"Burn-in" of SST-90 significantly lowers Vf

[easilyled]

I wonder why the "burn-in" isn't done by the manufacturer.
They could then claim lower Vf, and greater efficiency.

 

[Techjunkie]

The burn in is lowering the Vf, and so pulling more current.... and I realise that more current increases the output and also affects the tint slightly according to the specs. Is this burn in also affecting the tint and brightness, or is it just pulling more current to achieve the same output as it was new? - if it were restricted, to the same current as before, would it have dimmed down and colour shifted slightly after burn in?

Higher current draw and the brightness/tint changes that come with it are achieved at a lower Vin for a burned-in emitter. This can be an advantage when direct driving the emitter and battery sag and resistance were keeping the circuit from achieving optimal Vin prior to burn-in. If supply/resistance were optimized for maximum current prior to burn-in, disaster could occur in the form of thermal runaway after the emitter as burned-in and Vf is then lower than Vin.

A constant current driver would simply have to deliver less voltage to the emitter after it has burned-in than it did before it was burned-in, but there would be negligible change in brightness as current would remain the same.

A burned-in SST-90 being driven to 9A will yield the same lumen output immediately after it was burned-in as it did when it was driven to 9A before it was burned in. Over time, the "lumen maintenance" of an LED drops, meaning that at a given current, less lumens are output than at an earlier time in the LED's life.

Running a high-output LED at high currents reduces lumen maintenance faster than running it at low currents. It is arguable that aggressive burn-in accellerates the initial drop in lumen maintenance, compared to a prolonged burn-in.

I would only recommend intentionally burning-in an LED if it were to be used in a direct-drive setup, to achieve a desired battery/resistance configuration against a more predictable Vf. If a constant current driver were used, then burning-in would only afford the small advantage of less total Wattage consumed by the LED and is probably not worth the time invested to perform an intentional burn-in.

I have heard that the yellow phosphor that coats the LED to change it from blue to white is slightly consumed over use, so it is also arguable that at the same current, a burned-in LED may have a slightly cooler tint than prior to burn-in. By my own experience, I thought that the hotspot of my 4500K SST-90 was too yellow when I first began to experiment, but no longer find that to be the case. I attribute that change more to the higher current, different reflectors and my own perception than phospor consumption though.

 

[flashfiend]

So you are saying my 3-mode direct drive sst-90 does have a PWM driver in it and it is adding resistance to the circuit? I thought in high mode it would just be completely direct driven with insignificant amounts of resistance introduced into the circuit. Btw, I'm doing a burn-in right now and it 'feels' like it is drawing more current mostly indicated by heat and not output.

I would assume, as you have, that on the brightest mode, the PWM that controls the FET is at 100% duty cycle, so the circuit is constantly closed. The FET itself adds some resistance though, and as we've seen in the video, the slightest amount of resistance changes the numbers significantly in this high current, low voltage application. In other words, compared even to your high mode, direct drive without the driver would draw even more current.

I built a few keychain torches with MC-Es and observed the same difference between the direct drive ones without the PWM only driver and the ones with it (and an MCE only pulls 2-3A at the same Vf that the '90 pulls 5-9A).

Short of unassembling your torch and getting a clamp meter between the PWM driver and the heatsink, the closest to actual measurement that you can take (by introducing as little extra resistance into the circuit as possible) is to test as I have with a piece of Aluminum sheet metal or some very heavy gauge copper and a clamp meter. Whatever number you measure, you can safely assume that without the test equipment in your circuit, the emitter is drawing even more current than you measured.

If you find that you are seeing 6A or more to the emitter and you are still underwhelmed, then you should consider trying different reflectors or optics. That big giant emitter makes getting a tight focus a real challenge (especially with the dome in tact).

 

[Techjunkie]

I wonder why the "burn-in" isn't done by the manufacturer.
They could then claim lower Vf, and greater efficiency.

Over time, the manufacturer could improve the manufacturing process to reduce or eliminate the need for burn in. Burning in the LEDs would require heatsinking, time and other costs, which would raise the unit cost.

So you are saying my 3-mode direct drive sst-90 does have a PWM driver in it and it is adding resistance to the circuit? I thought in high mode it would just be completely direct driven with insignificant amounts of resistance introduced into the circuit. Btw, I'm doing a burn-in right now and it 'feels' like it is drawing more current mostly indicated by heat and not output.

