P7 Heatsinking.

[davidt]

Ok I finished my 3c p7 CSXPI and powering it with nimhs. I'm also using the heatsink from fenix-store. The batteries aren't even fully charged and the current is 4 amps at the batteries! After running for 10 seconds the color has shifted to a noticeably blue tint and dimmer. The dome of the led is too hot to touch at 10 seconds.

I read somewhere when using arctic alumina the object being epoxied must have pressure applied to it for an hour before pressure is removed. I guess my question is do you have to apply pressure the entire time on the led when waiting for epoxy to cure? If so how do you guys do it.

If the problem isn't epoxing the led to the heatsink than what could it be?

So far the led isn't damaged yet, everytime I turn the light off and let the led cool, when turned back on brightness is normal again.

I don't want my entire project to be a failure. Please help.

 

[Greg G]

You're the second person lately to report over-driving a P7 off 3 "C" NiMH's.

First off, 4 amps is way too much current. Don't run it anymore until you figure out a driver for it.

Thinking out loud here, but I wonder if the C NiMH batts push more current than their D cell big brothers? Because my 3 D NiMH M@G in DD is doing great.

Hopefully others will chime in with an answer.

 

[davidt]

Elektrolumens built his p7's with direct drive using 3 c nimh. His seems to run fine.

Do leds draw more current as they heat up? Because when I fire up my mag the current is measured to be 3.2 amps, as it heats up the current jumps to 4 amps at about 10 seconds when the led dome is extremely hot.

 

[TexLite]

What compound are you using between the heatsink and the body?

If moderate pressure was applied to limit the thickness of the AS,and heatsink compound is in use between the body and heatsink,then I'm at a loss.

3.2a should be no problem whatsoever,I've ran these at 3.6a with no tint shift or other problems.

Do you know what the Vf is when running?

I would use one of the linear regulators available,they are reasonable and should keep the current within specs.It would also add multiple levels,which would make the light much more user friendly.

Michael

 

[LumenHound]

What charger are you using and what voltage have you measured at the leds when you fire it up? What bin are your P7's? How rested are your cells before powering up the P7's?

 

[davidt]

What compound are you using between the heatsink and the body?

If moderate pressure was applied to limit the thickness of the AS,and heatsink compound is in use between the body and heatsink,then I'm at a loss.

3.2a should be no problem whatsoever,I've ran these at 3.6a with no tint shift or other problems.

Do you know what the Vf is when running?

I would use one of the linear regulators available,they are reasonable and should keep the current within specs.It would also add multiple levels,which would make the light much more user friendly.

Michael

I used arctic alumina between the led and heatsink. I never used arctic alumnia before so this is how I did it. I applied enough to cover the heat sink pedestal completly while trying to keep the layer as thin as possible. Next I applied some to the base of the led completly covering the base while trying to keep the layer as thin as possible. Next i plopped the led onto the epoxy covered heatsink then i pressed down on the led slightly to let the epoxy between the heatsink and led base stick.

I've been trying to drain my cells so that it won't drive the led so hard anymore. I'm guess vf is the voltage drop across the led. I can't let my led run long enough to get a stable reading because if i let the voltage reading stablize than the led would over heat. But I stuck the meter probes to the led legs and held them on the legs with my fingers. With vbatt at 3.9 the vf is initally at 3.45 volts than when the led is too hot the voltage is about 3.35 volts. I have about 10 seconds to measure before the led overheats.

Lumenhound, I am using a tenergy universal smart charger here is the link http://www.batteryjunction.com/unsmchfornib.html. I taped my cells together to from a 3.6 volt battery pack. When I ran my p7 the first time my cells wern't even entirely charged. The light on the charger was still red indicating charging wasn't complete. I think the voltage was about 4.2 for the entire pack. Now I drained the cells down to 3.9 volts and the led still over heats quickly. With vbatt at 3.9 vf is initally at 3.45 volts than when the led is too hot the voltage is about 3.35 volts.The amp reading starts at 3.2 and jumps up to 4.0 over a period over 10 seconds. I got my led from fenix store so I trust the bin is labeled correctly, but it was listed as CSXPI.

