The red flash

[Jasen Betts]

What can cause a green led (just a cheap 5mmx1mm through-hole led) to
light-up red a few times and then fail?

it was antiparallel to a similar green led at the time and that LED
continued to function correctly

the LED supply was a 12V square wave (from a 555) at approx 5HZ
through a 2uF capacitor and 560 ohm resistor,

the power supply was a switched mode wall-wart 240VAC in isolated
12VDC out.

the LED connections were exposed, could static discharge or an
active-to-ground loop current (through the suppression caps in the PSU)
do that sort of damage?

 

[Jamie]

What can cause a green led (just a cheap 5mmx1mm through-hole led) to
light-up red a few times and then fail?

it was antiparallel to a similar green led at the time and that LED
continued to function correctly

the LED supply was a 12V square wave (from a 555) at approx 5HZ
through a 2uF capacitor and 560 ohm resistor,

the power supply was a switched mode wall-wart 240VAC in isolated
12VDC out.

the LED connections were exposed, could static discharge or an
active-to-ground loop current (through the suppression caps in the PSU)
do that sort of damage?

Not seeing the circuit I am picturing a possible charge pump effect
taking place? Or, it could of just been a bad LED.

Jamie

 

[Jasen Betts]

Not seeing the circuit I am picturing a possible charge pump effect
taking place? Or, it could of just been a bad LED.

_______ //
| | .-->|--.
| 555 3|---||--/\/\/--| |----.
|___1___| | \\ | |
| `--|<--' |
| |
`------------------------------'

It was a bad led, (perhaps mistreated) began lighting red and failed after
perhaps 10 minutes, of run-time. I thought it surprising to get red light
from a green LED.

 

[Randy Day]

What can cause a green led (just a cheap 5mmx1mm through-hole led) to
light-up red a few times and then fail?

it was antiparallel to a similar green led at the time and that LED
continued to function correctly

the LED supply was a 12V square wave (from a 555) at approx 5HZ
through a 2uF capacitor and 560 ohm resistor,

the power supply was a switched mode wall-wart 240VAC in isolated
12VDC out.

the LED connections were exposed, could static discharge or an
active-to-ground loop current (through the suppression caps in the PSU)
do that sort of damage?

Could it be a bidirectional red/green device
that they oscillate to display yellow occasionally?

 

[John Tserkezis]

What can cause a green led (just a cheap 5mmx1mm through-hole led) to
light-up red a few times and then fail?

I've seen LEDs glow (or should I say smoulder) a different colour when
you drive them WAY over spec before they get hot and burn out.

Could be a short somewhere, preventing the limiting resistor from doing
its job.

 

[asdf]

_______ //
| | .-->|--.
| 555 3|---||--/\/\/--| |----.
|___1___| | \\ | |
| `--|<--' |
| |
`------------------------------'

This acts like a charge pump voltage doubler. When pin 3 is low the cap
charges at vcc (minus led dropout), when it goes high, the cap is
connected in series to vcc with the right polarity, therefore one of
the leds gets vcc+vcc-led_dropout, roughly 22V.
That led likely failed because the limiting resistor value was too low;
you either raise its value or (better) change the circuit.
Try this one. When the 555 isn't inserted both leds should light.

 

[Teodor Väänänen]

I've seen LEDs glow (or should I say smoulder) a different colour when
you drive them WAY over spec before they get hot and burn out.

Could be a short somewhere, preventing the limiting resistor from doing
its job.

Speaking of LED failure modes... I remember this time back in school
when a classmate just had to figure out what would happen if you reverse
polarized a green LED at 50V.

Yes, I had the presence of mind to turn my face away from it, but that
piece of epoxy that shot away from the LED when it disintegrated stung
when it hit my hand.

So for future reference, if you are going to demonstrate (or experiment
with) failure modes of LEDs, take precautions. And I assume that the
magic smoke (if any) given off isn't too healthy either.

/Teo.

 

[ehsjr]

What can cause a green led (just a cheap 5mmx1mm through-hole led) to
light-up red a few times and then fail?

Does this phenomenon occur only on 4/1 ?

Ed

 

[Michael Black]

What can cause a green led (just a cheap 5mmx1mm through-hole led) to
light-up red a few times and then fail?

Did it ever light up green?

In the old days, it was much easier to get scrap LEDs than good ones.
Certainly it was cheaper. I always tested them with voltage, because I
could never trust that they'd work, or be the right color. It wasn't
uncommon to find that they'd barely light.

If you never lit this one up, then I'd say you either got a mixed up one,
or it was never supposed to be green. Of course, that's harder to do
these days when many LEDs are packaged in the same color as the actual
light emission.

Michael

 

[[email protected]]

Did it ever light up green?

In the old days, it was much easier to get scrap LEDs than good ones.
Certainly it was cheaper. I always tested them with voltage, because I
could never trust that they'd work, or be the right color. It wasn't
uncommon to find that they'd barely light.

If you never lit this one up, then I'd say you either got a mixed up one,
or it was never supposed to be green. Of course, that's harder to do
these days when many LEDs are packaged in the same color as the actual
light emission.

Most of the LEDs that I've been using recently (all SMTs) are clear, with the
polarity markings in green, no matter the wavelength.

 

[asdf]

This acts like a charge pump voltage doubler. When pin 3 is low the cap

Self correction: it doesn't. I didn't pay attention to the leds being
connected to ground, not vcc/2 or something else. Looks like part of my
brain was thinking about H bridges and the other about operationals
Anyway, while my proposed modification is correct, the reason why the led
failed remains unknown. Maybe a defective part or an error somewhere.

 

[Michael Black]

Speaking of LED failure modes... I remember this time back in school
when a classmate just had to figure out what would happen if you reverse
polarized a green LED at 50V.

Yes, I had the presence of mind to turn my face away from it, but that
piece of epoxy that shot away from the LED when it disintegrated stung
when it hit my hand.

So for future reference, if you are going to demonstrate (or experiment
with) failure modes of LEDs, take precautions. And I assume that the
magic smoke (if any) given off isn't too healthy either.

It's not the "explosive nature of LEDs" that counts. It seems to be the
epoxy. There was a time when power diodes weren't so small as the 1N4000
series, and I had a bunch of fairly big epoxy encased power diodes. I
remember putting some of those in backwards and they seemed pretty deadly.
In both cases, it was the fact that the actual component inside would
heat things up enough and fast enough that the epoxy package would break
up and be propelled out.

Though I'm not sure if that was as spectacular as the time I put a
tantalum capacitor in backwards.

Michael

 

[Jasen Betts]

Does this phenomenon occur only on 4/1 ?

I hadn't considered the date

It was several weeks previous, and then there was an earthquake that
destroyed and scattered the evidence.

 

[Jasen Betts]

Did it ever light up green?

yeah, it worked as intended on several occasions for about about 30 minutes
total, then it switched sides.

I keep a CR2032 cell on my bench for testing leds and always use it to
confirm the polarity and test the parts, the cell's 3V output and
internal resistance make it well suited for testing LEDS. A CR2016
would be better sized but the 2032 was free, it came from a scrapped PC

I think I got it from futurlec last year some time