply #10 on: March 02, 2020, 12:33:43 pm »
at one time many cheap blinkys were configured so that even a momentary loss of battery contact was enough to turn the lamp off; really dangerous in the case of a back light. I don't think there are any like that these days but you never know; it is something that I try to remember to test for.

cheers

Kim

  • Timelord
Re: Can LEDs dim?
« Reply #11 on: March 02, 2020, 12:40:41 pm »
at one time many cheap blinkys were configured so that even a momentary loss of battery contact was enough to turn the lamp off; really dangerous in the case of a back light. I don't think there are any like that these days but you never know; it is something that I try to remember to test for.

Seems just as common that momentary loss of contact turns them on these days.  Which is much better when you're riding along, but not so much when they turn on randomly and drain the battery.  The cheapo light on my trailer seems to like switching on in response to large changes in temperature (this is why it's on the trailer).

The moral of this story (apart from the ones about dynamos being less hassle, and you getting what you pay for) is that it's a good idea to have two rear lights.
Careful, Kim. Your sarcasm's showing...

Re: Can LEDs dim?
« Reply #12 on: March 02, 2020, 01:07:23 pm »
IIRC I settled on 'about three' as about the right number when my light collection was infested with bounce=off lights...  It was not uncommon to be down to one after a half-hour ride....

cheers

Re: Can LEDs dim?
« Reply #13 on: March 03, 2020, 09:47:01 pm »
However, since LEDs are very non-linear in their brightness to current curve, LED lamps that provide a dimming function tend to do so by flashing on and off at high frequency, varying the ratio between on time and off time.  This is more power efficient, and can be done by simple digital circuitry.

It's rare that I disagree with anything Kim says, and the rest of her post is spot on. However, LEDs are fairly linear for the brightness to current curves, and are slightly more efficient at low currents. The voltage - current curve of an LED is very non-linear, and that makes regulating the current more difficult than switching them on and off (at a frequency the designer thinks is fast but is in fact slow enough to be annoying to a lot of people).

At really low currents, like <1% of max, LEDs can be less efficient or just stop emitting light at all.

http://www.farnell.com/datasheets/2034713.pdf
Quote from: Kim
Paging Diver300.  Diver300 to the GSM Trimphone, please...

Re: Can LEDs dim?
« Reply #14 on: March 04, 2020, 09:12:56 pm »
LEDs go dim with age and heat.  The 5 deg C LED on my fridge has been on for 6 years.  If I reset the temperature to 3 deg, that LED is MUCH brighter because it's never been used.
And Darkness and Decay and the Coronavirus held illimitable dominion over all.

Kim

  • Timelord
Re: Can LEDs dim?
« Reply #15 on: March 04, 2020, 09:22:50 pm »
They'll last for ages if you substantially under-drive them.  But nobody usually wants to do that (especially not molishers of lighting or chinesium electro-tat).

Indicator blinkenlights *should* have an easy life (unless someone's skimped on the current limiting resistor), but that's probably cancelled out by using whatever LED was going cheap on the Shenzhen market that week.  In the case of a fridge, it's reasonable to expect it'll be still working (if not quite as bright) at the 10 year point when all the plastic parts start to fall off.


ETA: 6 years is the 50k hours you tend to see quoted as the expected lifetime of lighting LEDs.  I've just looked at the datasheet for the last set of indicator LEDs I ordered from a reputable manufacturer, and they don't quote a lifetime at all.
Careful, Kim. Your sarcasm's showing...

quixoticgeek

  • Mostly Harmless
Re: Can LEDs dim?
« Reply #16 on: March 04, 2020, 10:05:34 pm »
They'll last for ages if you substantially under-drive them.  But nobody usually wants to do that (especially not molishers of lighting or chinesium electro-tat).

Indicator blinkenlights *should* have an easy life (unless someone's skimped on the current limiting resistor), but that's probably cancelled out by using whatever LED was going cheap on the Shenzhen market that week.  In the case of a fridge, it's reasonable to expect it'll be still working (if not quite as bright) at the 10 year point when all the plastic parts start to fall off.

