LED lighting

Can't find the recent discussion on this topic, but spurred on by it, I bought a reel of LEDs with adhesive backing, running off 12 volts and trimmable to length. This was because one of my basement fluorescents packed up, with a faulty fitting. A new one is £18 and tubes about £5. These LEDs cost £7 for 5 metres (600 in quantity, or about 1p each) I cut off 600mm to go under the shelf over my computer, fed it with 12 volts from a redundant 'wall wart', and Shazam! Let there be light, and there was light, and Lucifer was consigned to the outer darkness. Now, simple unrectified/unregulated power supplies may cause the LEDs to flicker when looking directly at them, but the reflected light appears steady. As LEDs are diodes, they only conduct every half-cycle of the mains, so appear to flicker at 25 Hz. Rectified power supplies, which provide DC, should not have this problem.

Geoff

I must be doinfg something wrong, so some clever bloke tell me what it is.

LED strip light on e-bay quotes 14.4 watt/metre consumption and 420 lumen/metre output. That is 29 lumens/watt. The wall wart will not be 100% efficient either, some are dreadful.

GE fluorescent tubes quote 2850 lumens for a 36watt tube. That is 79 lumens/watt. seems to me I'm better off sticking to tubes, but others rave about LED so where am I going wrong?

Various reasons, some are:

The difference of about one stop is not huge (2:1 ratio) and the eye will probably adapt to the difference

LED 'white' light is very blue and all in the visible while some fluorescent light is in less efficiently detected parts of the spectrum.which can create the impression of greater visual brightness.

LEDs throw all their light down and out, while the tube sends half of it up to bounce off the luminaire or ceiling, losing a fair percentage in the process unless you have an expensive mirrored luminaire

Neil

What Neil said. It's like the difference between a plain bulb and a spotlight. Checking with my photographic lightmeter reveals an illumination difference of 4 x between the 4 foot fluorescent tube and the LED strip, with the tube light being brighter. However, the eye perceives little difference (on a logarithmic scale) the LEDs are mechanically robust, and consume far less energy, besides costing less in capital outlay. If you need more light, a second or third etc., parallel strip may be fitted for a few more pennies.

Geoff

Neil, do you mean these?

https://www.youtube.com/watch?v=riYfs7yz5AI

Geoff

Hi Geoff

Have you mounted your LEDs to a heat sink? I made some under cupboard lights out of LED strips and they deteriorated overtime which I put down to heat, The ones for my workshop I mounted on an aluminium stip and they seem to have lasted better.

Cheers Michael

Indeed, heat (temperature) is the enemy of the LED, so most high power LEDs are mounted on IMS. And the general rule for electronic components is that every 10C rise halves the life and doubles the failure rate.

The flicker will be at 50 Hz with half wave rectification. With full wave it will be 100 Hz but at a much lower level.

Russell.

Michael, no.

After being ON all day, they are still cool to touch (Stuck below a wooden shelf) whilst the supply is only warm. If it proves to be a problem, replacements are quick and cheap, or aluminium strip can easily be inserted.

Geoff

I'm thinking of trying these for a machine light - on a mill. I'm happy with the one I've mentioned before on the lathe.

RwAAOSwe7BW0Blx">http://www.ebay.co.uk/itm/Black-30W-1200LM-U2-Motorcycle-Car-Bike-Motorbike-LED-Laser-Fog-Spot-Headlight-/262308258436?hash=item3d12ca6284:gRwAAOSwe7BW0Blx

These because I know from others that the "glass" needs to be shaped in that sort of fashion to get an even pool of light and with luck they may have done it properly.

The box they come in states 12-60v AC or DC. Actual consumption 6 to 9w. Construction is interesting. The back screws off and it looks like the led is positioned somehow and the potted in clear epoxy also forming the lens. Then some black epoxy on top of that.

People often talk about lumens but there is a need to get one's head around these 2

**LINK**

**LINK**

and what we are really interested in is the illumination levels at the "surface" we want to light = lux. Frankly I don't think that a lot of Chinese led strips or leds are rated correctly. They put out a lot of blue and the eye sensitivity factor isn't accounted for which makes them sound brighter. The 3 colour output can make measurements difficult as well. A meter might record the blue completely where as out eyes are not very sensitive to it.

John

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Sorry, Russell, yes.

John, white LEDs are actually three differently coloured devices in one package, possibly with two blues, since they need to seem brighter, as stated, making four. Then a phosphor is applied overall, which glows white. I will have a look at them using my photographic filters, to see if anything is revealed. NB, this is off the cuff, I haven't checked it for accuracy.

Geoff

Hi Geoff,
Are you sure that ALL white LEDs are made from three coloured LEDs ? I was under the impression thet white LEDs were actualy UV LEDs with a coating of phosphor to convert the UV to white light.

Les.

Les, I found this just now.

https://en.wikipedia.org/wiki/Light-emitting_diode

Geoff

Geoff,

Count the connections to the LED ... If you only have +ve and -ve, then it's almost certainly as Les says.

A short wavelength Blue emitter, which activates a yellow fluorescence in the phosphor. ... The combined light is approximately white.

RGB packages have more connections.

MichaelG.

They aren't exactly UV led's and a phosphor as I understand it. The blue is sometimes called dental blue which is getting towards UV.

This is what tends to come out of a typical higher than usual power led - it's on the wiki page linked to

The blue is what excites the phosphors for the other colours. I've not seen any with a blue phosphor but have seen then with an extra green and an orange. The blue is often rather high in order to obtain a higher colour temperature and has to be because our eyes are not very sensitive to it. It's a lot lower once the colour temperature is down to around 5000k. It's still a spike even then but usually around the level of main colour hump or lower.

Just add that sometimes LED's and lights are available in more than one colour temperature and the lower colour temperature always outputs less lumens but is also often more efficient. All down to our eyes inefficiency at the wavelength of the blue used. As the excess blue ones had a short term effect on my eyes I wont go near them any more and even sometimes wonder about PC screens but suspect they use a different blue.

John

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Edited By Ajohnw on 27/08/2016 09:51:26

Edited By Ajohnw on 27/08/2016 09:52:06

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Nice exposition of what I said, John

Posted at the same time Michael.

If anyone is curious about led's this site has good data sheets - makes very good led's too.

**LINK**

They even make one now since last time I had an interest that is said to dim following black body radiation patterns, sic. Interesting to see what the blue does to achieve that and no way is it the same as black body radiation really.

John

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"Great minds ..."

MichaelG.

Well now, Michael, the LED packages are two-terminal. The photographic filters reveal one main light spot in the centre of each package using any colour. The LEDS are on a reel, probably soldered & mounted automatically. Obviously, the diodes must be the same way round, ie anodes together, cathodes together. So, they are probably as Les said. Thank you, Les.

Geoff

Possibly unlikely in this case Geoff but other things in the led package may make it happy with ac or dc either way round. it doesn't take much in them to do that. They are very likely to have something in them to limit the current each one takes.

John

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Edited By Ajohnw on 27/08/2016 20:48:38