sst-90 3d dd turning blue @ 2.5 amps

[holdthat1]

Im using 3 tenergy d cells. Each battery is @ 1.35 volts. Within 20-30 seconds the light turns to a blue tint. Now I have another set of tenergy d cells charged at 1.25 volts. Current draw is 1.75 but no blue tint. It was my understanding that the sst-90 would turn blue if driven over 10 amps. So why is my sst-90 turning blue at 2.5?

sst-90-w57s-f11-gn201







Thoughts?

 

[LEDAdd1ct]

How is/what is your method of removing heat?

 

[irv_usc]

My guess is that you don't have a good thermal path to your heatsink. Even with the 3 cells the die is probably overheating pretty quickly.

 

[fyrstormer]

I don't think it's the amps that's doing it, I think it's the volts. Changing the supply voltage going into any LED will cause a tint-shift, up towards blue and down towards red.

1.25V x 3 = 3.75V, right on target.

1.35V x 3 = 4.05V, quite a bit higher than the emitter's Vf.

It's the same reason manufacturers of inexpensive, non-fully-regulated lights say not to use rechargeable lithium batteries -- even if the cell can't output more amperage than the emitter can handle, the voltage will still exceed the emitter's design spec. For small cells you can get away with it anyway, because the drain will cause the voltage to sag close to the emitter's design spec, but with D-size batteries, the voltage is going to be pretty stable even with a drain of a couple amps.

One last thought: is the limit on the SST-90 10 amps or 10 watts? I ask because 4.05V x 2.5A = 10.125W. And in any event, even if the limit is 10 amps, the waste heat from running that emitter at 10 watts is going to be significant. So even if everything else I said is dead wrong, I'm going to agree with everyone else and say it could be the heatsinking is insufficient.

 

[Der Wichtel]

I had bad experience with some old arctic silver thermal adhesive.
The emitter popped off after some time. Luckily I saw that right away and switched the light off. Bought some new arctic silver and everything was fine.

After that I compared the old arctic silver thermal adhesive to the new one.
The old adhesive was softer and took more time to cure whereas the new adhesive was very solid and took just a few minutes to cure.

I didn't store the adhesive in the fridge. Maybe that was my problem.

 

[PCC]

My guess is that you don't have a good thermal path to your heatsink. Even with the 3 cells the die is probably overheating pretty quickly.

My thoughts exactly. I was getting blue from 4 D cells but could run all day, within thermal limits, with 3 on a similar build.

 

[Lumencraft (Matt)]

Im using 3 tenergy d cells. Each battery is @ 1.35 volts. Within 20-30 seconds the light turns to a blue tint. Now I have another set of tenergy d cells charged at 1.25 volts. Current draw is 1.75 but no blue tint. It was my understanding that the sst-90 would turn blue if driven over 10 amps. So why is my sst-90 turning blue at 2.5?

sst-90-w57s-f11-gn201

Thoughts?

Hi, I would stop using it right away. I am about 99% sure this is due to a bad thermal path. The most common reason for this in my XP is the epoxy layer is too thick.

Arctic Silver epoxy is IMO the best thing to use by a land slide. I have lost emitters using Arctic Alumina or combinations of other things in high power mods.

What heat sink are you using? What epoxy did you use?

 

[ma_sha1]

Totally agree with theromo adhesive being the problem.
Properly bonded SST-90 could withstand well over 15 AMPs,
I went as high as 17 AMPs for 15 seconds W/O seeing blue.

 

[holdthat1]

I'm using a heat sink from britelumens. I used artic silver thermal adhesive to attach the led. So, I'm assuming if there's a bad thermal path that im screwed or is it possible to remove the led from the heat sink without damaging it? So the voltage of the batteries is not the cause of the blue tint?

 

[Lumencraft (Matt)]

I'm using a heat sink from britelumens. I used artic silver thermal adhesive to attach the led. So, I'm assuming if there's a bad thermal path that im screwed or is it possible to remove the led from the heat sink without damaging it?

Yes, I would say the LED probly did not sit down on the pad correctly I have had this happen before to and had to redo it.

First in order to remove the led from the sink without damaging it you will need to take it out of the light and set it on somthing metal ( I set it on my vice, but not clamped in). Next use a propane torch or similar to heat up the sink. DONT get crazy with the heat. You will need to get it fairly hot though, about as hot as it takes to melt solder. Proby 4 seconds on 2 sides of the sink for a total of 8 seconds MAX. This will break down the AA.

Next while the sink is still fairly hot, gently grip it with a pair of pliers and use a knife or similar to lift the led out of place, then push the wires etc up through the holes.

I have done this several times and it does work if you are careful. If you got the sink hot enough the led will come loose fairly easily.

Next of course be sure to clean all the old epoxy off the sink. If you have to you can use paint stripper, this will NOT damage the anodizing. I just rub the leds on a flat file to clean them, and then clean the sink and the led base with rubbing alcohol.

After you have soldered the new wires onto the led its a good idea to do a dry fit. I put the LED down into the sink and press on each side of the emitter (not where the wires are but on the other 2 sides) with my knife.