 

[Bimmerboy]

It's possible your P7 could have an unusually low Vf. The "I" rating isn't very high to begin with. The starting current of 3.2A sounds like it's pointing in that direction.

Also, C and D NiMH can dump a healthy amount of current over much of their runtime (all other things equal, D will put out more), so not having a full charge may not have mattered as much in this case.

10 seconds though? Sounds like the kind of thing that happens when something's wrong with the heatsinking. Did you try to squeeze out as much AA as possible while it was curing? How long did you hold pressure on the emitter while the AA was curing? Is it holding the emitter firmly?

Do leds draw more current as they heat up?

Yes. And without some form of current regulation, there is always the risk of "thermal runaway" if you're pushing things close to the edge. A LED's Vf drops as it begins to overheat, which causes it to flow more current, which makes it get hotter, dropping the Vf even further, and so on.

Edit: More discusson took place by the time I was able to finish this post. It's now sounding more like there was way too much AA used. Just a little dab on the emitter base is all that's needed, then you try to squeeze most of it out when curing it to the heatsink.

 

[James35]

If it's overheating at only 10 seconds, the problem isn't going to be the heat transfer between the heatsink and the host body. It's gotta be between the LED and the heatsink.

I've only been into modding flashlights for about 5 months, so take my words with a grain of salt. In the world of computers, we "lap" (fine sanding) the heatsinks for ultra smooth finish. And then we use a tiny bit of Artic Silver thermal grease (not epoxy) between the CPU and the heatsink. If too much Artic Silver is used, the CPU temperatures go up. We put it on so thin that we apply it with a razor blade. (The razor blade is angled not in the scraping direction, but as if you are brushing it on.) Essentially, the goal is to have as much metal to metal contact, with the Artic Silver only filling in the microscopic cavities. This makes for the most efficient heat transfer. On a side note, most people don't know that Artic Silver actually settles in and performs better after the first day. Day 1 temperatures are the highest, then it levels off at day 2. Not sure why.

So we know that metal to metal contact will be better than Artic Silver grease. We also know that Artic Silver grease performs better than Artic Silver epoxy. So I was quite surprised to hear about how much Artic Silver epoxy was being used by flashlight modders. Some apply the epoxy to the heatsink first and let it cure. This keeps the (positive?) emitter base from shorting out on the heatsink (negative). I was also surprised to see many lips and divits in some of the heatsinks. For example:

My P7 emitters haven't arrived yet (been nearly a month from Kaidomain), so I'm not sure what they look like on the bottom. But I don't really know why this heatsink isn't flat across. There are 2 depression sections that don't make any sense to me.

 

[Bimmerboy]

But I don't really know why this heatsink isn't flat across. There are 2 depression sections that don't make any sense to me.

I believe the larger depression is to help center the emitter, as it's base fits into it. The small one in the center makes no sense though, and definitely cannot be helping to maximize heat transfer.

 

[Bimmerboy]

Lumenhound, I am using a tenergy universal smart charger here is the link http://www.batteryjunction.com/unsmchfornib.html

Hmmm... is 4.2V pack voltage a bit on the high side considering the charger wasn't done yet? Might seem like a stupid question, but did you check the charger to make sure it's set to 3.6V, and not higher?

Edit: Never mind... it doesn't have such a setting. It autodetects pack size.

 

[davidt]

It's possible your P7 could have an unusually low Vf. The "I" rating isn't very high to begin with. The starting current of 3.2A sounds like it's pointing in that direction.

Also, C and D NiMH can dump a healthy amount of current over much of their runtime (all other things equal, D will put out more), so not having a full charge may not have mattered as much in this case.

10 seconds though? Sounds like the kind of thing that happens when something's wrong with the heatsinking. Did you try to squeeze out as much AA as possible while it was curing? How long did you hold pressure on the emitter while the AA was curing? Is it holding the emitter firmly?