No amount of resistance is insignificant in a low voltage high amperage circuit. Consider the three different readings I got in my second video. The reason each measurement differed greatly was becaue each different method of measuring introduced a different (slight) amount of resistance to the circuit.

 

[flashfiend]

So have I shot myself in the foot by getting the extra modes? Meaning, with the modes I won't get full output or even have a chance to get the 6 amps I'm looking for? Hopefully the DeOxit w/ LED burn-in can get me close. I'm bummed.

 

[Techjunkie]

So have I shot myself in the foot by getting the extra modes? Meaning, with the modes I won't get full output or even have a chance to get the 6 amps I'm looking for? Hopefully the DeOxit w/ LED burn-in can get me close. I'm bummed.

Not necessarily. If you eliminate as much other resistance as possible and the emitter's Vf drops after a burn-in period, you might even find that you're sending 7-9A to the emitter after it warms up, provided that your battery pack is up to the task.

 

[Lumencraft (Matt)]

Thanks for the info, and I love that you added vids:thumbsup:

 

[mvyrmnd]

Just a quick point about your tailcap measurements...

I used a second multimeter today to measure the resistance of the measurement circuit of my primary multimeter, to compare it to the tailcap resistance.

For example, the tailcap in my Solarforce L2 provides 5 milliohms resistance.

The measurement circuit of my multimeter provides .5 ohm resistance.

That's a full 100 times greater resistance than the tailcap!

Using a multimeter to measure tailcap current is a VERY rough guide at best!

 

[Techjunkie]

Just a quick point about your tailcap measurements...

I used a second multimeter today to measure the resistance of the measurement circuit of my primary multimeter, to compare it to the tailcap resistance.

For example, the tailcap in my Solarforce L2 provides 5 milliohms resistance.

The measurement circuit of my multimeter provides .5 ohm resistance.

That's a full 100 times greater resistance than the tailcap!

Using a multimeter to measure tailcap current is a VERY rough guide at best!

Exactly. Also, bear in mind that your tailcap has a switch in it. Mine is a deanodized Mag tail with a gold plated short spring, both treated with deoxit gold.

Even the aluminum sheet metal bridge that I used in the clamp meter tests has more resistance than my tailcap, which was obvious when it didn't turn the emitter blue but the tailcap did.

 

[zelda]

from my sst-50 with 3 SCHR cells observation:

When I measure at the "tailcap" with a quality 1mm2 multimeter-cable I got 3.2A.
The same way with a 4mm laboratory cable without test probes like this one: https://www.distrelec.com/ishopWebF...r/is/102651/and/id/is/01/and/series/is/1.html
I got 5.7A
Put enough force to the plugs with multiple cells when measuring!

zelda

 

[uluapoundr]

I just put together a DD SST-90 3D Mag. I initially tested the amps at the tailcap using a Sperry MM. Using 3 Tenergy NiMH, got only 2.0-2.4 amps. Slipped in 4 C-cell Tenergy NiMh and was getting 9.0 amps. Been doing 15-30 seconds burn-in sessions, using the 4 cells and running on the MM, then letting it cool down. Using the tailcap immediately turns the light blue. Prior to Deoxit the 3 cell was pulling 3.4 amps and the 4 cells 12.0 amps. With deoxit treatment, the 3 cell is now at 3.9 and 4 cell 12.8. Looks like the emitter is burning in. At 12.8 amps, this light is super bright, brightest single LED I've ever seen, makes my P7s look like stock mag bulbs. This bin is only WM bin, what an awesome emitter!

 

[Illum]

its natural for LEDs to lower their Vf when heated...but considering in how short time these LEDs lower its Vf draws more concern about the current regulation of the power supply overtime, as well as the longevity of these LEDs in a fixed lighting configuration

 

[mvyrmnd]

So just to clarify something... If I wanted to burn in an emitter, what would be the best procedure?

Would I hook a battery directly to the emitter and feed it 4.2V, and keep it heatsinked well enough that it doesn't burn up?

 

[uluapoundr]

Another update. Continued running the SST90 3D mag build using 4 C NiMh with measurements at the tailcap at about 10-12A using standard MM wires. Would run in 30 second sessions till the head of the mag heated up, then let cool. Couldn't use the tailcap as the currents were too high and light would turn blue in a few seconds. Used the MM to create some resistance for the cycles. Did a dozen or so cycles. Took measurements with the 3 D NiMh and was getting almost 5A at the tailcap. Switched out the MM wires for thicker wires and took measurements again, it's 5.8A. Light OTF looks about twice my P7 D bin, pretty awesome!