COB Panel Mount
A COB panel is the cheapest way to buy a lot of light. The one this bracket is built around is a bare aluminium board, 8.6 × 4.4 in, 70 W of 6500 K white, sold as a flood-bulb retrofit with no housing, no driver and nothing to hold it on to anything. Two problems follow from that, and this project is both of them: something has to carry the board, and something has to pull 70 W of heat off the back of it.
The bracket is the easy half. Read the heat section before you print anything.
Run one of these at its full 70 W with nothing on the back and it will get hot enough to discolour the phosphor in minutes and de-solder its own leads not long after. No printed part goes against the back of the board. PLA gives up around 60 °C and an unsinked panel goes well past that. Heatsink first, bracket second.
Parts
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The panel and its cooling:
- 12 V 70 W COB panel, 8.6 × 4.4 in, 6500 K — the board everything here is sized around
- Aluminium heatsink, 100 × 25 × 10 mm — sold in fives, and eight of them cover one panel, so buy two packs
- Thermal conductive glue — silicone, cures hard, permanent. This is the one to use. A 10 g tube is about one panel’s worth of thin film and not much more
- Double-sided thermal tape, 15 mm wide — the removable alternative, and two strips per sink, because the sink base is 25 mm
- 99% isopropyl alcohol — nothing bonds to a board with flux residue still on it
Power:
- 24 V 5 A power supply — 120 W, 5.5 × 2.1 mm barrel
- 20 A / 300 W CC-CV buck converter — 6–40 V in, 1.2–36 V out, two-pack. The constant-current half is the entire reason it is on this list
- 5.5 × 2.1 mm barrel jack sockets and pigtails — 20-pack, panel-mount sockets plus flying leads
- 18 AWG two-conductor silicone wire — six amps at 12 V is real current, and thin hookup wire will warm up before the LEDs do
The bracket:
- PETG filament — not PLA. The reason is under Print settings
- M3 heat-set threaded inserts — a printed thread will not survive being taken apart twice, and this one gets taken apart
- M3 screw, nut and washer assortment
- #8 × 1/2 in wood screws — mounting to a stud, a joist or a shelf
- Drywall and hollow-wall anchors — mounting to anything else
- Digital caliper — measure your own board first. “8.6 × 4.4 in” is a product listing, not a tolerance
Worth having:
- Infrared thermometer — the only honest way to find out whether your heatsinking is enough
- 40 mm 12 V fan — if you insist on running near full output. There is no 12 V rail anywhere in this build, so run it off the second module in the converter two-pack, set to plain constant voltage
- MOSFET PWM switch module — 10-pack; dims the panel from a 3.3 V ESP pin
- Inline rotary PWM dimmer — dims it with a knob and no code
- ABS filament — only if the fixture will genuinely live at full power, and only if your printer is enclosed
Heat, which is the actual project
Seventy watts is a soldering iron. It goes into a piece of aluminium about the size of a postcard, and every one of those watts has to leave through the back, because the front is busy making light.
Eight of the 100 × 25 × 10 mm heatsinks laid side by side across the short dimension of the panel cover 200 mm of its 218 mm length, which is about as close to full coverage as that extrusion gets you. That is a good stack, and it is still not enough for 70 W in still air. Passive cooling at the rating is not on the table.
So underdrive it. Set the converter’s current limit around 2 A and the panel draws about 24 W — a third of its rating, and inside what that stack can shed. You lose light more or less in proportion to the current — you get a little back, because a cool LED is a more efficient one — and what you buy is a fixture that still works next year instead of one that dims for a month and then dies. Twenty-four watts of bare 6500 K emitter is still more light than you want to stand in front of.
Two things about how it hangs, both of which cost nothing:
Fins vertical. A finned extrusion mounted with its channels running horizontally is a decorative aluminium block. Convection needs the air to be able to climb out of the fins.
Air behind it. Do not bolt the bracket flat to a wall with the heatsinks in the gap. Stand it off, or point it out into the room. A sealed cavity behind the sinks undoes the sinks.
Print settings
There is no STL in this folder yet. What follows is what the part wants whenever one lands, and it holds just as well for a bracket you model yourself in the meantime.
| Setting | Value |
|---|---|
| Material | PETG |
| Layer height | 0.2 mm |
| Perimeters | 4, about 1.6 mm of wall |
| Infill | 30–40%, gyroid |
| Supports | Only where the geometry needs them |
PETG, and this is the one decision on the page that is not negotiable. The bracket never touches the LED board, but it does bolt to a heatsink that will happily sit at 60–70 °C, and it is holding something heavy. PLA softens right in that range, and a PLA bracket does not snap — it slowly leans, and you find out when the panel is pointing at the floor. PETG holds to about 80 °C. ABS if you are running the panel hard and have an enclosed printer.
