LED lighting

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Excellent point, and a very good reason not to buy cheap ones of dubious origin. Ditto CF lamps. At least with a GLS lamp (General Lighting Service, ie incandescent) there is very little to go wrong or combust.
I had a CF lamp explode a few years ago. Blew the side out of the ceramic base when I switched it on. Went with a hell of a bang and sent a shower of sparks and other smoking debris everywhere. It also destroyed the lampholder. I used to have one on a timer to come on when I was on holiday, but after that I went back to a 60w bulb!
I bought a couple of LED spots from Homebase the other day - SES fitting allegedly equivalent to 35 watt halogen. Neither lamp worked in 3 different fittings. The fittings work with other lamps and the contacts seemed to be correct sizes. Took them back to store and tried them in one of their fittings and behold - there was light.
Now baffled as was neighbour, who is an electrician. One suggested possibility was that my fittings had the wiring reversed, which would make no difference to halogen or CFL but might to LEDs which require DC. But can this be correct since the LED lamps must have circuitry to convert AC to DC? And doesn't the AC supply reverse polarity about 50 times a second anyway, so how could it make a difference?
I have a couple of LED lamps with ES fitting working OK and a couple of GU10s.
My voltage tends to be a bit high - I have seen it at 251v but it is currently around 243v so I doubt that is the issue ( and the lamps were undamaged ).

Any clues? I may have to try rewiring the fittings.
Got a product spec, or part numbers?

The design of the internal circuitry should convert the incoming AC into whatever the LEDs need (DC, about 2.5V at quite a high current), and the LEDs won't put up with much abuse if they are to have a decent lifetime.

My bet is that the centre contact isn't quite reaching the terminal in the bulb holder when the lamp is fully screwed in.
No, I took the bulbs back but I'll look on the website.

My thought was the same but comparing the LEDs with the old halogen lamps there was no obvious difference. And as I said I tried them in 3 fittings of 2 different types. Actually I remember now buying the same type of lamp ( but I think a different brand ) from Homebase a year or two back with the same result. I took them back but didn't try them in the store.
Very odd. I'll buy one tomorrow and try again.
My bet is that the centre contact isn't quite reaching the terminal in the bulb holder when the lamp is fully screwed in.

This ^^
I've had it with the odd (not CFL/LED) lamps over the decades. Usually there is something that fouls the holder, like a blob of solder at the top of the brass cap, or the screw thread in one or the other part isn't quite long enough. If the centre contact is bendupable that will often sort it, but I've been known to take a file to things on occasions :)
I'm a bit confused about light output units (I could look them up). Some units measure total light output, others measure the amount of light which falls on a unit area at a specific distance. This is important because if the lamp concentrates all its light output in one direction, the brightness in that direction can be quite high while the total light output is quite low.

Normal old-fashioned GLS (general lighting service) tungsten bulbs radiate their light more-or-less in all directions, while the LED bulbs I have seen are more like spot lamps - creating a puddle of light in the forward direction but providing very little general light. This is how I think manufacturers get away with saying things like "equivalent to a 60W lamp" - yes, but only if you compare it at the spot under the lamp, not the overall room illumination.
The lumen value is the total light emitted. The candela value measures light intensity over some spherical cone. The EU is now obsessed with "useful lumens" for gu10s, only considering those emitted over a 90 degree cone.
I think it means the base of the cone (apex at the light source) is a section of a sphere, with its centre of curvature at the light source.

Useful illumination for a street lamp (for example) is the puddle under the lamp. Useful illumination in a room (like with the standard lamp I have on now and which illuminates the whole room, with light coloured walls and ceiling) is over the entire sphere. A bulb with a 90 degree radiation cone would be bloody useless.
This is the problem I've had with the few changes from 50W halogen to LED I've tried. They both have the same(ish) candela values in the centre of the beam but the 38° beam of a halogen has a much greater spill outside that nominal angle (and more total lumens). Hence the halogen is more of a floodlight whereas the LED is more spot.
When you are using them for room lighting, as opposed to displaying goods in a shop window for example, you need something more like a floodlight. I think a 75W 'equivalent' LED with a 50-60° beam would probably be a better substitute for a 50W halogen ... but when I last looked they don't make them.

If starting from scratch of course you could use more lamps, closer together, to get the illumination.
Can you [lease explain to a numpty how a cone can be spherical?
A spherical cone is constructed as follows :

Take an area A on the surface of a sphere.
Join every point of A to the centre of the sphere.
What results is a spherical cone.

An example is to draw a circle on the earth's surface, join every point of it to the earth's centre, and then cut along those joins and extract it.

An ideal spotlight at the centre would radiate the spherical surface of such a cone uniformly, if you scooped out all the dirt and rock and water.
If starting from scratch of course you could use more lamps, closer together, to get the illumination.
But it would still feel like being in a cave, due to the poor colour rendering index.
Aldi currently have a range of dimmable LED bulbs at £5.99 each, I bought three standard round pearl bayonet types to try in a three-bulb pendant fitting with "470 Lumen" 7W, 40W equivalent) rating (the fitting is not currently on a dimmer by I have plans).

They are branded "Muller Light" (yoghurt??), and there were ES, and minature ES and bayonet, available in various formats including some at 13W (75W equivalent). "Made in China for Aldi Stores Ltd".

Also for £5.99 were pairs of non-dimmable bulbs.

What can I say: I fitted them and they work. Will report on the dimmability when tested. Yes, I will keep an eye on them to make sure they don't burst into flame.