1.5W From A 1206 Resistor - China's Found A Way

[killingtime]

I bought some 7" LED bar fog lights from China. They were cheap but I thought there's not much that can go wrong with them. Some LEDs and a resistor right?

Noticed that the metal housing wasn't getting hot while they were on. Not a good sign as it means the LED heat wasn't making it to the finned housing. Decided to do a tear down and look at what was going on. There are 3 1W 'bead' LED emitters in series with a 1206 SMD 12 Ohm resistor, on an aluminium PCB (x2 to make a 6 LED array). No thermal paste on mine...

These emitters run 3.4 Vf @ 350mA, so that's 10.2V @ 3.5W. Working backwards from the resistor value, that's a design voltage of 14.4v, which makes sense as car alternators charge a 12v lead acid battery at this voltage, except the power dissipated by the series resistor is 1.5W. That's 6x the rated power (for a 1206), but it works. Tested for 24 hours on a variable supply (after I added thermal paste).  The issue is to get to the LED PCB you have to destroy the polycarbonate cover. It's siliconed in and so thin it just breaks, so not a good buy. Many reviews of these style of lamps failing at random as well on the usual websites. I'll bet it's the thermal paste. China do a 7" version of this light that can be un-bolted and re-assembled for another few $. That's the one you want. Stay away from anything that looks as though it's glued together. It can't be serviced.

[ebastler]

Well, 1206 resistors rated 2.4W are available, e.g. the Vishay RCP series. I don't think they expect you to use thermal paste; heat conduction is via the terminals, and there is a package option with larger terminal area on the bottom of the resistor. Although that's most likely not what your fog light uses, since those resistors cost actual money...

Once you get proper heat transfer from the LEDs and the resistors to the aluminum PCB, where does the heat go from there, considering the polycarbonate enclosure? 

And, heat management aside: Aren't fog lights meant to be quite precisely collimated, to let them penetrate the fog somewhat? Those bare LEDs and decorative little reflectors can't be great in that respect. You probably need to spend more money to get an actual metal enclosure for cooling, and a more meaningful optical geometry.

[killingtime]

>>where does the heat go from there..
The black enclosure is painted metal. It's not steel as it's too light. Some aluminium alloy at a guess. It's a shame they cheaped out on 2 cents worth of paste. The thing would actually have been useful. Just the front is clear poly.

>>Aren't fog lights meant to be quite precisely collimated..
The unit produces a wide spot with some spill, although not quite bright enough for road use. I think these would have been good on a bike.

[Doctorandus_P]

I bought a similar 12V "car" light with a size of 82x82mm and a matrix of 4x4 LED's and it's quality is also so bat that I'm divided by starting to use it and see how long it lasts, or throwing it in the bin right away.
They appear to be supposedly be water thight, but only about 3/4 of the circumference is glued, so they are not suitable for any wet environment.

But about the power rating of an 1206 resistor...
That is not a fixed number. Those SMT resistors are too small for either radiation or convection of the heat, so that only leaves conduction through the mounting pads. And Aluminium substrate is a quite good heat conductor and heat sink.

And then, when some chinese *&^%$#@! company fully encloses the aluminium heat sink in plastic, then the Aluminium PCB becomes more of a heat spreader instead of a heat sink. Moulding "heat sink ribs" in the plastic enclosure is useless, and pretty much makes it a fake and misleading product overall.

Also, datasheets of such resistors assume mounting on an FR4 PCB and possibly amended with extra copper as a heat sink. It would be interesting to see the real temperature of those resistors (for example measured with a thermal camera).

[killingtime]

The model below is much better. You can unbolt them, and the PCB has a buck regulator on it that keeps the LED array in CC mode as the voltage drops. This one is bolted to a 3D printed bike mount. 10W instead of 6, so a bit brighter as well. Only a few $ more.

[WI_Hedgehog]

The model below is much better. You can unbolt them, and the PCB has a buck regulator on it that keeps the LED array in CC mode as the voltage drops. This one is bolted to a 3D printed bike mount. 10W instead of 6, so a bit brighter as well. Only a few $ more.

Gottalink? 

[killingtime]

Gottalink? 

https://www.aliexpress.com/item/4001162937633.html

Pick the one titled: 1pcs-10led-178mm