Yes. And without some form of current regulation, there is always the risk of "thermal runaway" if you're pushing things close to the edge. A LED's Vf drops as it begins to overheat, which causes it to flow more current, which makes it get hotter, dropping the Vf even further, and so on.

Edit: More discusson took place by the time I was able to finish this post. It's now sounding more like there was way too much AA used. Just a little dab on the emitter base is all that's needed, then you try to squeeze most of it out when curing it to the heatsink.

I didn't know you were suppose to apply the led with enough pressure to force out almost all the AA. And leaving as little AA as possible.

The only pressure applied was me setting the led on the heatsink appling enough pressure for epoxy between the led and heatsink to stick, than I just set the entire thing aside to cure.

I was suspecting improperly mounting the led was the problem. I didn't think if mounting the heatsink to the body was done improperly would cause the led to heat up so quickly. I was thinking it had to be the led not being able to release heat to the heatsink quickly enough.

Dang, I was having doubts on how to mount an led with AA, but I was too impatient to to post the question on the cpf and wait for an answer.

Well here is my crummy attempt at mounting an led:

Too much AA?

 

[JamisonM]

My P7 emitters haven't arrived yet (been nearly a month from Kaidomain), so I'm not sure what they look like on the bottom. But I don't really know why this heatsink isn't flat across. There are 2 depression sections that don't make any sense to me.

I believe the smallest depression was where the heatsink was held while it was being machined. The second is where the base of the emitter sits and also centers the heatsink.

davidt, maybe it's possible you applied too much thermal epoxy? If not, did you apply thermal epoxy between the heatsink and the battery tube?

 

[davidt]

I believe the smallest depression was where the heatsink was held while it was being machined. The second is where the base of the emitter sits and also centers the heatsink.

davidt, maybe it's possible you applied too much thermal epoxy? If not, did you apply thermal epoxy between the heatsink and the battery tube?

Yeah I used AA between the sink and body, enough for the AA to overflow and get on the threads of the body when inserting the sink into the body.

After reading responses to my question I believe too much AA was used between the emitter and heatsink. I'll have to try and remove the emitter tomorrow and remount it with as little epoxy as possible.

Oh well the light is pretty kickass for 10 seconds I can use it before the led overheats.

 

[Norm]

David I'm using exactly the same set-up as you, looking at my heat sink and LED there is no visible AA, maybe the thinnest line on one side, the amount of AA I use would be about match head size or a little less. My light did draw about 3.5A when directly driven, I am now using this driver http://www.kaidomain.com/WEBUI/ProductDetail.aspx?TranID=5025
gives me 2.8A with batteries straight off the charger down to about 2.1A by the time the batteries are due to charge. Try to remove the LED straight away the more the epoxy sets the harder it gets.

​

Norm

PS Driven at 3.5A it got nice and warm but not too hot.

 

[Al Combs]

Ok I finished my 3c p7 CSXPI and powering it with nimhs. I'm also using the heatsink from fenix-store. The batteries aren't even fully charged and the current is 4 amps at the batteries! After running for 10 seconds the color has shifted to a noticeably blue tint and dimmer. The dome of the led is too hot to touch at 10 seconds.

I read somewhere when using arctic alumina the object being epoxied must have pressure applied to it for an hour before pressure is removed. I guess my question is do you have to apply pressure the entire time on the led when waiting for epoxy to cure? If so how do you guys do it.

If the problem isn't epoxing the led to the heatsink than what could it be?

So far the led isn't damaged yet, everytime I turn the light off and let the led cool, when turned back on brightness is normal again.

I don't want my entire project to be a failure. Please help.

I also bought my heat sink and CSXPI P7 from the FenixStore. Mine was for the D cell MagLite. I don't think the size of battery would make a difference as far as drawing too much power. D cell NiMH's can easily provide more current than C cells do.

