As subject... my old LED lamps in the ensuite ceiling had 'chimneys' (lengths of 4" ducting) poking through the rockwool insulation, to ensure they didn't overheat.
Given that the new replacement GU10s only draw 4W, versus 35W with the old halogens, is it OK to do away with venting? Or is it more about promoting longevity of the LEDs now?
Not sure how the relative power of the two lamps translates into heat output- obviously the light output is way different. Does it make any difference if the LED lamps are 240V rather than 12V? Logic tells me the higher voltage ones will give out more heat?
It's the other way around, but the key difference is that a GU10 can run at almost 300C, whereas an LED will be killed by running much above 100C case (125C junction), and will have short life unless somewhat cooler. So if we look at the temperature rise above, say, 25C @ ceiling, that's 275C verses 75C, or a difference of almost 4x.
So to a first approximation, if you don't change the cooling of the fitting, max power of an LED you can run is about 1/4 of that of a filament GU10, and if you want full life and efficiency, you want to go with even less than this. If you reduce the cooling, then the LED power dissipation will need to be even less still.
I suppose it really depends on how efficiently the voltages are dropped and of course the number of leds andwhat sort of thermal mountings they are on. I suspect some of the cheaper ones are not as efficient, or not as robustly built, but its still early days yet. Is there any reason why you cannot leave the vents there then?
That is all well and good but one also needs to know, assuming the psu is internal, how hot it makes the leds, and whether the design of the bulb is good in that it allows efficient cooling of the leds. Brian
The power dissipation specified includes power lost in the internal power supply.
Thermal design is one of the most difficult parts of designing a power LED - how to get the heat transferred out from the LED chip and dissipated from the case, keeping the LED as cool as possible. GU10 and MR16 form factors are about a bad for this as could have been designed (specifically designed to run hot enough that much of the heat is radiated away), so retrofitting LED designs into them is not at all optimal.
Can do; and looks like I probably will: just that a fully integral blanket of rockwool's got to be more effective, and I have sometimes noticed a draft coming down through the old downlighters.
But they also tolerate that heat at high temperatures very much better and a fair amount of it gets away as thermal IR radiation.
Rubbish! They *HAVE* to be on a decent heat sink to survive at all!
About half the power that goes into them ends up thermalised. Some of the LED dies have output light intensities high enough to degrade the yellow phosphor and darken the plastic if the heatsinking is inadequate. (it is one of the ways that cheap ones fail or degrade with time)
Basicaly a quartz halogen lamp can survive having an envelope temperature of 300C and still work fine. It might set fire to your ceiling but the bulb will be entirely happy with that arrangement.
An LED bulb will suffer shortened life from electrolyte boiling in its capacitors if the ambient gets much above 110C. Even with a proper heatsink and in free air they still cook their PSUs if hung in a conventional room lamp socket with the electronics above the LEDs.
Basically you need free airflow for CFLs or LED if they are not to cook their own electronics into oblivion.
The DC PSU is at least 90% efficient (maybe nearer 95%) the current generation of LEDs are about 40% efficient and the rest is hot air.
OP would be wise to leave the air vents in place for cooling or expect to replace the LED bulbs very frequently indeed.
It's worth noting that halogen lamps need to run the envelope at, IIRC, 250 dec C _minimum_ in order to allow the halogen cycle to function. Normal glass envelope material softens at these required run temperatures, hence the use of 'quartz glass'.
It's also worth noting that the function of the halogen cycle is to allow higher filament temperatures _without_ blackening of the glass envelope due to the resulting evaporation of tungsten onto the inner wall of the envelope. The halogen 'recycles' the tungsten back onto the filament (unfortunately, not onto the parts it originally came from) provided the bulb temperature remains above c 250 deg C.
The 'life extension' arises out of the avoidance of premature reduced luminous efficacy due to build up of tungsten on the envelope rather than making the filament last longer.
Yes, the wattages might be a lot lower than for a filament lamp of similar luminous output but the maximum temperature is going to be a good 150 to 200 deg lower which reduces the thermal gradient which means reduced power dissipation for a given surface area.
It's the need to dissipate the heat at the lower upper temperature limits of LEDs that restricts the luminous output due to this temperature constraint of the LEDS themselves (there's a limit to how much wattage can be safely dissipated via the confines of a "Bulb Shaped Lamp".
CFLs can tolerate much higher discharge tube temperatures by using a mercury amalgam to optimise efficiency at these relatively high temperatures compared to the normal running temperature of a linear fluorescent tube which can use plain mercury vapour which doesn't need to be run up to its optimum temperature in most indoor lighting situations.
The luminous efficacy of mercury amalgam at optimum temperature is slightly lower than ordinary mercury at its optimum temperature. the benefit comes when the mercury amalgam has run up to a temperature where ordinary mercury lamps would suffer a considerable drop in efficiency. The penalty is the famous 'run up time' required for CFLs to reach full brightness (around half a minute or so).
Funnily enough, ISTR that mercury vapour discharge lamps had a similar efficiency figure (not sure whether this was purely for the UV emmision spectra or whether it included the conversion losses of the phosphor coatings used to convert their output into visible light).
The lamps may be dissipating a lot less heat but they have a much reduced temperature range within which to achieve the required dissipation.
Even a 5W LED bulb benefits from being in a ventillated fitting. My one and only 'suck it and see' purchase from Asda (at a bargain price of a fiver) seemed to be suffering when fitted into a wall light fitting that had no such cooling vents so I transplanted it into a desk lamp which had such vents to allow it to safely use a 60W tungsten filament lamp. I could probably get away with a 10 or 12 watt LED lamp in the desklamp fitting (but not the wall light fitting).
I have had a fair few and of the ones I have tried the Philips replacement spotlamps are the most convincing by far. You cannot tell them apart from real incandescent spotlamps apart from the fact that they don't fuse the entire lighting circuit if one blows. And so far touch wood they have been well behaved in the kitchen 1 year and counting. I think you can (could) get them for about £8.
Apparently they were the end of line the new ones are £26!!! :( But dimmable. (and don't look as nice on Amazon)
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