Incandescent lamp AC vs DC

[D Yuniskis]

Hi Greg,

I was just thinking of the old discs you put in a lamp socket to make them last. A
diode I guess. I have to make and modify many lamps and microscope
lamps to convert the AC to DC for light noise problems. If your working

Could you please elaborate on this? ^^^^^^^^^^^

 

[D Yuniskis]

Hi Don,

The way I heard it, more notably incandescents have a slight
trend of having shorter life expectancy with DC than with AC.

--> DC shorter than AC

The explanation is that a very small percentage of the tungsten
vapor near the filament gets ionized by the small amount of UV
produced by the filament, causing a drift of tungsten vapor
towards the positive end of the filament.

That causes the negative end of the filament to suffer slightly
more from evaporation than the positive end.

Of course, there are other reasons why the ends of the filament
may suffer unequally from evaporation:

1. In gas-filled lamps, there is often a convection current. If
the ends of the filament are unequally affected, then one end can
get hotter than the other, or be surrounded by gas lower in tungsten
vapor than the other end, or one end may have more wind from the
convection current than the other.

Suggesting that the actual orientation of the filament
(wrt gravitational field) would exacerbate the problem)?

2. An incident of mechanical shock may have stretched the ends of the
filament unequally, causing the ends to have unequal temperature.

3. One end may have more filament radiation reflected back to it than
the other.

=====================================

On the other hand, if the filament is thin enough and the AC frequency
is low enough, then the filament's temperature can vary significantly
over each half-cycle of AC. Increase of evaporation during temperature
peaks will outweigh decrease of evaporation during temperature dips.
That will cause life expectancy to be shorter with AC than with DC.

--> AC shorter than DC

<grin> I suspect *the* solution is to just invest in a
small sh*tload of spare bulbs and live with the consequences!
:-/

Or, find a solid state alternative and some way to position
it at the correct focus :-/

(sigh) Nothing is *ever* easy.

 

[Don Klipstein]

I thought your plan was to run it on DC anyway.

DC would surely have to be kinder to the filament by removing the cyclic
thermal stress.

Sylvia.

Except that my experience suggests that thermal cycling is less damaging
than most people think.

I can say why specifically for a couple of issues:

1. It is well-known that incandescents often burn out during cold starts.
That leads to reasoning that starting is what does the damage.

However, there is another explanation: The filament evaporates
unevenly. A thin spot that runs hot develops. The higher temperature of
the thin spot causes it to evaporate faster and become thinner at an
accelerating rate.

Such a "fatal hot thin spot" does incur a temperature overshoot during a
cold start, due to less mass and tungsten's positive temperature
coefficient for resistance. Such a "fatal hot thin spot" becomes easy to
melt during a cold start.

However, by the time a filament with such a condition becomes unable to
survive a cold start, its hours are numbered. This bad condition
accelerates worse than exponentially during steady operation.

2. I did an experiment with a thermistor type soft-starting "stick-on"
"button" ("my words") that claimed to double the life of an incandescent
lamp. I found that when it was fully warmed up, it dimmed the lamp enough
to account for 50-60% increase in life expectancy. (I used to post 50-55%
back when I was working with a rule of life expectancy being inverse
proportional to voltage to the 11th-to-maybe-12th power, as opposed to
most likely the 12th.)
Should the device actually meet its claim of doubling life, it only
improves life by 25-33% by soft-starting according to the one experiment I
did.
(In my experiment, current draw was reduced 2%, power consumption of
lamp and device combined was 2% less than lamp alone, and light output was
reduced almost 12% - based on the life-extension-figure that I remember
better. Actual voltage measurements from the experiment I posted in
sci.engr.lighting shortly after the experiment several years ago.)

I am now aware of some railroad crossing flashing signals that put a
resistor in series with the lamps for the first whatever fraction of a
second into a flash, and I have posted before as to my doubts when
someone else posted about their existence. However, I still doubt the
soft-starting gains a lot.