If you get the led to sit down perfectly most of you are most of the way to a good thermal path. When you mix the Arctic silver epoxy be sure to spread only enough on the sink to make a very thin layer that fully covers the base. No need to put any on the led too, just one or the other. I also fill the wire gaps slightly with the AAE so this will give additional heat dispensation (no I personally have not had it short any thing out).

If you get the thermal path repaired these emitters are capable of being driven well beyond 9 amp with no color shift.

Good luck.

 

[Fulgeo]

I'm using a heat sink from britelumens. I used artic silver thermal adhesive to attach the led. So, I'm assuming if there's a bad thermal path that im screwed or is it possible to remove the led from the heat sink without damaging it?

You really shouldn't use Artic Silver thermal adhesive to attach the emitter to an aluminum heat sink. Here is a quote from the manufacture's page "NOTE: Even though Arctic Silver Thermal Adhesive is specifically engineered for high electrical resistance, it should be kept away from electrical traces, pins, and leads. The cured adhesive is slightly capacitive and could potentially cause problems if it bridged two close-proximity electrical paths." This is probably not the problem because you have an anodized heat sink which affords you a bit more electrical isolation. In the future use Artic Alumina thermal adhesive from the same manufacture. That being said I would think you have either applied too thick a layer of adhesive as another poster mentioned or you are not making proper contact to the heat sink. Does the emitter look level on the heat sink? Is it loose? Can you move it with your finger?

 

[holdthat1]

I don't have a torch but I do have a heat gun. would that get the job done? the led does look leveled and I can not move the led with my finger.

 

[Der Wichtel]

capacitive doesn't mean conductive. There shouldn't be any problems for LEDs. Arctic Silver Thermal Adhesive shouldn't be used on high impedance traces like on led drivers.

 

[Canuke]

capacitive doesn't mean conductive.

Correct. Capacitive coupling presents as an open to DC circuits. It's when you are dealing with high frequency AC, including sinewaves and digital signals, where the capacitive coupling presents a problem, as capacitors tend towards acting like a short with higher frequencies.

 

[holdthat1]

you guys were correct. I barely stuck a flat head screw driver under the led then pop! It came off the heatsink with ease. Now besides painter thinner, what else could I use to remove the epoxy? Goo gone alcohol ect?

 

[Fulgeo]

you guys were correct. I barely stuck a flat head screw driver under the led then pop! It came off the heatsink with ease. Now besides painter thinner, what else could I use to remove the epoxy? Goo gone alcohol ect?

Very carefully scrap the epoxy off with a sharp edge. If this does not work you might also be able to lightly sand it.

 

[cmacclel]

I have lost emitters using Arctic Alumina or combinations of other things in high power mods.

Every light I build and have built has used Artic Alumina. I have yet to encounter any problems. Are you sure your problem with the Artic Alumina?

Mac

 

[cmacclel]

You really shouldn't use Artic Silver thermal adhesive to attach the emitter to an aluminum heat sink. Here is a quote from the manufacture's page "NOTE: Even though Arctic Silver Thermal Adhesive is specifically engineered for high electrical resistance, it should be kept away from electrical traces, pins, and leads. The cured adhesive is slightly capacitive and could potentially cause problems if it bridged two close-proximity electrical paths." This is probably not the problem because you have an anodized heat sink which affords you a bit more electrical isolation. In the future use Artic Alumina thermal adhesive from the same manufacture. That being said I would think you have either applied too thick a layer of adhesive as another poster mentioned or you are not making proper contact to the heat sink. Does the emitter look level on the heat sink? Is it loose? Can you move it with your finger?

Brightlumens heatsink does not contact the LED positve / negative connection points on the bottom of the emmitter.

Mac

 

[cmacclel]

you guys were correct. I barely stuck a flat head screw driver under the led then pop! It came off the heatsink with ease. Now besides painter thinner, what else could I use to remove the epoxy? Goo gone alcohol ect?

You problem may be your preperation. Did you wipe the bottom of the emitter and heatsink down with alcohol before epoxing? I remove dried epoxy with a razor blade. You can also very carefully put a piece of sand paper down on a flat surface and clean the bottom of the led that way. I use 600 grit.

Mac

 

[Fulgeo]

Brightlumens heat sink does not contact the LED positive / negative connection points on the bottom of the emitter.

Mac

I have purchased 5 of Brightlumens heat sinks. 3 for the SST-50 and 2 for the SST-90. I agree with your statement. I have never had any problem with them. They work well. I think when most people glue their emitters down they will "squish" out a bit of the epoxy and it stands a good chance of flowing across the connection points on the bottom of the emitter even if you do not see it. I have never had to pull one back off but I am fairly confident it does happen. Out of curiosity just now I pulled out a spare SST-90 I had laying around and put the VOHM to it. Just discovered that the center pad of the SST-90 is neutral. Neither positive or negative. I thought that all/most emitter's center pad was a connection point typically positive like the SSC P7? I also have always used Arctic Alumina. I know that it is not as good as Arctic Silver thermal adhesive in heat conductivity. I just never trusted it over the Arctic Alumina because of the formally posted quote from the manufacturer.