Designing and Making a LED Light Fixture for a Planted Tank

SuperColey1

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VaughnH;38276 said:
My tentative guess is that moisture between the star mount and the heatsink may have been involved, or the star mounting is not well done by the assembler. Tomorrow I will figure out what next. Any suggestions? Is there a relatively easy way to electrically isolate that one - with a mica shim, for example? Or would it then overheat?

Not overly sure what the 'grounded' malarky is as I am no electrician. lol

All mine are screwed in using the same screw as you and the heatsinks are well and truly touching the stars :) Each LED is seperated though as the heatsink/LED combos are mounted on an acrylic sheet :)

complete%20board%20unit.jpg


As for the moisture issue I think the 'splashguard' is the key thing. Will stop moisture coming into the unit, stop the fans creating more evaporation by cold air escaping downward, keeps all the fans cold air running through the luminaire, and as a 'just in case' I have my fans set to come on 15 minutes before the LEDs to dry out any moisture there may be in the unit.

I was told never to wire high power LEDs in parallel, always to do them in series!!!

A couple of threads I was using and getting good info/feedback from:

UK Aquatic Plant Society Forum • Login
Diy Whole Tank Led Lighting Retrofit - Tropical Fish Forums

On the strobing side of things, when I initially set mine up 6" above the water as the T5HO setup had been it sent my eyes 'wappy'. Its fine now it is 15" above the water much more like the 'glimmer/shimmer' of MH. One factor is the surface water movement I (and you) use. No surface disturbance removes the glimmer :)

AC
 

VaughnH

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SuperColey1;38285 said:
Not overly sure what the 'grounded' malarky is as I am no electrician. lol
It is highly desirable to have the negative lead of a DC power supply at ground potential - connected to the ground terminal for the house wiring. This keeps the positive voltage at +whatever the power supply voltage is, rather than just that voltage above the negative terminal. (Or, that's what I've been told)

All mine are screwed in using the same screw as you and the heatsinks are well and truly touching the stars :) Each LED is seperated though as the heatsink/LED combos are mounted on an acrylic sheet :)
This means your heatsinks can be electrically "hot", not a problem with low voltage circuits, but that would be a problem with my 48 volt circuit.

As for the moisture issue I think the 'splashguard' is the key thing. Will stop moisture coming into the unit, stop the fans creating more evaporation by cold air escaping downward, keeps all the fans cold air running through the luminaire, and as a 'just in case' I have my fans set to come on 15 minutes before the LEDs to dry out any moisture there may be in the unit.
I chose not to use a splash guard because it would obstruct the cooling air flow, which, for my unit, exits from the aquarium facing side of the fixture. And, since the fixture is about 5 inches above the aquarium, I can't see how moisture from the tank collects there - it is too well ventilated for that. (I think)

I was told never to wire high power LEDs in parallel, always to do them in series!!!
It isn't quite that simple, in my opinion. You need to wire them in series as much as you can, in order to keep the same current through each LED, but without having to supply a constant current driver for each LED, which would be extremely expensive. Then, the series strings can be wired in parallel, and the more of them in parallel the better, because then if one string stops working, the increase in current through the other strings isn't so big. I have only two strings in parallel, so the current through the remaining operating one doubles if one string quits. That forced me to keep the operating current below 500 mA for each string in order to stay below the 1000 mA maximum current allowed, in case one string quits operating. If I do this again I will use 12 volts DC instead of 48 volts, which will give me more options for wiring them, plus 12 volt supplies are much easier to find and generally cheaper.

A couple of threads I was using and getting good info/feedback from:

UK Aquatic Plant Society Forum • Login
Diy Whole Tank Led Lighting Retrofit - Tropical Fish Forums

On the strobing side of things, when I initially set mine up 6" above the water as the T5HO setup had been it sent my eyes 'wappy'. Its fine now it is 15" above the water much more like the 'glimmer/shimmer' of MH. One factor is the surface water movement I (and you) use. No surface disturbance removes the glimmer :)

AC

Thank you for the thoughts. Even when I don't go along with what is suggested, it stimulates me to think more broadly, and that helps a great deal in solving a troubleshooting problem. I would still be stuck, frustrated and sleeping poorly if not for the suggestions that have made it possible for me to figure out what was wrong and how to fix it.
 

VaughnH

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Problem solved - for now anyway. The "failure" was current leakage to ground through the "line" contact where the screws held the one LED to the heat sink. Looking very carefully I can see slight discoloration around the screw notch in the star mount, where it contacted the conical back face of the screw head.

Today I found some Arctic Silver brand alumina filled epoxy heat conductive cement at Frys. I thoroughly cleaned the heat sink in the contact area, cleaned the back of the LED (using alcohol), and reinstalled the LED using the alumna filled epoxy. After a half hour (it sets up in 5 minutes) I plugged it back in, and only the 6 LEDs ahead of the failed on lit up. So, I removed the attaching screws entirely. Plugged it in again, and it worked fine. That is when I looked for and found the discoloration.

Now I suspect that I will get this failure again, as another LED leaks current at that spot. If so, I will then mount all of the LEDs with the ceramic epoxy and not use screws at all. In the future I won't try to use the screws if I make another one of these.
 

shoggoth43

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I use flat head screws and nylon washers so that may be why I haven't run into that particular issue yet. There are some pastes and adhesives which will conduct heat but not electricity. I've seen them for the stars for ~7.50$ for 10 or twenty of them. Just stick them on and they'll even hold the star in place. Very nice.

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S
 

shoggoth43

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I use flat head screws and nylon washers so that may be why I haven't run into that particular issue yet. There are some pastes and adhesives which will conduct heat but not electricity. I've seen them for the stars for ~7.50$ for 10 or twenty of them. Just stick them on and they'll even hold the star in place. Very nice.