LED indicator lights have become rather bloody annoying.

https://hackaday.com/2020/02/20/we-ruined-status-leds-heres-why-that-needs-to-change/

I have a switch on my desk network lab, it has a blue status LED. Even with a 200gsm piece of card taped over the top, it's too fscking bright.

LEDs are an amazing invention, but we on 'arf abuse and misuse them...

J
--
Beer, bikes, and backpacking
http://b.42q.eu/

Kim

  • Timelord
Re: Can LEDs dim?
« Reply #17 on: March 04, 2020, 10:38:05 pm »
LED indicator lights have become rather bloody annoying.

https://hackaday.com/2020/02/20/we-ruined-status-leds-heres-why-that-needs-to-change/

I have a switch on my desk network lab, it has a blue status LED. Even with a 200gsm piece of card taped over the top, it's too fscking bright.

LEDs are an amazing invention, but we on 'arf abuse and misuse them...

In designs for my own use, I tend to use red to mean "power on" because red is a dim, inoffensive colour; green/orange/yellow to indicate that things are happening, and blue only ever to mean "something needs attention" because it's the eye-grabbing colour of emergencies.  And of course nothing is indicated by colour alone, because colour-coding is a work of Stan.

On the odd occasion I feel sufficiently motivated (or confident of getting the thing back together undamaged) to hack some existing device, replacement of blue power LEDs with something reasonable is about equal with dis-ambiguation of bi-colour status LEDs.  Otherwise, I tend to add however many layers of appropriately-coloured insulating tape are needed to bring the brightness to a sensible level (sometimes you can do this between the LED and the enclosure, which is neat and saves faff).


Related:  I bought some 5mm blue LEDs from eBay the other week, because I wanted to use one as a fan failure indicator on a computer thing.  They were insanely efficient, I think I ended up with a 1.5k series resistor (at 5V) to make it a reasonable brightness, and remember this is for something that's supposed to be attention-getting.


ETA: There's a special place in hell for those people who design a front panel with back-lit icons to denote whatever, then illuminate it with a wavelength the human eye is incapable of focusing on.
Careful, Kim. Your sarcasm's showing...

Re: Can LEDs dim?
« Reply #18 on: March 04, 2020, 10:48:44 pm »
They'll last for ages if you substantially under-drive them.  But nobody usually wants to do that (especially not molishers of lighting or chinesium electro-tat).

Indicator blinkenlights *should* have an easy life (unless someone's skimped on the current limiting resistor), but that's probably cancelled out by using whatever LED was going cheap on the Shenzhen market that week.  In the case of a fridge, it's reasonable to expect it'll be still working (if not quite as bright) at the 10 year point when all the plastic parts start to fall off.

LED indicator lights have become rather bloody annoying.

https://hackaday.com/2020/02/20/we-ruined-status-leds-heres-why-that-needs-to-change/

I have a switch on my desk network lab, it has a blue status LED. Even with a 200gsm piece of card taped over the top, it's too fscking bright.

LEDs are an amazing invention, but we on 'arf abuse and misuse them...

J
FWIW I concur with much of that hackaday piece.
LEDs are up there with smart/mobile touch screens which you have to touch in a minimum of 12 places, before you get to the first number you want to enter on the keypad or return a call via 'Recents'.
Unnecessary shit for those of us who use mobiles for making calls.
Do we have a rant thread?

Re: Can LEDs dim?
« Reply #19 on: March 10, 2020, 09:04:40 am »
I'm sorry, I thought this was the rant thread. ;)

Re: Can LEDs dim?
« Reply #20 on: March 11, 2020, 08:26:34 pm »
Plusnet ADSL routers have two settings for the blue LED, which is nice.

Unfortunately, the choice is between Very Bright or Trinity Test At The Moment Of Ignition.
And Darkness and Decay and the Coronavirus held illimitable dominion over all.