Help: Using copper pipe cap as heat sink

BrianMc

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Since another has opened the door on design input, I also seek counsel.

(Mad! Completely nuts!, No NOT that sort of counsel, or a lot of us would be in trouble, right?)
:crackup:

You can skip to the bottom to the issue and refer to the following sections to understand why I am asking, or where I messed up. :eek:

Background on why I want to use copper end caps at all (FYI):
I would prefer a CNC aluminum head light unit for the minimal number of thermal interfaces and the 'look' but those are beyond my budget at the moment. I also wish to try a unique arrangement of XPG's to double up a usable light and an experimental platform for road beam testing. I want the finished lights to look like a commercial setup and be amenable to redesign later. Lastly, in case one or a few readers wished to follow the same path, I thought at least a limited supply of reasonably priced bits would be good to base these lights on.

So I decided to try two of EL34's stash of MR11 (not MR16 as listed) bullet Marwi metal halogen light heads, and an array of matching handlebar and helmet mounts to add mounting positions to the mix of testing. He also has the older funnel or bell shaped head units and a reflector/heatsink machined to fit a P7, and you can piece out a P7 light kit as you like, as some of you know. However, I wanted a multi XP-G design with the new LISA2 pin lenses. I wanted room for either a Bflex or Maxfflex driver board behind the light engine array so the bullet style is it.

That leads to the reason for considering the copper pipe end cap as the starting point for the MCPCB mounting and heat spreader to the aluminum shell.

For those interested, Google this forum for copper and aluminum and read away. The summary is that copper is as much better at conducting heat from hotter to cooler zones compared to aluminum as aluminum is compared to steel. Silver is a bit better than copper but no where near the cost differntial. Since we are concerned with long term steady state and not short-term transient heat storage as in some flashlight applications, the higher heat storage per unit volume of copper is not particulary useful.

Copper is more expensive and heavier than aluminum. But pipe caps are mass produced and you can buy just what you need and not 4' or aluminum tube when you need 50 cm, so at the DIY level of cost, copper can be cheaper. The weight can be a deal breaker for some cycling applications. For non-competition or training use, as long as we aren't talking a solid copper CNC light, a copper heatsink is a worthy alternative to aluminum.

And so, we arrive at the use of copper pipe end caps. It so happens, that a 1" pipe end cap is a good first approximation of a nesting heat sink for the Marwi light with a wall to mount the led and flanges to move the heat to the body of the light and at least one builder has used this method successfully it in a triple Q5 application with some cut away to allow for the mounting slot.

http://forums.mtbr.com/showthread.php?t=295664

Go to post 24.

LED thermal management 101 (background for issue):
However, if you Google this forum on lapping heat sinks or stars, there is an issue with the end cap and MCPCB mounting that needs to be addressed for optimal thermal path. Although the thermal path from the mounted LED to the MCPCB is the most critical, I have elected to trust Cutter for my mounted XP-G's (as so many here have, so they are what they are, but I hope there are some VIAs from the thermal pad to the bottom surface). That leaves me with the MCPCB to heat sink and heat sink to body interfaces to optimize. It is the first interface that isn't as simple as it first appears.

The MCPCB is a heat spreader and metal-metal contact between the spreader and heatsink is an order of magnitude better than thermal glues or greases, which are orders of magnitude better than an air gap. So you want as much metal to metal contact as possible, the thinnest layer of thermal compound or epoxy, and no air gaps.

MCPCB's are punched or cut and by definition are not flat. Some DIY's have lapped these while being careful not to rock them and flare the edges. (if anyone has lapped the 10 mm round ones and has a good technique, now's the time to come forward). :grin2:

Even new heat sinks may not be ruler flat. Even if flat, unless they are carefully polished, they have microscopically large peaks and valleys (polished equals microscopicaly smaller peaks and valleys). Thermal compounds and epoxies fill these gaps and fill the air gaps between the heat sink and MCPCB. But if the layer is thick in order to compensate for a considerable lack of flatness, then the thermal resistance at this interface will be high and can prevent heat leaving the MCPCB as fast as needed to keep the LED in operating range let alone as close to ambient as possible.