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VaughnH

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The problem is with the conical back face of flat head screws. That puts a very high pressure on the edge of the star mount notch, so if the electric conductive layer is anywhere close to the surface there, the screw can contact it. Like:
LEDScrewProblem.jpg


Using round head or pan head screws, with the flat back faces on the heads would solve that problem, but they would contact the extra solder pads on the surface of the mount, so the non-conductive washers would be essential then.
 

SuperColey1

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Could you not get some silicon and coat the screw head in it then screw it down? or maybe use a fibre/rubber washer to seperate screw from star?

AC
 

Philosophos

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I was just about to post the same; countersink a little then add a washer.

-Philosophos
 

VaughnH

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I would never try to countersink a mounting notch in a LED star mount. That is virtually impossible to do, and actually impossible when you are my age. I can see no reason to use the screws at all. Just glue the LEDs to the heat sink with the alumina filled epoxy thermal adhesive. If you ever have to remove one, it presumably isn't working anyway, so destroying it to remove it isn't a problem. And, as long as you don't run the LED above 700 mA or so current, the epoxy thermal adhesive should be more than adequate for conducting the heat away from the LED. Remember, the heat generated will be proportional to the current squared, so 700 mA generates half the heat that 1000 mA generates.
 

Philosophos

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It's doable, but true enough, it's a pain. If they were removed regularly, I'd probably still do it my self. I forgot about the obscenely long life on LED's just enough to make a stupid post :D

-Philosophos
 

shoggoth43

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I'd have to look at mine and take a look to see how much the pads are overlapped by the washers. Since I have two pads unused on the stars it doesn't really matter much if they're covered. Seems easy enough and they were cheap. The epoxy might be simpler in the long run though as long as you don't feel the need to mess with what you have by deciding you need a slightly different color or whatnot. Upgrades are also easier, but that probably doesn't apply here since we aren't trying to bleach anything here. I've popped heatsinks off of video cards that were epoxied on with a screwdriver as well, so that may just end up being the simplest method.

Here's the adhesive stuff I was talking about.

Luxeon Star LEDs, High Power LED Products & Accessories
Pre-Cut, Thermal Adhesive Tape for Luxeon Emitters (10) [LXT-E-10]

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S
 

VaughnH

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Those adhesive self sticking tape "dots" look pretty good, but I think they should be about .50" in diameter, instead of .32". It may be that the extra handling of the star mounted LEDs needed to attach them with the tape would make it more likely that you would leave a greasy fingerprint on the little plastic lens. That wouldn't be good. Still, the alumina epoxy stuff is a bit of a pain to handle too.
 

Philosophos

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This is where jewelers pliers or tweezers and a mounted magnifying glass are a big help. Detail work needs the tools for it; I've found they help a lot when I've used them.

-Philosophos
 

shoggoth43

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I think the smaller sizes are probably for putting the LEDs on the stars in the first place and they expect you to do a reflow job on it to do the soldering. The larger sizes attach the stars to your heatsink or whatnot so that would probably be the size to go buy. Fingerprints wipe off easy enough. It's not like the MH bulbs which might shatter due to oily prints since most of the LEDs heat is headed backwards to the heatsink.

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VaughnH

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I haven't found any good illustration or description of the star mounts. Base on my interpretation of what I have read, they must be aluminum plates, coated with a dielectric substance on both sides, with solder pads deposited on one surface of the dielectric. If that is true, the mounting notches expose the aluminum core to contact with the mounting screws. And, that means a short from a solder pad to the core could cause a short to the mounting screw. Is that an accurate analysis? Also, the top surface dielectric coating appears to stop about .010" short of the edge of the aluminum core, which means the connecting wire is extremely close to contacting the core after it is soldered on. (All of this from looking at the Cree stars) And, this makes me think I will run into these shorts again, probably many times.
 

shoggoth43

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From what I can tell:

Aluminum plate
Non conductive coating applied ( probably with hole in center for LED heat disipation )
conductive coating split down the middle of the star left/right ( probably with hole in middle again )
Non conductive coating exposing lower metal coating into three solder pads per side with the aforementioned hole.
LED in middle w/thermal compound and leads going to aforementioned solder pads ( there should be 3 plus and 3 minus pads ).

The solder pads are usually rectangular so I try to run my wiring in on the "long" side to minimize any possibility of a short if the bare wire runs past the pad. If you come straight in from the side like a wheel spoke you might run into that stray wire strand touching the base of the star. The long side method will hopefully make that less likely but it's still an occupational hazard. Obviously any excess solder or wire strands touching the metal screw will make the heat sink live and cause issues. Nylon washers will not help that kind of problem either. I would be loathe to suggest nylon screws to hold it in place due to the possible heat you might have to deal with. I also suspect they never envisioned conical head screws with the design or the conducting/nonconducting layers would be layed out a little different to give more clearances. Not much we can do about that unless you can somehow scrape the coating back. I haven't bothered to see if the star plate is live, but I would be very surprised if it were.

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S


VaughnH;38340 said:
I haven't found any good illustration or description of the star mounts. Base on my interpretation of what I have read, they must be aluminum plates, coated with a dielectric substance on both sides, with solder pads deposited on one surface of the dielectric. If that is true, the mounting notches expose the aluminum core to contact with the mounting screws. And, that means a short from a solder pad to the core could cause a short to the mounting screw. Is that an accurate analysis? Also, the top surface dielectric coating appears to stop about .010" short of the edge of the aluminum core, which means the connecting wire is extremely close to contacting the core after it is soldered on. (All of this from looking at the Cree stars) And, this makes me think I will run into these shorts again, probably many times.