# Electrical Analysis of LED behavior



## IMSabbel (Oct 1, 2012)

I have been playing around with some equipment in my lab, and thought I might share the results. 

My main interest was in looking just how low the lighting threshold of LED really is, and the behavior at very low currents and voltages.

For this, I used but a good sourcemeter to power the led, as well as a Hamamtsu GaAs PIN diode with a Keithley 6517b for light detection. This setup allows pretty precise measurements of the led current and voltage (up to 1A), and very precise determination of the optical output of the LED.

The way the setup is made does NOT allow for direct comparisons of efficiency, as it is not a real integration sphere (it is optimized for low light, where an integration sphere would show too much loss). THerefor, LEDs with different emittance angles will show different readings. So this is not replacement for the White Led Lumen Testing thread.

What can be done, on the other hand, is show relative efficiencies, dark currents, etc. Due to the limit of 1A of drive current, and the fact that these scans were done in logarithmic scale, LED heating does not play any significant role (outside the die. The heatsink can be considered room temperature for the whole measurement, but the Die is of course acting on much shorter timescales in its heating).

Most of the work I did up to now was making the labview and building the hardware setup, so I only had two loose LEDs sitting around to test it on. A SST-90 of unknown bin, and a good old CREE XR-E Q4 on a star.

So here are some result (click for big graph):




Voltage / Current graph. At high voltages, both leds show a very similar behaviour, only with the SST being shifted to lower forwads voltages. Vastly different behavior at low drive voltages (below 2.1V), though. At these voltgages, current through the SST-90 is over 5 orders of magnitude higher than throught the CREE leds, while also showing a vastly different leakage characteristic (The cree mimics a normal diode, while the SST seems to have _something_ else going on, with the "round hill" in the graph.





You can see a much steeper correlation between forward voltage and optical output on the SST compared to the cree. The XR-E starts to light up at lower voltages, but is quickly overtaken by SST with its nearly an order of magnitude bigger die. The SST then reaches 1A current at about 3.3V, while the XR-E does take nearly 4. Below 10fA, the readout current is noise to to practical limitations (coupling in the sourcemeter cables). We can already see that the efficiency of the SST has to be lower at low currents than the Cree, as at 2V the cree produces 10 times as much light at less than .1% of the current.




Ok, now to the meat, light output vs current through LED. This graph has been normalized to 350mA as 1 for the output, due to the non-comparability between different LEDs. Effectively, nearly 13 orders of magntiude of brightness could be captured. Here we can see something very very interesting: While the Cree produces light with currents as low as 10nA, the SST produces ZERO light up to about 0.3mA, and is only reliably generating light at about 0.6mA. And with zero I really mean it (output has to be below 1000 photons per second (the error level)).




Looking at the same data in linear scale is much more boring. The expected is visible: The old XR-E is pushed quite hard by the 1A, showing reduced gains with current, while the SST90 is still pretty linear in its current scaling




Efficiency of both leds, normalized to the peak. Dynamic range not quite as big as before, due to the more noise characteristics of the sourcemeter meassurements. You can see that there is a much wider band of efficiency at lower currents for the CREE, while the SST simply nosedives at lower currents. It drops to only 10% of its peak intensity at 5mA current!
If somebody is interested, they could also use the low current data to evaluate the moon mode efficiency for driver free lights (QTC, etc). 




Finally, the efficiency at linear scale. The gliches are due to syc problems during range switches, please ignore (I would redo it if it were to peer review, but I do not care here  ). Now you can clearly see the difference in peak efficiency. Cree with 1mm^2 die is best at about 15mA, while the 9mm^2 SST-90 die has its peak at about 250mA.

The 60% of peak range for the Cree is 0.4mA-425mA, while the SST90 has from 220mA- (not meassureable).

If anybody has interest in getting data for other LEDs I could take a look, now that the setup is made it would not be too much of a hassle to check out other LEDs.


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## mvyrmnd (Oct 1, 2012)

Very interesting stuff, thanks!


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## AnAppleSnail (Oct 1, 2012)

Wow! I'm able to get visible light from LEDs on 2 alkalines and a 10-kOhm resistor. But I don't have the lab tech to analyze the current and voltage.


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## Gunner12 (Oct 1, 2012)

Really cool data. I made a little 9v battery light, 2 5mm white LED in series hooked up to various resistors (100Ω to 10MΩ), and the lowest mode through the 10MΩ resistor is only visible at night. The LEDs are probably getting ~0.4 µA but I didn't have a setup to measure the light output.

What was the lowest current where you could tell that the LED was lit? Also, did you do calculations on current per die area? The SST-90 has a 9mm^2 die, so that might have affected the output at lower currents a bit. It'll mean the SST-90 is almost one magnitude closer to the XR-E per die area.


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## SemiMan (Oct 1, 2012)

Awesome data!