Finally: The copper end cap issue:
This is true for all heat sinks, so why am I reviewing all this for the copper end cap? First, for anyone familiar with these, the end wall or floor of the cap is far from being planar, let alone polished. Second, is the teacher in me: I spent a lot of time tracking all this down and thought it would be nice for onter newbies and not so newbies to know. Last, I need any misunderstanding to be corrected and there are willing parties here.
:wave:

I have two manufacturers' examples, and though this is a small non-random sampling, 100% is a high frequency, so it may be a common problem. I have two different logos stamped in the copper and ghostly images of them clearly visible inside the cap bottom. Copper is maleable and ductile which means it can be molded and bent to shape, handy to modify the end cap to fit the inside of a bullet-shaped light body. It also means the pressing has pushed up the center bottoms of these caps. I can detect the raised parts around these logos inside the pipe caps and the recessed circumference near the side wall. Not even close to flat, let alone polished.

So has anyone addressed this issue to get a flat mount for the MCPCB?

The XP-G has a high thermal limit and a slower heat decay, so is this is a lot of fuss over a slight reduction if lumens/watt?

Does this solution sound feasible:
Unless I learn better here, :naughty: I plan to cut off the bottom of one 1" end cap, remove any 'edge' of the former 'wall' remaining, then sand the inner side of the bottom flat and lap it to a mirror finish. Then, lap the MCPCB's. Mark, drill , and tap mounting holes for screws, and use minimal thermal paste on the MCPCBs.

As it happens, I need a slightly larger diameter than a 1" pipe end cap to meet the inner wall of the body from just behind the former seating for the M11 bulb to the plane of the LED and lens mounting and a little beyond. So I plan to use that the removed wall, (which is a short cylinder after the bottom is cut off). It will be cut lengthwise to make a split cylinder that can be expanded and used to make the heat sink larger in diameter to meet the inside of the body at that point.

I am also mounting the LED's asymetrically, so I need a thicker thermal path to the opposite side of the light. (Cross section of heat sink should be about 1/4-1/2 the distance from the LED to the body, a rule of thumb in this forum, under heat sink size rule of thumb thead, I think) I will use a second pipe cap's bottom for this added thickness and it's walls will be beveled to match and meet the bullet shape towards the rear of the light to help heat flow through that part of the light's body.

Assuming I am a sucker for all the hand work, any thoughts?

I can assemble the heat sink in two ways:
Each light will use about 3.5 watts on the street on 'highs' (350 mA) and 10 on the trails (1 A), about 80% of that, as heat.

I can solder the the heat sink as one assembly and have difficulty mounting the MCPCB's and thinning out the thermal paste for best thermal path because they are close to the sides at the bottom of a 8 mm deep 'well'.

Or I can make the end cap bottom a removeable part of the heat sink bolted and thermal pasted to the second part with lapped mating surfaces, rather than soldered in place. Then the MCPCBs and lens can be mounted in the open to aid thinning out the thermal paste.

In the first case, I get the excellent thermal path of solder through the heat sink but likely a poorer path at the MCPCB's. In the second design, I get easier modification, likely much better MCPCB thermal path, but I add another resistance interface to the thermal path to the body.

In both cases, I plan to hone the heat sink to the body and use thermal paste to reduce the resistance at the last thermal interface before the air. (CNC will start to look good, during this process, I'm sure).

Does anyone have experience or a gut feeling about how the 2-piece heat sink would work compared to the 1-piece?

My gut says the smaller and hotter interface needs the most attention. So the 2-piece is the better solution thermally and for future mods.

What do you think?
:thinking:
 

Szemhazai

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lampka1.jpg


lampka3.jpg


lampka2t.jpg


Cree XR-E emitter glued directly into the cap and ledil 21,6 mm diameter optics - the wiring this is hard but it is small and reliable lamp.
 

formantjim

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Hi BrianMc I have been using copper end caps for all my lights from 3/4" through to 1 1/4" with great success.
Here is a picture of 9 XPE's being mounted on a copper end cap it has been sanded by wet and dry sandpaper on a flat glass table top(when my wife is not looking). A thin layer of Arctic Silver is applied and then the MCPCB's are screwed to the end cap.
file:///C:/Documents%20and%20Settings/All%20Users/Documents/Bike%20light%20parts/IMG_0013.jpg
For all the smaller lights ie 3 or 4 up and MCE Pipe Lights I then cut vertical slots with a hacksaw after sanding and drilling as one cannot hold it in the vise properly once this is done
file:///C:/Documents%20and%20Settings/All%20Users/Documents/Bike%20light%20parts/IMG_0350.jpg
Here's an example of an MCE Arctic Silvered and screwed to the copper end cap. The copper end cap is a very tight push fit into the pipe coupling and provides excellent heat transfer to the brass chrome plated tube.
Here is a selection of the lights built with the same technique.
file:///C:/Documents%20and%20Settings/jim/My%20Documents/My%20Pictures/LED%20light%20JF1234/LED%20light%20JF1234%20158.jpg
 

HEY HEY ITS HENDO

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looks like the pics are on your pc hard drive......
you will need to use a photo hosting site eg photobucket or imageshack, etc
and upload your pics to them
then you can post a link here, to the hosted pics ..
click on this icon
insertimagekk4.gif
when posting your reply
insertimagekk4.gif
and paste the url (adress) of the pic
 
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formantjim

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Thanks I thought this forum had the same or similar upload functions as MTBR but obviously not.

