Under Cupboard Halogen Lamps don't last long

[JohnW2]

Thanks john, I knew if anyone can explain, it would be you ...

You're welcome. I hope that it was vaguely understandable!

... and as he is in usa maybe hes in bed.

You may need to think a little about which way the earth rotates

I do beleive it prolongs the lamp life, possibly not that much though and do promote people to use there dimmers at 90% but have no facts it works, only what dimmer firms claim, and some claim 50% extra lamp life with just 10% dimming, but i suppose they would be biased.

As I said before, I'm quite certain that it's qualitatively correct, the only question is about the degree of improvement of life expectancy at lower voltage. I haven't done the sums but if Porque's figures are even remotely correct, I would imagine that a 10% dimming (I presume you mean of brightness) would result in a lot more than a 50% increase in lamp life.

Kind Regards, John

 

[Porque223]

Well, I'm glad that issue is put to bed!!!

 

[bernardgreen]

The thermal shock ( the rapid change in temperature ) to the filament when the lamp is turn on and off is a major factor in lamp life duration.

With a coiled filament the magnetic effect of the high in-rush current creates a mechanical shock as well.

Filament indicator lamps were often kept glowing dull red when OFF by a resistor across the switch. Life time of lamps was much longer.

When controlled by solid state switches the pre-heated filament had a much lower in-rush current when the lamp was turned ON and this prolonged the life of the driver chips as well as the lamp.

 

[bernardgreen]

I wonder if the vibration from cupboard doors slamming shut has an effect on lamp life duration.

 

[JohnW2]

I do beleive it prolongs the lamp life, possibly not that much though and do promote people to use there dimmers at 90% but have no facts it works, only what dimmer firms claim, and some claim 50% extra lamp life with just 10% dimming, but i suppose they would be biased.

As I said before, I'm quite certain that it's qualitatively correct, the only question is about the degree of improvement of life expectancy at lower voltage. I haven't done the sums but if Porque's figures are even remotely correct, I would imagine that a 10% dimming (I presume you mean of brightness) would result in a lot more than a 50% increase in lamp life.

On reflection (and having now done some sums), that 50% might not be far off - I had forgotten how little reduction in voltage would be necessary to get a 10% reduction in brightness ...

...if one assumes Porque's figure of brighteness being proportional to voltage to the power 3.5 (which sounds very credible), and if the dimming were achieved simply by reducing voltage (it's obviously more complicated than that in practice), then a reduction in voltage of only about 2.76% would be needed to get a 10% reduction in brightness. Using Porque's 'life expectancy' figures, that would translate to an increased life between about 39% and 46%.

Kind Regards, John

 

[JohnW2]

The thermal shock ( the rapid change in temperature ) to the filament when the lamp is turn on and off is a major factor in lamp life duration. ... With a coiled filament the magnetic effect of the high in-rush current creates a mechanical shock as well.

Yes, I don't think that has been mentioned, and it's probably the most important consideration of all. Filament lamps (and, indeed, many other electrical and electronic things) almost invariably fail at 'switch on', in the case of filament lamps almost certainly due to the very high 'inrush' current due to the low resistance of the filament before it heats up. Over the decades, I must have witnessed the demise of 1000+ filament lamps, and only a tiny handful of them have just 'died' whilst switched on and operating normally - nearly all have failed at switch-on.

Filament lamp life (at a given voltage) is therefore crucially dependent on how frequently it is switched on from cold, and similar consdierations apply to many other things.

Kind Regards, John

 

[westie101]

I was talking to an instructor from a flight training school yesterday. Noticed that their aircraft have been fitted with LED landing lights, so commented on it.

He told me the fitting of a single light to that type of aircraft was approx £300 but they should recoup the cost within 2 years.
Owing to the high risk of failure they were changing the filament bulbs every 50 hours to avoid failiues that would ground the aircraft.

 

[JohnW2]

I was talking to an instructor from a flight training school yesterday. Noticed that their aircraft have been fitted with LED landing lights, so commented on it. He told me the fitting of a single light to that type of aircraft was approx £300 but they should recoup the cost within 2 years. Owing to the high risk of failure they were changing the filament bulbs every 50 hours to avoid failiues that would ground the aircraft.

That makes sense. Filament lamps are inherently fragile, particularly in what I assume is a high-vibration environment and, of course, failure is not particularly predictable.

Kind Regards, John

 

[JBR]

As has been mentioned, bulbs blowing at switch-on is really quite common. Of course, I have the same problem.

Having read through this thread, wouldn't it be possible to produce light switches that, when switched on, gradually increase the current over a second or two to full value?

Yes, I know, dimmer switches! But when we had a dimmer switch in our living room I would routinely turn it on from the 'low' setting, yet bulbs would still blow at switch-on.

 

[Porque223]

Owing to the high risk of failure they were changing the filament bulbs every 50 hours to avoid failiues that would ground the aircraft.

There is an aircraft level of safety, so-called six nines, which is a 99.999% likelihood that the aircraft will not fail during its mission length.
In principle you can work backwards from this 50 hours to figure the mean time between failures of these lights.
If the bulb MTBF is 50 hours there is only a 1/e (.37, 37%) chance of succeeding for a 50 hour mission and a 90% chance for a 5 hour mission.

 

[JohnW2]

Having read through this thread, wouldn't it be possible to produce light switches that, when switched on, gradually increase the current over a second or two to full value?

Anything is possible, and that certainly would be, but it would be appreciably complex (and something more to go wrong). It might actually be more practical to incorporate some 'soft start' functionality within the lamps themselves. However, lamps are relatively cheap, so (unless reliability is a crucial issue, or lamp replacement very difficult) there's probably a limit to how much complexity/effort/cost could sensibly be justified in order to reduce the frequency with which they have to be replaced.

Kind Regards, John

 

[bernardgreen]

The wave form produced by most dimmer has a sharp change of voltage where the sine wave is chopped 100 times a second. This fast change of voltage and hence current in the filament causes a sharper magnetic effect and movement in a coiled filament. This mechanical vibration may reduce the life of the lamp.

 

[JohnW2]

The wave form produced by most dimmer has a sharp change of voltage where the sine wave is chopped 100 times a second. This fast change of voltage and hence current in the filament causes a sharper magnetic effect and movement in a coiled filament. This mechanical vibration may reduce the life of the lamp.

I can't argue with that as a theoretical proposition - but any such effect would be at least partially counteracted (in terms of lamp life) by the fact that the average voltage and current were reduced. I haven't got a clue which of thsoe effects would win with relatively modest degrees of dimming, but I suspect (maybe wrongly!) that the reduced voltage/current would win with high degrees of dimming.

Kind Regards, John

 

[JBR]

The wave form produced by most dimmer has a sharp change of voltage where the sine wave is chopped 100 times a second. This fast change of voltage and hence current in the filament causes a sharper magnetic effect and movement in a coiled filament. This mechanical vibration may reduce the life of the lamp.

Thanks. That explains why my dimmer didn't help.

I suppose the cheapest method would be to manufacture and supply 'over-rated' bulbs. That is, the filament made just a little thicker. I'd be happy to put up with slightly dimmer bulbs that don't blow as often. Perhaps there could be two (thicker) filaments in parallel per bulb to make up the difference in light output.

 

[333rocky333]

There is an aircraft level of safety, so-called six nines, which is a 99.999% likelihood that the aircraft will not fail during its mission length.
In principle you can work backwards from this 50 hours to figure the mean time between failures of these lights.
If the bulb MTBF is 50 hours there is only a 1/e (.37, 37%) chance of succeeding for a 50 hour mission and a 90% chance for a 5 hour mission.

John W2

When you get a minute