6in1 LED Diode Faults

[Don Allen]

I have been using 18x18W 6in1 IP65 LED Wall washers for several years now and have had to start doing repairs, as they are good value for money and worth keeping in service. The photo of a =141092302&filters[recent]=1&sort=1&o=0"]MH LED 2009K 6in1 LED Diode is taken with only the U/V channel driven at a very low level. LED1 is showing some red which should not be there and I have learnt that it is a sign of an LED that is going to eventually fail. There are two strings of nine LED's in these fixtures as they have a 32V power supply. The other LED string shows no red bleed.

 

Most of the faults are caused by overheating as the units are sometimes set up on 44 degrees celcius days and have to be powered up when the temperature is still in the mid 30's, as the show is due to start.

 

These have 6x3W LED die in them, are nominally called 18W LED's.

 

What experience can other repairers offer about troubleshooting 6in1 or other "x"in1 LED's please ? Am I making correct observations and assumptions on troubleshooting LED's ?

 

I used to have some Eaton 3W LED PAR's that had separate RGB LED's and I learnt that if one LED failed in a string, then it paid to replace all of the LED's in that string for reliability. I have been able to replace just the faulty 6in1 LED's in a string, usually only one, then it is a long time before another will fail. Hence the request for other technicians experience with LED fixture repairs thanks.

[timsabre]

High ambient temp is unlikely to cause LED damage - more likely bad thermal coupling of the LED to the heatsink during manufacture.

 

Do the 2 strings have separate drivers or are they connected in parallel? If in parallel, replacing odd LEDs might cause problems, if the forward voltage of the replacement LED differs slightly then one of the strings will get a lot more drive current than the other, this will cause premature failure.

If they have separate drivers (unlikely) then this will not be a problem, in fact you could bridge out the faulty LED colour to get the rest of the string working.

 

[Don Allen]

All of the LED's that I have replaced have had heatsink compound underneath them, but there has not been much heatsink compound between the aluminium backing of the LED board and the body of the wall washer.

 

It is two string in parallel, only one driver per colour.

 

I am using 6in1 LED's supplied by MS Lighting as spare parts.

[timsabre]

You want as little heatsink compound as possible with them screwed down really tight.

 

The thing about forward voltage is all about batch control or "binning", if the manufacturer pays extra to get leds with all the same forward voltage then you'll be ok. Over time sometimes they change the binning or even the supplier so it pays to watch out that the spares you get are exactly the same as the ones in the fixture.

[bigclive]

The number of connections and the tracks in the picture suggest the LEDs may all be wired in series which would make voltage matching much less critical.

[timsabre]

I think they're wired in 2 parallel chains of 9 leds. I've had trouble before where I swapped one led in one of the chains with a spare LED from a different batch. This had a lower forward voltage which caused the other side not to light, and the side I'd replaced to get twice the intended current.

[electronicsuk]

I think as you've already identified, the biggest enemy of these things is heat. I repaired some similar Miltec RGBW pars recently and the contact between the heatsink and the bonded PCB from the factory was appalling. Minimising the amount of thermal paste between surfaces is a good thing to an extent but in some cases there aren't enough screws between the PCB and heatsink to ensure even pressure and therefore good thermal contact all round, so judicious application of the paste can be required.

 

The biggest help, if you don't already have one, is a 100W soldering iron with a wide chisel tip. This should allow you to heat all the pads on one side of the LED simultaneously, making lifting them off incredibly easy. Even with the heatsink still attached, a 100W iron will have the solder flowing in a few seconds.

[Gerry]

Slightly changing the subject but is Thermal adhesive tape as good as thermal paste?

Wondering whether I could use the tape to stick COB led s to a heatsink.

Cheers

Gerry

[electronicsuk]

Slightly changing the subject but is Thermal adhesive tape as good as thermal paste?

Wondering whether I could use the tape to stick COB led s to a heatsink.

Cheers

Gerry

 

I think it's generally worse but it should be easy to find out, just compare the thermal resistance values between the two, usually degrees C/watt w/mK, the higher the better. Personally I've used a thermally conductive adhesive paste in the past as it holds the emitter and makes it much easier to solder.

[Gerry]

The adhesive paste is rather permanent.

I thought the tape might be easier to use, and less messy than normal paste, for testing purposes before the COB is fixed in it's final position.

Cheers

Gerry

[bigclive]

I think they're wired in 2 parallel chains of 9 leds. I've had trouble before where I swapped one led in one of the chains with a spare LED from a different batch. This had a lower forward voltage which caused the other side not to light, and the side I'd replaced to get twice the intended current.

 

Darn! That does make things difficult.

[DrV]

If each chain has an individual series resistor that will help to even things out

[electronicsuk]

The adhesive isn't as bad as you'd think. Certainly, for small emitters (circa 12w) they're very easily removed by gently prising away with a screwdriver and no damage is left to the board. Larger emitters could be more problematic. Without checking, I believe the product I use is Electrolube TCOR. However, their website also lists a TCER which apparently has a lower bonding strength and may be what you're looking for.