Are Joule thieves hard on LEDs?

[carbonzit]

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.

 

[crutschow]

As long as the average current is below the rated value you are probably ok. But to reduce the LED peak current you could place a small cap across the LED. That will absorb some of the spike energy and smooth the current flow into the LED. Try a value somewhere between 0.01µF and 0.1µF to start.

 

[MrAl]

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.

 

[carbonzit]

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.

I take that as high praise, coming from you. Thanks.

So without (hopefully) overthinking this, when we talk about LED current, and especially LED overcurrent, what are we really talking about? What I mean by this is that there's the instantaneous current, as shown in my simulation above, which is clearly well above the rated current for these devices. But since it's a pulsed current, certainly there's a certain amount of de-rating that should be done.

So I'm wondering what the best way is to think about this current. One thing that occurs to me, since I have at least a passing familiarity with calculus, is to take the integral of this current (that is, the volume of those spikes above). Is this a valid measure to work with? or even a useful one?

And we haven't even really touched on the nature of those spikes, with their sharp rise times, and what potential effects that might have. Even if that very tip of the spike only lasts for nanoseconds, it's still up there in the stratosphere so far as instantaneous current value goes (some of them go over 400 mA).

Thanks again for your informative reply.

 

[ARandomOWl]

If I am not mistaken, finding the definite integral of the curve between 2 points (which is the area and not the volume) will give you the charge transferred in the given time.

Sorry, I can't help with anything else.

 

[carbonzit]

Area. Area. Area. Where did I come up with volume?

Time to re-read that calculus textbook ...

 

[crutschow]

So I'm wondering what the best way is to think about this current. One thing that occurs to me, since I have at least a passing familiarity with calculus, is to take the integral of this current (that is, the volume of those spikes above). Is this a valid measure to work with? or even a useful one?

And we haven't even really touched on the nature of those spikes, with their sharp rise times, and what potential effects that might have. Even if that very tip of the spike only lasts for nanoseconds, it's still up there in the stratosphere so far as instantaneous current value goes (some of them go over 400 mA).

You certainly can take the integral of the current over time to get the average current. However the light output may not correspond to this average calculated current due to the non-linear characteristics of the LED as mentioned by MrAl.

It's true the spikes have very high current, but they are so short that likely the thermal time constant of the LED chip keeps it from heating appreciably due to the spike. That's why the average current is probably adequate to determine if the LED is being overdriven (since it's primarily a thermal concern).

Note: Putting a cap across the diode with this circuit does not stress the transistor since the peak current is being delivered by the transformer inductance when the transistor is off.

 

[ARandomOWl]

You can find volumes of revolution using integrals, but that is beyond this topic.

 

[crutschow]

You can integrate the current waveform in LTspice to get the average current (see "Waveform Arithmetic" in help file).

Edit: This integration also gives the RMS Current, but the Average Current is more appropriate for determining LED power dissipation since the LED voltage drop is non-linear and does not change much with current. RMS current would be used with a linear resistance to determine power dissipation.

 

[mynameisdan]

With the average current idea, does that mean I could put 1000mA through an LED if I pulse for 10uS and wait for 990uS? 0.01% duty cycle?

 

[crutschow]

With the average current idea, does that mean I could put 1000mA through an LED if I pulse for 10uS and wait for 990uS? 0.01% duty cycle?

That's the general idea. But at that current the voltage will rise significantly due to the series ohmic resistance of the LED and thus the average power dissipation will be higher than you would get with 10ma steady-state. And the average light output will likely also be noticeably less.

 

[carbonzit]

You can integrate the current waveform in LTspice to get the average current (see "Waveform Arithmetic" in help file).

Yes, pretty simple: zoom to waveform of interest, then <Ctrl>-click on plot label. Thanks.

In my case, it shows an average current of about 40 mA (RMS = ~90). How does that strike you for a (nominally) 20 mA device, considering the pulsed nature of the drive?

 

[crutschow]

Yes, pretty simple: zoom to waveform of interest, then <Ctrl>-click on plot label. Thanks.

In my case, it shows an average current of about 40 mA (RMS = ~90). How does that strike you for a (nominally) 20 mA device, considering the pulsed nature of the drive?

It means that the average power dissipated is at least twice the rating. Not good for the life of the diode.

I noticed that adding a cap in parallel reduces the average current.

 

[carbonzit]

Well, remember that my question wasn't "how can I reduce the current to my LEDs": it was whether circuits such as these are unduly hard on the little buggers. And I don't think that question has really been answered yet. If the answer is "yes", then I might consider some current-reducing techniques.

But since there probably tens of thousands of these things being used out there, at least to judge by the number of web pages devoted to the subject, and since I haven't seen early LED mortality mentioned out there at all, I'm not going to lose any sleep over it.

Of course, maybe in a couple-three years everybody's favorite Joule thief light will suddenly go Poof! ...

 

[crutschow]

And, of course, we've been looking at a simulation. The real circuit (especially since the inductors are often home-made and of unknown parameters) could (and likely does) have significantly different current than the simulation.

 

[audioguru]

The datasheeets for all name-brand LEDs specify the max allowed pulse current and the duration of the pulses.
But if you bu cheap no-name-brand LEDs from E-Bay then nobody knows if they can survive high current pulses.

 

[carbonzit]

Well, I buy my LEDs from Digi-Key, so no problem there. What, did you ASS-U-ME I got mine from eBay?

 

[ericgibbs]

The datasheeets for all name-brand LEDs specify the max allowed pulse current and the duration of the pulses.
But if you bu cheap no-name-brand LEDs from E-Bay then nobody knows if they can survive high current pulses.

As 'audioguru' as pointed all the information you are asking for is stated in the datasheet for the LED type that you are using.
If you carefully read what he posted "BUT IF you buy from eBay", its not possible to be sure of the LEDs specification.

Get and read the datasheet for the LED type you are using.

 

[carbonzit]

As 'audioguru' as pointed all the information you are asking for is stated in the datasheet for the LED type that you are using.

No, it's not.

For instance, the datasheet for this white LED (Cree, sold by Digi-Key), says nothing at all about pulse parameters.

On the other hand, the datasheet for [url="https://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=516-1377-ND]this red LED[/url] does state a value for the "peak pulsed forward current" (100 mA) in the absolute maximum ratings section, but it says nothing at all about pulse duration.

So no, not very helpful. I'm sure there's more detailed literature out there for some LEDs, but it doesn't seem to be available for all types.

 

[ericgibbs]

If you READ the datasheet, it refers too Brief I-024 for pulse conditions for the RED led.

EDIT:
For the WHITE led, looking at the manufacturers data sheet, it gives more information.
You just have to make the effort to look for it.