LED spotlights

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Does anyone have experience with fitting LED's in place of low voltage spot lights?
I'm doing a fair bit of house refurbishment and am attracted to LED's which use less power than incandescent spot lights. They appear to be made up of clusters of single LED's in a unit which replace the original spot bulb.
Can these be dimmed, or is it possible to turn off the individual LED's in a cluster to achieve the same effect?
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I tried some of the mains GU10 versions of these and was rather disappointed. One failed within a week and the other 2 lasted a couple of months. I may have been unlucky as the supplier maintained they had not experienced similar failures elsewhere. However if you look in showrooms you will often see ones where some of the LEDs have failed - I was in B&Q the other day and every one on display was like that. Also, you should be aware that for example, with the GU10 variety which I think was about 2 watts, they are equivalent to a 20watt incandescent so they are relatively dim. I don't believe they are dimmable - and certainly not individually switchable. I'm sure they will be good in the fullness of time - I certainly hope so. In the meantime I'm using the low energy fluorescents for the GU10 fittings
HTH
Chris O
"Andrew Vevers" <http://www.environmentalconsultants.co.uk wrote in message

spot
a
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made
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LEDs aren't dimmable, it's all or nothing. Interesting idea - being able to switch off individual LEDs. Not sure they offer that yet.
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LED's
spot
LED's
WRONG - I once had an LED alarm clock whereby you could adjust the intensity of the 7 segment LED digits !!!
Interesting idea - being able to

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voltage
be
intensity
You sure they weren't LCD digits?
Diodes are either on or off, you cant alter their intensity.
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original
No, they were LEDs ! - This was long before LCDs were available (>20 years ago)

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to
to
individual
Basically true - but you can vary the forward current which does change the intensity.
LJ
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re : dimming of LEDs ... the LED itself will vary slightly depending on the forward current, if the current is not pulsed. The normal method of dimming is to switch the LEDs on and off very quickly, varying the ratio of 'on' to 'off'. Known as pulse width modulation. It's used in aircraft at least for varying the LED backlights and switch illumination from "so dim you can't see it unless it's really dark all around" to "readable in direct sunlight" which is pretty bright (lots of realtive contrast and stuff I know) So LEDs can be dimmed, just needs a suitable controller. No doubt they'll be on the market sometime for household lights. In the meantime, try googling or sci.electronics.basic for suggestions. hth Neil
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    "Andrew Vevers" <http://www.environmentalconsultants.co.uk writes:

However, they aren't any more efficient, so if you fit a 5W one, you will get same light as a 5W spotlamp. Their only benefit is if you want a very narrow spot beam, or if you want one of the coloured ones. They can't be made to produce as much light as an incandescent spot because they would get just as hot, and LEDs are destroyed by such high temperatures. LED technology is improving, but the highest efficiency LEDs are far too expensive to use in such lamps.
--
Andrew Gabriel


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We tried them at work in our rest area. Straight swap for GU10s. They are OK for accent lighting but not very good otherwise. I have used Cold Cathode lamps at home. They use the same power as LEDs but get brighter the longer they are on. No they are not dimmable and added side effect is they run cooler to the extent that they can be removed by hand and they are barely warm. More of the energy used is converted to light not heat. The downside is they cost 9.99 trade price but should run upwards of 10 years before they need replacing.

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Many thanks for all responses on this. I'm at a stage where I'm about to have several rooms rewired and I'd planned to have ceiling spot lights with what I've assumed to be low energy LED's. I'd prefer not to have conventional incandescent spots, but once the fittings are in I'm stuck.
Given the price of the new LED's I can see it might be a costly mistake.
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An LED's brightness can be dimmed entirely from reducing its current, no pulse width modulation is required.
LED's are hugely efficient in comparison with all other light emitting devices. A typical LED will have a forward current in the region of 20 - 30 mA (milli amps) with a PD of around 2V. Also they are so efficient that they generate 2 tenths of bugger all heat. They are so efficient because there is no filament, it's just a very tiny current passing through a gas that glows which has a huge resistance in comparison to a usual 'bulb'.
The only problem is that they traditionally don't provide such a wide wavelength of light as a bulb, or as much of it.
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Yes. There are reasons to drive LEDs with pulse width modulation, but it's not because the LEDs require this.

This is just so wrong. Bog standard LED's are right down the bottom of the efficiency scale, right alongside filamant lamps. The highest efficiency ones don't even match the efficiency of fluorescent tubes, and are incredibly expensive.

There's no gas in them -- I wonder if you meant gallium arsenide (GaAs)?

The white ones are all fluorescent and could in theory produce any of the same colours as can designed in a fluorescent lamp. However, the manufacturers always push them towards the higher colour temperatures, as that gives them the higher lumens/watt figure in a market where the lumens/watt figure is considered a competitive advantage.
The reason they can't be made to give out as much light as a filament lamp is that being only the same efficiency, they would get just as hot. However, the semiconductor junction is destroyed long before it reaches the temperature of a filament lamp, so it has to be operated at a much lower power than the equivalent sized filament lamp.
Where LEDs have some potential to win is where you want a very narrow beam, which they can do without the use of lossy reflectors and unwanted light spillage you would get from a filament lamp, and where you want one of the native LED colours, which they can do without the filtering losses you get with a filament lamp.
--
Andrew Gabriel


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Why is it then that so many cars (tail lights) and traffic lights are starting to use LED clusters?
I doubt a filament bulb would even glow using just 20mA at 2 volts - 40mW
Scuzzle.
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I believe that LEDs are physically more robust than filament bulbs, so may last longer on vehicles. Also cost (manufacture, installation) is probably an overriding factor. The LED needs to get the 2V from somewhere - with an efficiency of 70% there will still be some losses to get from vehicle 12V/24V supply to the LED power. Presumably the tail-light uses several LEDs in a cluster, so uses more than 2V 20mA ... I don't think that efficiency is very high on the priority list for vehicles. Reliability, safety, cost perhaps more so.
One of the reps for LEDs told me that they are used extensively on railway crossings simply because they withstand gunfire (small bore amateurs) whereas the old filament bulbs would smash (thus showing a target shot) and need to be replaced every few days.
regards, Neil
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there
what !!!! ?
Presumably the tail-light uses several LEDs in a cluster, so uses

and
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wrote in message

I refer to the vehicle supply of battery 12V or 24V, which must be reduced to the current limited 2V or so for the LED itself. If six 2V LEDs are connected serially to 12V, operation is not optimal. Usually there is some form of circuit which provides the LED with the appropriate power. The losses in this power supply circuit may be in the region of 30% of the power provided to the LED. regards, Neil
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the beams are incredibly narrow -- i'm sure you must have noticed !!!
CG

starting to

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One of the big problems with LEDs is that they have a wide viewing angle most LED lights for cars and general indication are formed by using clusters of LED with a device called an ODD (optical deflection device) glued to the front to focus this light forward. LEDs typically have a life span of about 8 years before the colour of them starts to degrade. Degradation can also be because by over driving them, ie allowing too much current to flow through them. The main issue with CAR lights, and the reason that we have only just seen them in full production in the past five years is that the colour of the LED is provided by doping the silicon. The get the right RED car light has been difficult to consistantly acheive.
The LED light is becoming the customers choice over the filament lamp mainly because of its superior life span and its robustness in mechanically challenging enviroments.

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