"Colin Stamp" wrote in message
...
On 27/02/2013 01:07, Arfa Daily wrote:
"Colin Stamp" wrote in message
o.uk...
Hi all,
I tried out some Ikea LED candle bulbs today, wondering if 90 lumens
would be enough:-
http://www.ikea.com/gb/en/catalog/products/30217927/
I thought I was impressed. 90 lumens turned out to be a little bit on
the dim side, but they look quite reasonable in the fittings, which were
intended for halogen candles. The light comes out in about the right
places IYSWIM, The colour is fine and they're cheap.
Then came the realization. The sodding things strobe really badly. Now
I've noticed it, I can't get away from it. Anything that moves in the
room turns into bloody set of freeze-frames, as does the whole room
every time you move your eyes!
Is this a common thing with cheap LED bulbs? I'm all worried about the
impending Ebay purchase of Chinese replacements now. The only other LED
bulbs I've got are two brands of GU10 and they're fine, so maybe it's
just Ikea ones that are crap...
Cheers,
Colin.
The way to get the maximum brightness and longest life from LEDs, is to
pulse drive them. They're actually not very good at handling high
frequencies, so they tend to get driven at fairly low pulse rates, and
that's why you see the strobing. It's been driving me mad for years with
LED car tail lights where the situation is even worse because they use
the fact that they are pulse driving them, to open up the mark-space
ratio to something like 50% when they are a tail light. This gives the
impression of half brightness. When the car owner treads on the brakes,
the pulse drive tightens right up to give high brightness, but the
strobe effect becomes less noticeable. Those lights that they stick on
the top of traffic cones are also LED based, and also strobe for the
same reason. Possibly, the better quality LED lamps like your GU10s, use
better quality LEDs that can be driven at a higher pulse rate, or maybe,
they are DC driving them ?
Arfa
Are you sure that's right? I've made a few things with high power LEDs
and I've never seen it mentioned on a data sheet, either for the LEDs
themselves or the dedicated driver ICs. Everything is quoted for
steady-state current. Where PWM is mentioned (e.g. for dimming) the data
sheets normally suggest that the actual output to the LED is smoothed to
prevent strobing.
I got it originally from a long paper on driving LEDs for best efficiency,
from one of the big manufacturers' websites - though I'm struggling to
remember which one now, but it was one of the big boys like Cree or Osram or
Kingbright. As far as I remember, it had to do with light output from the
die being proportional to current, but if you tried to feed it with a steady
current that was high, heating became an issue, and that this led to fairly
rapid deterioration of the light output. By pulse driving, you could force a
current of several times the maximum steady state current, through the die,
producing very bright 'flashes' of light, without the problems of heat
deterioration. This, I recall it saying, could lead to 50% lifetimes of over
100,000 hours, compared to as little as 10,000 from DC driving.
I'm sure that I downloaded the paper and archived it somewhere. I'll see if
I can find it.
The Ikea ones all pulse in unison. I haven't measured the rate, but I'd
put a lot of money on it being mains frequency. Just cheap, absolutely
minimal drive circuitry, I reckon.
Cheers,
Very possibly ...
Arfa
Colin.