LED candle bulbs
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. 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, Colin. |
LED candle bulbs
"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. |
LED candle bulbs
On 27/02/2013 10:28, Arfa Daily wrote:
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. Ah, Sounds like the kind of thing they do for infra-red comms type applications where they keep the duty cycle low so they can drive the emitter with high peak power. The detectors are fast enough to see the short pulses so you get a big improvement in effective transmitter power. I didn't think the human eye behaved the same way, but I could be wrong... Still, I guess it's something that'll only get used on mega hidh-end, "ultra-reliable" lights. Probably not something I need to worry about on Chinese cheapies. Cheers, Colin. |
LED candle bulbs
Colin Stamp wrote:
On 27/02/2013 10:28, Arfa Daily wrote: 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. Ah, Sounds like the kind of thing they do for infra-red comms type applications where they keep the duty cycle low so they can drive the emitter with high peak power. The detectors are fast enough to see the short pulses so you get a big improvement in effective transmitter power. I didn't think the human eye behaved the same way, but I could be wrong... Still, I guess it's something that'll only get used on mega hidh-end, "ultra-reliable" lights. Probably not something I need to worry about on Chinese cheapies. The principle lets them use cheaper dies for the same apparent output and life. As the human eye is closer to being a peak level sensor than an average level sensor when light is pulsed, everyone is happy, except those who can see the flickering. -- Tciao for Now! John. |
LED candle bulbs
On Wed, 27 Feb 2013 14:01:21 +0000, John Williamson
wrote: The principle lets them use cheaper dies for the same apparent output and life. As the human eye is closer to being a peak level sensor than an average level sensor when light is pulsed, everyone is happy, except those who can see the flickering. My headlamp strobes on low power, but not on high. Something to beware of when viewing rotating machinery. I reckon it's around 40Hz. |
LED candle bulbs
On 27/02/2013 23:50, Grimly Curmudgeon wrote:
My headlamp strobes on low power, but not on high. Something to beware of when viewing rotating machinery. I reckon it's around 40Hz. Which make is it? - so we all know not to buy one... Andy |
LED candle bulbs
On Thu, 28 Feb 2013 10:41:38 +0000, Andy Champ
wrote: On 27/02/2013 23:50, Grimly Curmudgeon wrote: My headlamp strobes on low power, but not on high. Something to beware of when viewing rotating machinery. I reckon it's around 40Hz. Which make is it? - so we all know not to buy one... One of these http://www.ebay.co.uk/itm/170875795055 If it weren't for that, I'd buy another one, it's an excellent lamp, only spoiled by the usual cost-cutting measures, like a nasty hinge. |
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