I had a problem that from your picture looks like you might have as well. The heat sink was improperly machined. The dimple in the heat sink used for centering the LED, was slightly smaller than 8 mm. I don't remember the exact size, but it caused the LED to rock when placed on the heat sink. I was too impatient to wait and bring it in to work to cut on a lathe. I cut, or rather scraped the LED with a razor blade until it fit in the depression. I reasoned (besides my impatience) cutting the heatsink could possible cause problems with the LED's anode touching the ground. It took almost an hour and I had to stop several times because of the cramp in my hand. Like I said, I didn't want to wait to see 700 lumens. My dear wife would walk by and shake her head at the crazy man with his new toy.

There is a slight gap here were the Arctic Silver epoxy I used though touching the copper slug, does not gush out the sides of the plastic case. This gap appears to be much smaller than on your picture.

That aside, the 3.2 amps you report initially it still beyond the LED's absolute max rating. I realize several people have reported more than 4 amps initially that have no problems. Well besides a few hundred hours life instead of the rated 50,000.

I wired a 0.1 ohm 5 watt resistor in series with the LED. I Arctic Silver epoxied it onto the back of the heat sink. On 3 fresh off the charger D cell NiMH's that read 4.29 volts, I get 2.8 amps. Well give or take 50 ma. After an hour or so of use, it drops to 2.1 amps. I tried shorting across the resistor while looking at the wall. Honestly I couldn't tell the difference. I reasoned if I can't see it, why replace the LED after 100 hours for something that isn't there. When a good constant current regulator comes along, maybe I'll switch to that instead.

 

[davidt]

Sweet, I just removed the led from the heatsink . Took me 2 hours using a utility knife and a butane torch to heat the blade, I removed 1 week old AA. Although I did remove some important anodizing from the heatsink.

Here are the results:

Heatsink:

Led:

No damage to the dome, only the black part is chipped a little.

Because some insulating anodizing was removed, how do I attach the led to the heatsink? How do I keep the led anode from touching bare aluminum? If I just apply the correct amount of AA on the anode slug would that be enough insulation?

 

[Al Combs]

Sweet, I just removed the led from the heatsink . Took me 2 hours using a utility knife and a butane torch to heat the blade, I removed 1 week old AA. Although I did remove some important anodizing from the heatsink.

Here are the results:

No damage to the dome, only the black part is chipped a little.

Because some insulating anodizing was removed, how do I attach the led to the heatsink? How do I keep the led anode from touching bare aluminum? If I just apply the correct amount of AA on the anode slug would that be enough insulation?

If you made a scrape that dug below the anodizing, I think you could assume being a low point, there's no real danger of its touching. It would have to be a high point to do that. Of course there's always the chance the LED's copper slug having an irregular shape might have its high point rest in the valley of the gouge. A little sand paper action on the slug would go a long way to minimize that possible risk. It would also give the epoxy a better surface to adhere to.

As a test after you glue it back on, make an intentional scrape on say the back where it won't show. Try your ohm meter between that spot and the LED anode. If you still don't trust it, someone recently proposed using hair as a spacer on a bare heat sink.

BTW, since you have the glue off now, does yours rock when you put the LED on the heat sink and wiggle it? Mine did and I felt it worth the time to fix it.

 

[Norm]

I had no problem with LED fit.
David it looks like most of the damage to the anodising is around the edge of your heat sink, the only part that can make contact is the slug which sits into the depression, I'd say you will have no problem, but as Al said check with a meter to be sure.
Norm

 

[davidt]

At about ten o'clock position a very tiny flake of anodizing came off within the depression. I was worried this tiny little bare aluminum exposure would fry my led if not insulated.

Would the led fry if the base wasn't insulated?

The led seem to fit well with my heatsink, no rocking what so ever. I have not glued the led down yet, but I did hold the led to the heatsink with pressure and tried what al suggested with the meter and everything seems fine.

 

[Greg G]

The recess in the heatsink needs to be large enough for the metal slug on the bottom of the LED to fully seat all the way down into the bottom of it. It can't held up off that surface. If it is, the epoxy film will be too thick and the heat transfer to the heatsink will be insufficient to keep the LED cool.

If a modder has a pair of good dial calipers it can be checked easily.