Flush ceiling light overhaul heatsink calculations

I am cooking a flush ceiling lamp overhaul for a friend. The lamp is an old design which should look apparently unmodified. It consist in a cylindrical metal base of ∅≈6.1" inner diameter, or a bit more than 15cm wide, and an inch or so of height. The body is a ∅≈9 × 2.75" (∅≈23 × 7cm) cylinder of white glass, with a very short neck which goes in the metal base. I measured its neck inner diameter at 15.3cm. I've borrowed the following pictures for a better idea of how it is:

This is not an ad but since I borrowed the pictures, here's the link: https://www.jossandmain.com/lighting/pdx/bodalla-2-light-flush-mount-j000258865.html

His lamp is similar to the above one, but the white crystal body is relatively wider and strictly cylindrical. In the center of the base is a long, top and bottom threaded bar of ∅≈9.5mm. The body is held in place with a big wing nut.

Given the above limitations I've planned a compounded heatsink made with a 100 × 100 × 18mm base, and on top of and glued near its corners 4x columns made with 2x stacked ∅47 × 19mm sunflower heatsinks each (a total of 8 sunflower heatsinks over the base).

On top of each column will go one of these 10S 3535 copper DTP MCPCBs with 3x SST20 and 3x LH351B emitters driven at ≈650mA, which is ≈12W of power per column.

Using this simple celsia heatsink calculator, my numbers barely look good:

My combined heatsink volume is 443.71cm³.

Any thoughts? This is all theoretical right now but but the main problem, as I see it, is that the body will not help with convection.

I already told my friend some fans could help, albeit he doesn't likes the idea (he's mentally a tad short-sighted, mind you).

Cheers :-)

If you convert it to LEDs you will not need any heat sinks. There are conversion kits available that include the LED plate and driver. The aluminum base for the lamp is sufficient for heat dispersion. Just choose the one that will fit inside the space. It looks as though 18 W would be the maximum that will fit and this design is within your dimensions.

Example: https://www.ebay.com/itm/12W-18W-24W-36W-220V-Energy-Saving-LED-Light-Source-Retrofit-Home-Ceiling-Lamp/283505705457?var=584632024251&hash=item420241c1f1:m:mEDT-Cguscle3bkpotNCR4w Price delivered to the US is less than $5 but 220V lights don’t work here.

At 48 watts of decent LEDs this will be too bright to look at

Thanks for the suggestion Vegas LED Fan, that stuff is way off my intentions concerning power and colour rendering, though (fixed link: https://www.ebay.com/itm/12W-18W-24W-36W-220V-Energy-Saving-LED-Light-Source-Retrofit-Home-Ceiling-Lamp/283505705457). It also is absurdly cheap, I wonder how much actual lifespan do these modules endure and the quality level of their electronics.


20 sq.cm per 1W - this formula works well if you need longlife.

I can’t assess the lifetime of this particular replacement unit but my experience with LED lights purchased directly from China has been excellent. I converted my home to all LED more than three years ago and have had just one bulb fail so far from all of the bulbs and fixtures I purchased. It may be simpler to just buy a brand new flush mounted ceiling light. I have one similar to the picture below in my second floor hallway. I’m not one who obsesses about CRI, just about getting enough light so I generally use cool white fixtures. There are many to choose from in different sizes and lumen outputs. I paid about $12 for the one I bought. When I looked them up on eBay I had difficulty finding the same one shipped from China. Most of the listings are for US sellers so designed to work on 110V, the standard in the US.

Thanks for the present suggestions and advices, the project is on the way. I sort of think the stuff will need some cooling. There's no space for axial fans as I see the problem, but since the body is tall enough some blowers may work well extracting hot air from the inside and ejecting it upwards like chimneys, this would force fresh air to go in from below around the wing nut.

Fan orientation compels me to use ball bearing units. I've picked a couple possible choices:

Any feedback for these fans? Hope they still move enough airflow even if nicely undervolted.

Cheers ^:)

Hi Bark, I don’t really trust the calculator because it doesn’t put the most important aspect of a heatsink: surface area. Big chunk of metal will only cool the heat source to certain equilibrium temp. The heat needs to get out.
Looks like the formula assuming a perfect sphere surface area?
Smaller volume with more surface area will cool faster. Forced air convection can further save weight and size tremendously at the expense of extra power required by fan or pump.
For long life maintenance free operation, passive cooling is always the best. If you want to go with forced cooling, in my opinion, intermittent air pumping is much more reliable than fan.


Finally decided to go for a fan cooled setup. My previous fan arrangement hesitations have been discarded. Decided to set 4x Delta 5020 blowers just on top of the base heatsink (the drivers will go somewhere else), sucking air through its fins and blowing it sideways. At 100 × 100 × 18mm is a perfect fit. Will very slightly undervolt the fans to ensure them being barely audible even in silence.

I've recently made a final diagram, the thing is gonna look ;-) really crowded. Once I have it all setup I may take some photographs for the sake of it. :-D

Cheers :-)

Update for the sake of it.

After some delays here and there, its finally taking shape. The sunflower heatsinks received were in fact ≈∅53 × 22mm (with one of 'em ≈0.65mm :facepalm: short in height). Hand machined the base heatsink the best I could (without drill bank), and this is how it looks after all properly aligned and glued with Tian Mu grey thermal goo:

The central hole is for the base's shaft. The holes on the underside are meant for the blower fans' sucked out air to get through, right now blocked with grey goo remnants but that is to be cleaned. The base heatsink open sides will be enclosed.