I think there would be a lot of interest in seeing how newer high efficiency parts behave at low currents, i.e. XPG2, XTEAWT, XML, and even RebelES. Not necessary at really low currents, but down to a 100uA or so. It would be useful to the CPF community to see where the optimum current point are for maximum efficiency and where to start switching over to PWM current control. I am looking at putting a spectrophotometer in the budget, but not sure I am going to be able to justify the cost of one that is accurate at low light levels as that is not the primary business requirement.

Semiman


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## IMSabbel (Oct 1, 2012)

Gunner12 said:


> Really cool data. I made a little 9v battery light, 2 5mm white LED in series hooked up to various resistors (100Ω to 10MΩ), and the lowest mode through the 10MΩ resistor is only visible at night. The LEDs are probably getting ~0.4 µA but I didn't have a setup to measure the light output.
> 
> What was the lowest current where you could tell that the LED was lit? Also, did you do calculations on current per die area? The SST-90 has a 9mm^2 die, so that might have affected the output at lower currents a bit. It'll mean the SST-90 is almost one magnitude closer to the XR-E per die area.



I could see the cree light up at about 500nA, but that was only while shading it with my hand and putting it directly in front of my eyes in a lit lab. With dark adapted eyes, I think I would have been able to see it quite a bit lower. Now the tiny dies of the 5mm leds could have less leakage current and show up earlier.

The behavior at high current of the SST-90 pretty much scales with the die size. You can see that in the Led Current / Voltage graph: At high voltages, the SST-90 curve looks just like the XR-E curve, just moved up by about an order of magnitude. At low currents, something in the SST is build differently. You can really have half a mA flowing through them without any current. Maybe there is some kind of protection diode in it which leaks, etc.


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## sidecross (Oct 1, 2012)

Thank you! :thumbsup:


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## IMSabbel (Oct 1, 2012)

Seeing that it has been a looong time since I bought any LEDs without a flashlight around them, I decided to get myself an assortment of star layers for testing.

I ordered anything interesting I found at led-tech.de_

Nichia NSPW-500DS
Nichia NSPW-510DS
YolDal YZ-WS5S20N
Cree MX3AWT-A1-2B-Q5
Cree XBD-AWT 00G50 
Cree XPEHEW-U1-0000-009E8 CRI > 90 
Cree XTEAWT-00-0000-000000H50 
Cree XPGWHT-L1-1T-R5
Cree XMLAWT-0000T6051
Luxeon LXML-PWC1-0100
Samsung Ultrabright SMD 1-Chip
0603 Ultrabright SMD (1.6x0.8mm) 
0.5 Watt Power LED 

I am especially interested in seeing a comparison between the 5mm LEDs and the power leds at low currents (moonshine mode, etc.), plus the different die sized with same package (cree XP-E,G,M-L).
While I am still limited to 1A, I did feel little reason to get any of the SSTs or arrays, as I would not be able to reach the interesting region for them anyway.

Sadly, they do not have a XP-G2 available. Would have been interested if there were differences.
I do not know when I will find time do do this stuff (and the order time), but now that everything is set up it is no more than an hour of work including evaluation for each LED.

If anybody is interested in something in particular they can PM me.


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## Tobias Bossert (Oct 2, 2012)

Good work!

The efficiency diagram seems to show lumens per ampere. Could you add a diagram for lumens per watt please?

The cree-tool allows to calculate those values for currents above 100mA only. The reason might be the spreed of behaviour at low currents: The manufacturer wouldn't be able to guarantee the behaviour at low currents precisely.

It would be of interest for many of us to know the behaviour at low currents of recent LED typs such as XB-D, XT-E, XP-G2 and XM-L. "low currents" means not as low as 100nA but in the region of 1mA to 100 mA. Below that level you just can see the emitting LED but no more any illuminated objects.


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## IMSabbel (Oct 2, 2012)

Tobias Bossert said:


> Good work!
> 
> The efficiency diagram seems to show lumens per ampere. Could you add a diagram for lumens per watt please?
> 
> ...



First , it shows Lumen/W(in arbitrary units) on the Y axis (LedU*LedI), and Led-Current on the x-axis. Sorry if it was not sufficiently shown in the labels.

Second, I think they do not show for low currenty simply because of the more complex behavior. 100mA seems to be well above the peak for all CREE leds, so they just have to fit a decay, and not the more complex rise and peak behavior. SST-90 is definitively different from all CREEs in that regard.

Thirt, my interest in ultra-low currents is totally academic. I know it is not really practical to drive a led with 1000 photons/s. But I do those measurements as logarithmic amp source scans, i.e. I take 500 points or so per order of magnitude current starting at 1nA, up to 1A. Has the advantage that I can get the 1mA-1A range, which is of practical interest, and the nA/uA ranges of academic interest in a single shot.


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## Tobias Bossert (Oct 2, 2012)

IMSabbel said:


> First , it shows Lumen/W(in arbitrary units) on the Y axis (LedU*LedI), and Led-Current on the x-axis. Sorry if it was not sufficiently shown in the labels.