Will try again.
 

BrianMc

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Thanks all. I have the pipe caps. The lights arrive Monday or Tuesday. So I can get started sanding and lapping.

Szemhazai:

Are the bottoms of the pipe caps you get (in Poland, right?) flat? Do you do anything to them to prepare them to accept a star? BTW I enjoy reading your posts.

znomit:

Your light was one of the first I read about and it helped convince me to build my own, and loved the Kiwi flavor of it. Even the cheap DX P7 with Geomangear support could not stop me after reading posts like yours. You sir, are a bad influence. Keep it up. :)

Remind me. Did you do anything special to prepare the heat sink/case and star interface to help heat transfer?

I also like the amber side light from the stray light that doesn't get out the lens. I almost designed amber markers on the headlights inspired by your design, but decided side markers were going to be a later build, front and rear. The amber 1/2 watt XP-E's at Cutter look too good not to try out.

formantjim:

I feel your pain. I had trouble too. Had to google this forum to find out how to do it. And will have to my next time, too. Your 'one more time' links took me to the host but no pictures. I don't know why.
Could you try again? Please? It soulds like I really do need to see them.

You do sand and lap the cap bottoms for mounting. Do you lap the stars, too?

If I understand you correctly, you are saying I need to think about the order of certain tasks or I won't be able to hold the sink in the vise to do it later, and that you found a good interference fit between sink and body was more than enough to get good heat transfer to the body.

I have been giving the order some thought. I will be out a lot of work if I have an essential step to do that I can no longer do. Like the chocolate cake I forgot to put sugar in. A bit late after its baked. It is one of the reasons for this discussion, I won't be able to foresee everything and just seeing another light or talking about it can be a big help.

In the Marwi lights, the front bezel tightened against the former MR11 lamp to seat it firmly. I am going to use that same force to push the sink firmly against the body. So that should do the trick without thermal paste? The shapes of the sink and body are compound curves, but I will get the match between them right.

I have a number of free panes of glass salvaged from picture frames that I plan to use over a plywood base to keep the marital peace.

HEY HEY ITS HENDO:

Quad die. P7 or MC-E but looks like the MC-E reflector. Was the inside bottom of your cap flat? Did you prepare either the pipe end cap or the star before fastening them together?
 
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znomit

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znomit:

Remind me. Did you do anything special to prepare the heat sink/case and star interface to help heat transfer?

I also like the amber side light from the stray light that doesn't get out the lens. I almost designed amber markers on the headlights inspired by your design, but decided side markers were going to be a later build, front and rear. The amber 1/2 watt XP-E's at Cutter look too good not to try out.

Nothing special, and given that it was hammered flat probably not a particularly good interface. I guess I used some brasso to polish it.
I don't know how important the lapping is. I've never bothered dressing stars.

Go easy with the side lights, unless you can mount the light clear of your eyesight.
 

Szemhazai

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BrianMc, it is 22mm pipe end cap, bottom in the center of the cap is flat. I've used emitters (no stars), if you want to use the stars you will have to cut them on the corners.
 

formantjim

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OK BrianMc.
Yes I do indeed sand and lap both the copper end cap and stars.
Yes the sequence of the tasks is important mark and drill the holes first, sand and lap the face then cut the slots. Using the face glass to finally push the copper in so an interference fit is obtained.
thelineup.jpg
[/IMG]
9xpesmountingoncopperen.jpg

copperendbeingdrilledbe.jpg

copperendwithcuts.jpg
 

BrianMc

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Thanks all for your feedback. :rock:

The take-home lesson here appears to be that if you are not fastidious about the interface does not mean the light won't work, but doing so (lapping etc) may still be a best practice.

formantjim

Thanks a lot for persevering pic posting. :twothumbs

We were right. Tell you sister you were right, Luke... oops Star Wars moment there! I really did need to see them.:kewlpics:

