Exceeding LED ratings

Pimpom Inscribed thus:

Current de-rated by a third, times 2 ! (2Hz = twice a second)

Infinite lifetime expected !

Very good. :-)

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Best Regards:
                          Baron.

Maybe my description of the arrangement was misleading. The 9x9 array is just the physical layout. The circuit could be all series or series-parallel, but for the purpose of my question, the full 60mA is to be pulsed through each LED.

That's a peak of 3 times the assumed max rating, 1.5 times average. Certainly not good for long-term use especially since the pulse frequency is low. OTOH, each pulse train lasts for only

4 secs with a few minutes of idle time in between each train. And the whole thing has to last for only two days.

Small LEDs have been pushed very hard for medical use, and although failures happened, most survived.

However, using LEDs with decent output within spec would be the more usual option.

NT

Yes, of course. For brevity, I didn't mention my reason for wanting to push the LEDs in my opening post. The requirement came at a time when I'd badly depleted my stock of red LEDs. I live in a remote place where there's no way to replenish my stock or get higher-powered types on short notice. I ran a test with the LEDs operating at 30mA peak but they weren't quite bright enough. I was wondering if I could get away with running them at twice that level. A couple blowing during the two-day period is manageable as I'll be in a position to do field replacements.

LED's are binned into intensity groups to make such use even remotely possible. Not having a tight enough bin causes all sorts of problems.

--
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I'm afraid I don't get your point. What does intensity binning have to do with surviving a higher-than-normal drive current?

If I were to use a group of LEDs as described below -

LED type = ordinary 5mm red diffused TH. No data but assumed to be ~20 mA. Several LEDs from the batch used for a few years as indicators at a few mAs. None has failed. Mounting: 81 LEDs evenly distributed (9x9) on 4.5"x4.5" PCB Environment: Outdoors, LEDs unenclosed, direct free air, shaded from direct sunlight, Ta

I reckon you should be fine, but I'd set them up in such a way that one dead LED doesnt extinguish another, which is very easy.

NT

My guess, and that's all it is, is that your chances of success are fairly high. You're likely only 2x over the continuous rating and your duty cycle is pretty low.

I posted a reply, but it has not shown up. Please excuse me if I end up posting the same ideas twice.

This is not an informed opinion. But will post it anyway. The problem is heating so what ever you use to limit current should not be on the same pcb as the LED's. If you are making the pcb than make the traces as wide as possible so they will act as heat sinks. Also use heavy copper if you can. If possible do not cut the leads of the leds. Let them stick out in the air on the other side of the pcb. Put extra holes in the pcb between the leds and solder bare wires in tbem to create more heat sink.

I assume you do not have enough leds to double the number of leds and reduce the current.

My opinion is that all the leds will survive, but let us know what really happens.

=20 Dan

Pimpom Inscribed thus:

That will teach me not to make assumptions. :-)

Based on the "60ma" through each led, I would reduce the cycle time to

1/3rd of a second. ie 1/3rd on, 2/3rd off. If you do put them in parallel, there is a risk that one or more will current hog and deteriorate rapidly with the others having to adsorb a greater amount of current.

I've seen led displays run at 50hz with a 3:1 current ratio. But they were effectively single devices.

--
Best Regards:
                          Baron.

I agree with the general consensus that your are safe.

What is the price of failure?

Cheers, John S

Perhaps 1000 deaths on an Indian railway

NT

In that case, don't chance it.

John S

It would be more than a little stupid to string them in series for more than a few reasons. Might even be more stupid to have even suggested it possible, but then again... we are talking about h*mo sapiens.

The Indians made railways? :-) Does a little guy sit on top of the 'engine' with a rug, making smoke signals?

Well, not when it comes to YOU! You have not demonstrated that you are sapient.

OTOH, AlwaysWrong, by putting them in series, the current is the same in each one. Since you are AlwaysWrong ( jack snipped-for-privacy@cox.net ), you lose (as you are so fond of saying).

You are stupid beyond belief! Meaning, you are not sapient.

Idiot. It also means that a FAIL open is a catastrophic failure, not that you, being the result of one, would know that term.

You obviously mis-read what I wrote.(haha) You are busted being stupid yet again.

You are stupid, and considering your retarded bloodline, it is quite believable!