Cree product characterisation tool derives for XR-E {CW} Q4 at Tj 25°:

220 lm @ 1A and 42,3 lm @ 0,1A. 
220 lm/A @ 1A and 423 lm/A at 0,1A. 
Ratio of lm/A values is 1 : 1,92

59,5Lm/W @ 1A and 147Lm/W @ 0,1A. 
Ratio of lm/W values is 1 : 2,47

Your diagram shows 44% @ 1A and 89% @ 0,1A. 
Ratio in your diagram is 1 : 2,02

There is no significant change of those ratios with temperature.

If your diagram really shows measured (relative) lm/W values, than your measurements doesn't fit to the values derived by the tool.
Please check.

I did relative measurements with an XM-L T6 in the range 5 mA to 1000mA and got verry good match with the values derived from Cree tool in the range 150 mA to 1000 mA (accessible range for XM-L in the tool).


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## IMSabbel (Oct 2, 2012)

Tobias Bossert said:


> Cree product characterisation tool derives for XR-E {CW} Q4 at Tj 25°:
> 
> 220 lm @ 1A and 42,3 lm @ 0,1A.
> 220 lm/A @ 1A and 423 lm/A at 0,1A.
> ...



I scavened that LED from an old dealextreme flashlight I bought a couple years ago, so go knows what is going on with that LED. Those two LEDs were proof of principle cause they were the only ones at hand. I should get my assortment of fresh, unused LEDs like I mentioned above soon, which I will look at in a more reproduceable way.


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## bshanahan14rulz (Oct 3, 2012)

Hobbyists generally look for a sudden change in the slope of the curve when they are looking at V/A graphs for laser diodes, and they call it the "knee." It is generally accepted that running a diode at currents around there will kill it in very short order. I wonder if this is specific to the geometry of the laser diode in comparison to the LED, or if the LEDs exhibit the same thing.

Either way, I can see that this thread is a keeper  Thanks for putting forth the effort just for us!


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## IMSabbel (Oct 3, 2012)

Ok, found the reason those ratios were off.

I made the most stupid beginner mistake imaginable: I have been working with femtoamperes so long that I forgot to take wire resitance into account. Those tripple shielded cables actually DO have a voltage drop at >100mA. 

Changed the setup to have 4-wire sensing, and found that the forward voltage dropped almost 250mV at 1A.

As I managed to fry my diode I am now waiting for some new ones to arrive. Ordered one with optometrical efficency (370-700nm, peak Efficiency at 550nm).


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## IMSabbel (Oct 5, 2012)

OK, got new PIN diode and a whole batch of leds. Those guys at led-tech are really funny, they have a printed label with my name on it on every pouch with a single LED in it 

So far, tested an (old) SSC P7, a cree MX-3, a YolDal 5mm led in warm white (damn nice color for a 5mm LED). Currently setting up an XP-D R4 (thats a pretty tiny LED, and damn bright, too).

Had a problem with my new diode - it is larger than the last one, so I hit 19.5mA at 1A with the P7. Limit of the Keithley is 20mA, so I added an aperture, causing a reduction in dynamic range (but still 11 orders of magnitude).


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## Steve K (Oct 5, 2012)

bshanahan14rulz said:


> Hobbyists generally look for a sudden change in the slope of the curve when they are looking at V/A graphs for laser diodes, and they call it the "knee." It is generally accepted that running a diode at currents around there will kill it in very short order. I wonder if this is specific to the geometry of the laser diode in comparison to the LED, or if the LEDs exhibit the same thing.



not intending to hijack the thread... but this is intriguing! My limited experience with laser diodes was that at low currents, the device wasn't able to lase, and would essentially just act as a LED. Can you explain what the mechanism is that would cause a laser diode to fail when operating at the knee of the curve?


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## IMSabbel (Oct 20, 2012)

Quick update: Lots of stuff prevented me from finishing up. I did take data from both 5 or 6 leds already, but still have to prepare the graphs and stuff.

Just as a quick gauge for low brightness: A small green tritium vial (in front of a white paper to account a bit for the isotropic character) did read as about 150pA in the photodiode. This means I can meassure down to less than 1/10000ths of the brightness of a tritium vial.

To get to this output, you would need to drive an XR-E with about 2uA. This gives you 3 weeks or lithium vial brighness for each mAh of a battery. 

If you take a lithium coin cell like this one http://www.microbattery.com/pdf/panasonic-specf/panasonic-lithium-br2020-br2032.pdf with 190mAh capacity, you get 10 years of operation - compareable to the tritium half-life, with better brightness retention.

A LED with smaller dies would be even better - as a comparision, the P7 I tested would have run out of juice at that brightness level from the same battery after only 3 days (due to capacity sag under the load), compared to 10 years for the XR-E.


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