Those are chromed brass sink u-bend parts? They do make for a compact package. And rugged. Look like a fun build. Hmmm: high power tailight? :devil:

lovecpf

Your cap's triangle-M logo is the same as on two of my caps from the local pumbing supply house. The two 'NIBCO' caps are a little shorter in height, larger in diameter, and came from Lowes. At one point, I was thinking of using slits in the side to allow splaying and contracting to follow the bullet shape better. Glad to know this method has real world testing behind it. Combined with a bit of bend in each leg this will do the milder front curve. For the legs aimed rearward, wider cuts (with double blades) near the edge to allow for the increased curve, and some sanding, I should be able to get the shape needed. There should be just enough spring left to push the legs against the body with the push from the bezel. I just really like the shape of these old Marwi heads and would like to find a DIY way other than a CNC aluminum heatsink to use them.

I have a project step-by-step list and Excel spreadsheet used instead of MS Project to keep things in order. A light recipe, I guess.

First I need to clear up my workspace for pics. My vise is about as photogenic as yours. My foot less so. :laughing:


Szemhazai:

Sorry. Missed the emitter driect comment in your first post, too amazed by the barbell design, I guess. :grin2: So you used something like Acrtic Silver Epoxy to fasten the emitter to the bottom center of the copper cap? Did you rig up a way to apply pressure to the emitter but not the dome to squeeze the layer of epoxy thin, or did you apply it to the back of the emitter scrapping it to a thin layer and sort of 'smoosh' it in place to get the layer thin?
:thanks:
 

Szemhazai

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BrianMc, I'm using my favorite 3M 467MP thermal adhesive tape - it is 0,05 mm thick and it is giving very good results for XR-E emitters.
 

BrianMc

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Sanding the bottom outside is not a problem.

I can feel the embossed brand stamp in both brands, and the centers of both are raised relative to the outer edges of the inside bottoms of the heat sinks/copper caps, and sanding is really not an option.

If you cut the sides mostly away as shown for one light, you could get a decent sandig job. I could take the sides off and resolder them back on.

I could go the tape or thermal compound route and hope the higher thermal conductivity of copper helps a fair bit. Three XR-E R2's and three XP-G R5's are about the same sized heat source, so the light will work.

Another idea is that liquids (except liquid helium near absolute zero) seek level. Gravity works. So what if I just places a coil of high silver content solder and some flux in the cap bottom and heat the cap up until the solder melts and flows into a flat surface and then let it cool? High silver content solders flow best. Silver is even a little better than copper in thermal conductivity so the mixed solder should be pretty close to copper, Maybe a bit better. No sanding. Just mount the MCPCBs and go.

Anyone have experience melting solder in a similar fashion to form a pad?

I'll have a go, but if there are tricks or pitfalls I'd like to know.

Wait! I mismeasured the bottom thickness, I thought two caps back to back would put be close to 1/4", but that's 1/8". Solves the lapping issue: I cut off an end cap bottom, lap it, and solder it in the bottom of the front cap before drilling and other mods.
 
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NigelBond

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If you use copper fittings like this I would suggest using type K fittings. Copper comes in three thicknesses K, L, and M. M is the common thinner wall stuff and K is the thickest. I don't think I've ever seen L copper before but it probably falls in between.
 

BrianMc

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NigelBond:

Thanks.

I googled K and L copper pipe and it seems to apply to residential (L thinner walled copper pipe) and K commercial. I can attest that the 3/4" copper pipe and 3/4" end caps that I also have, are only 1/32" thick whereas the 1" caps have 1/16" thick side walls and bottoms.

I am planning two caps back to back with extra soldered in because the LED's are on one side. A crude rule of thumb says heatsink thickness should be about 1/4 to 1/2 of LED distance to the body for flashlights. So 1/16 would be fine for the near side but, the rule suggests 1/4" to move heat to the far side. It is copper though so being at the low end of the rule should be fine. Once at the body, 1/16" of copper lines the inside of the body to dump the heat into almost the entire aluminum housing. The housings arrived today:

http://www.el34world.com/Misc/bike/BikesLights42.htm

I may be wrong, but I think that 3 XP-G's at 9.9 Vf and max current of 1 A, are likely dumping about 8 watt, if I keep them cool, the same amount of heat as a P7 at 2.4 A, or close enough. This housing has about the same surface area as a certain 'MS' P7 light, too.

I don't have a scale, but a hand test of the ViewPoint housings with 10 watt MR11 in place, say they are close to the weight of two unmodified pipe caps. Both caps will be shortend some, but will have the thicker bottom. I think that should be ample. Better be: they'll be heavy enough.
 
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