Now that I've started playing around in this very popular arena, I'm starting to wonder if these amazing little circuits are stressing out the LEDs they're driving.
I ask because when I looked at the current waveform for the LED in my current JT circuit, this is what I saw:
**broken link removed**
(Circuit and screenshot attached below)
Since I don't own an oscilloscope, I have to rely on a simulator here, so I'm ASS-U-Ming that it shows what happens in real life, more or less.
Notice that the LED is being allowed to take in 60 mA (2nd tick from the bottom of the waveform) for ~500nS. The absolute peaks of the wave go up as high as 350-400 mA! Of course, we'd never want to drive a LED this hard continuously, and since the drive is pulsed, presumably the device can take it.
But for how long? Are we damaging these diodes, eroding their little junctions little by little by feeding them such high pulsed currents? or are they rugged enough to take the punishment?
If anyone knows of any literature out there that addresses this, I'm interested.
Hi again carbon,
You really do ask some good questions, and that's good because the more you wonder about the more you will learn about electronic circuits in general. This question is a very good one too and it's a good idea to think about stuff like this when working with circuits of almost any type.
The best answer to this question is multifaceted, but i'll try to give you some idea what you are dealing with here in short terms.
As i am sure you know, an LED has an average current rating given to it by the manufacturer and it's usually around 20ma for these small Nichia type LEDs. The manufacturer also gives it another rating, a max (or peak) current rating. For a Nichia type LED this max is 100ma. Can we go over that without damage? We probably can, but if you read my sig line you'll get the idea. Unless we want to do literally thousands of tests on hundreds of LEDs of the type we are interested in, it is best to take the manufacturers recommendations and limit the current to the specified value. They are banking on the rating they give, so they must have done some significant testing on the devices so that they are confident enough to quote that particular value, and if they could go higher then surely would because that would make the product more appealing. After much testing they decided that 100ma should be the max, so that's what we should use. It's really that simple unless you care to test and test and test yourself, or take chances on blowing your LEDs without warning.
There are other specs too, and another interesting one is the spec on luminosity vs drive current level. We combine this with the average current flowing through the LED to come up with a number that we can call relative efficiency for the LED. This is very interesting because the efficiency actually goes down with increased current, and because we are talking 100ma compared to 20ma, the efficiency goes down quite a bit at 100ma. A rough figure would be that if we were to drive an LED at 100ma (which is 5 times 20ma) it would only put out about seventy percent of the light as we would get from 5 separate LEDs with all of them driven at 20ma. We'd be using the same current, but we'd get more light out of the 5 LEDs than the single one. And if that isnt bad enough, the 100ma LED would have to be run at a higher voltage than the 5 separate ones driven at 20ma each, so we not only loose light output we also loose electrical energy and so the overall efficiency suffers quite a bit.
What this correlates to with the pulsed LED is that a pulsed LED at some average current higher than the average current spec puts out less light and consumes more energy than an equivalent number of LEDs driven at less current.
It's not that we never want to pulse an LED, but we only do so when we really have to. If you want to reduce the peak current in your circuit you can use a Schottky diode in series with the LED and a electrolytic across the LED itself. If you try to put a cap across the LED without a diode too you'll stress the transistor and that will decrease efficiency quite a bit too.
Also interesting, driving two LEDs at 10ma each puts out about 120 percent of the light as a single LED driven at 20ma.