Orient the part so the load runs along the layers rather than trying to peel them apart — a printed part is weakest at the bond between one layer and the next. The screw bosses are where a printed bracket fails, and they fail by splitting between layers.
Wiring
Four wires, and nothing between the wall and the LEDs except the converter.
| From | To | Notes |
|---|---|---|
| 24 V supply, centre pin | Buck converter IN+ | Through a panel-mount barrel socket |
| 24 V supply, sleeve | Buck converter IN− | |
| Buck converter OUT+ | Panel positive pad | 18 AWG |
| Buck converter OUT− | Panel negative pad | 18 AWG |
Get the polarity right. An LED board does nothing at all backwards, which is at least a harmless way to be wrong.
Why 24 V into a converter rather than 12 V straight in. This panel has no driver in it — it is an LED array with a nominal voltage, and its forward voltage falls as it warms. Feed it from a fixed 12 V supply and that drop pulls more current, which makes more heat, which drops the voltage further. That loop is how these boards die. A constant-current supply breaks it: the module gives the panel the current you set it to and takes the voltage wherever it needs to go. The converter needs headroom above its output to regulate, which is why the supply is 24 V and not 12 V.
Both dimmers go after the converter, between it and the panel. The MOSFET module takes PWM on its trigger input from an ESP pin; the rotary one does the same job with a knob and no code, and since it is a barrel-jack pass-through you put a plug and socket from the 20-pack on the converter’s output to take it. Neither goes on the converter’s input — chopping a buck converter’s supply just makes it brown out and restart. Read the silkscreen on whichever board you use before wiring it: terminal ordering varies between batches of the same module.
Assembly
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Measure your panel with calipers before you print. Boards sold under this description vary by a millimetre or two, and a bracket that grips is a bracket that has to fit.
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Clean the back of the board with isopropyl and let it flash off. It arrives with flux and handling oil on it, and thermal adhesive bonded to flux is thermal adhesive that lets go in six months.
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Bond the heatsinks. Lay eight of them across the back, fins running the way they will end up vertical on the wall. A thin, even film of the silicone glue — squeeze-out is fine, a thick bead is not, because the glue conducts heat worse than the metal on either side of it. Weight the stack flat and leave it to cure.
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Solder the leads on with the 18 AWG wire and strain-relieve them at the board. The pads on these panels are the first thing to fail, and they fail by being tugged.
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Set the current on the bench before any of this goes near a bracket. See below. It is far easier to get a probe on loose leads now than on a mounted fixture later.
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Melt the inserts in, then fit the bracket to the heatsink stack. Inserts go in with the iron at PETG temperature and a slow, straight push — a rushed one sits crooked and takes the boss with it.
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Mount it into wood with the #8 screws, or into plasterboard with an anchor rated well past what the assembly weighs. Aluminium and screws add up faster than they look like they should. Leave the back open to the air.
Setting the current
The buck converter has two trimmers, one for voltage and one for current; the silkscreen says which is which.
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Wind both trimmers fully anticlockwise before you connect anything. These modules often arrive set wide open, and wide open into an LED array is a one-second mistake.
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With no load, bring the voltage trimmer up to about 12 V.
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Connect the panel — with a meter in series, if you have one that will take the current — and bring the current trimmer up slowly to your target. Start at 2 A.
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Leave it running twenty minutes and put the infrared thermometer on the heatsinks. If they are past about 70 °C, come back down on the current until they are not. That number is your fixture’s real rating, whatever the listing says.
Do not look into the emitter while you are doing this, and do not aim it at anyone. This is a bare 6500 K die with no diffuser of any kind over it, and even underdriven it leaves an afterimage from across a room. Point it at a wall while you work.
Troubleshooting
Nothing lights. Current trimmer still at zero, or the panel is backwards. Both are the same five-second check.
It starts bright and fades over a minute. Thermal droop, which means the heat is not getting out. Either the glue line is too thick, the sinks are not covering the board, or it is mounted somewhere with no air behind it.
One end of the board is much hotter than the other. You have not covered the whole board. The outer strip that eight sinks leave bare is fine; a gap in the middle is not.
The bracket has slowly gone out of square. PLA. Reprint it in PETG — there is no fix for a part that has already crept.
The supply clicks or shuts down. At full rating the converter pulls a bit over 3 A from the 24 V side once its own losses are in, and a cheap brick’s 5 A is a label, not a promise it will hold all evening in a warm garage. Come down on the current, or feed the converter from a supply with real headroom. One more argument for underdriving it.
Files & downloads
The source files for this project aren't in the repo yet — there's nothing to download
beyond the notes. They land in projects/cob-panel-mount/ when they do, and this
section fills itself in.