LED Downlights. GU10 vs MR16

[SeanB]

I trialled 4 12W Wun-Hung-Lo LED lamps recently, all of which blew up within 2 days of use. The light output was very good (more than the 9W LEDs that I am also trialling), the colour was 'neutral white', probably about 5000K.

The supplier was apologetic and made a refund, however I decided to open up and examine the failure in more detail. In short, the power supplies were under-rated for the LEDs used.

There's a full description and photos here:  www.axtsystems.com/dl2014/led_failure/

I did order the 7W 12V versions of those, they came branded HTTP Solar, but have a single large multichip module on the really poor heatsink. Tested one for an hour and measured, die temp was 100C. As I just wanted them for the led dies and the PSU  board it was no problem to take them apart. Thanks to Frankie for getting them at a good price for me as well. Took the lamps apart ( really poor mounting using the plastic light diffuser to hold the die down on the thin aluminium heat spreader, and using a drop of compound to transfer the heat along with having a milled slot to allow the wires to get to the middle of the sheet to go through the hole) and used epoxy to attach the die to an old P4 heatsink ( no fan needed here with 7W of heat, it was only needed for the 125W it had before) and put the inverter underneath. Now runs at under 40C for both the die and the inverter. The nice thing is the inverter is actually a constant current source that provides IIRC 350mA to the multiple series LED dies under the yellow phosphor blob. want more light use more little blobs up to a point. I have some driving a single die at 1W and some driving a 3 die one at around 3W.

[DrGeoff]

Nope. Current system is all CFL's, with GU10 240V fittings.

I was initially thinking of switching these over directly by fitting LED counterparts. At first I determined that some need to be 9W and others 12W for the replacements (about the same as the current CFL's).

I've trialled a few types, all with GU10 fittings and 3 LEDs (3W and 4W LEDs). The recent batch gave out good neutral light but exploded (document in previous post). Others have lasted over 6 months so far.

After these ones popped I thought I'd ask if anyone had had any experience to compare whether using a single decent 12V PSU for a room, and 12V LED fittings was a better option than using 240V GU10 fittings with a PSU crammed in each (which can get hot) so as to get maximum MTBF for the lamps.

[DrGeoff]

Good to hear about your measurements and retro fitting Sean.
I had to retrofit some wall lamps because they were rubbish electrical/thermal design. These were twin fluoro wall lamps (2x18W) which I retrofitted with some really nice 12W LED strips. I have a document about that as well (http://www.axtsystems.com/index.php?option=com_content&view=article&id=88:radiathermal&catid=48:fixingstuff&Itemid=81) which shows why they were simply exploding after about 6 months use. Also a fire hazard.
I'm kind of getting sick of having to debug and modify some of the crappy junk that is sold on the domestic market to be fitted into new homes. Honestly, there's more poorly designed rubbish out there these days and unfortunately it's not until after the failures start to happen that this is discovered.

[SeanB]

I can tell you why the lamp ballasts in that module failed. Very simply the original design was for 2 PL18/C integrated starter lamps running in series, with a magnetic 36W ballast in the centre along with likely a 4uF 275VAC PFC capacitor. Now they wanted electronic so placed in there a retrofit 2x18W ballast, and went to the 4 pin PL18/C so as to have the 4 wire connection. they did not read the datasheet for the ballast and that it must be mounted in a location where it runs at under 40C surface temperature, compared to the magnetic, with the allowed 80c above ambient to a maximum of 130C surface temperature allowed. Thus the ballast going bang.

I would always though mount those LED bars on a thick aluminium strip which has been ground flat on one side, and use heatsink compound to transfer heat to the bar to help with cooling on a thin backing sheet. the LED solution though probably gives the same light output and about half the power use though.