3. Traffic lights - incandescent lamps used in those go through usually a
cold start once every minute. My experience is that incandescents used in
those at least usually have risetime fast enough to indicate lack of
effort at soft-starting.
And traffic signal lamps appear to me to meet their life expectancy in
red and green traffic lights, exceeding for yellow, and to not greatly
exceed their life expectancy in Philadelphia's alley lights where the duty
cycle is about the same as that of red and green traffic signals and the
switching frequency is less by a factor of usually 1,440.
(And yes, I am counting only on-time for operating hours, so a 130V
traffic signal lamp rated for average lifetime of 8,000 hours should
have average life expectancy a little over 4.5 years, and a 120V one
should have average life expectancy a little over 2 years in red/green
traffic signal duty or Philadelphia alley light duty.)

- Don Klipstein ([email protected])

 

[Sylvia Else]

Except that my experience suggests that thermal cycling is less damaging
than most people think.

I can say why specifically for a couple of issues:

1. It is well-known that incandescents often burn out during cold starts.
That leads to reasoning that starting is what does the damage.

I would think that just means that starting is when the lamp is most
susceptible to failure due to the accumulated damage.

Sylvia.

 

[D Yuniskis]

Hi Don,

Except that my experience suggests that thermal cycling is less damaging
than most people think.

I can say why specifically for a couple of issues:

1. It is well-known that incandescents often burn out during cold starts.
That leads to reasoning that starting is what does the damage.

However, there is another explanation: The filament evaporates
unevenly. A thin spot that runs hot develops. The higher temperature of
the thin spot causes it to evaporate faster and become thinner at an
accelerating rate.

Such a "fatal hot thin spot" does incur a temperature overshoot during a
cold start, due to less mass and tungsten's positive temperature
coefficient for resistance. Such a "fatal hot thin spot" becomes easy to
melt during a cold start.

So, your point would be that AC drive tends to disturb where this
spot would want to otherwise develop? But, can that be relied upon?
Or, are manufacturing variations sufficient to render this moot?

However, by the time a filament with such a condition becomes unable to
survive a cold start, its hours are numbered. This bad condition
accelerates worse than exponentially during steady operation.

2. I did an experiment with a thermistor type soft-starting "stick-on"
"button" ("my words") that claimed to double the life of an incandescent
lamp. I found that when it was fully warmed up, it dimmed the lamp enough
to account for 50-60% increase in life expectancy. (I used to post 50-55%
back when I was working with a rule of life expectancy being inverse
proportional to voltage to the 11th-to-maybe-12th power, as opposed to
most likely the 12th.)
Should the device actually meet its claim of doubling life, it only
improves life by 25-33% by soft-starting according to the one experiment I
did.
(In my experiment, current draw was reduced 2%, power consumption of
lamp and device combined was 2% less than lamp alone, and light output was
reduced almost 12% - based on the life-extension-figure that I remember
better. Actual voltage measurements from the experiment I posted in
sci.engr.lighting shortly after the experiment several years ago.)

I am now aware of some railroad crossing flashing signals that put a
resistor in series with the lamps for the first whatever fraction of a
second into a flash, and I have posted before as to my doubts when
someone else posted about their existence. However, I still doubt the
soft-starting gains a lot.

But, presumably, "gains *some*" (or the series R practice would be
foolish)?

3. Traffic lights - incandescent lamps used in those go through usually a
cold start once every minute. My experience is that incandescents used in

Are you sure they really power off (completely)? E.g., that
perhaps they don't keep the filament "warm" though not visible?
(I don't know if operating the lamp at such reduced power
would make a difference -- I have a friend who does that
sort of thing, I should inquire)

Of course, bulb replacement has to be the driving force behind
the rise in popularity of LED traffic lights.

those at least usually have risetime fast enough to indicate lack of
effort at soft-starting.
And traffic signal lamps appear to me to meet their life expectancy in
red and green traffic lights, exceeding for yellow, and to not greatly
exceed their life expectancy in Philadelphia's alley lights where the duty
cycle is about the same as that of red and green traffic signals and the
switching frequency is less by a factor of usually 1,440.
(And yes, I am counting only on-time for operating hours, so a 130V
traffic signal lamp rated for average lifetime of 8,000 hours should
have average life expectancy a little over 4.5 years, and a 120V one
should have average life expectancy a little over 2 years in red/green
traffic signal duty or Philadelphia alley light duty.)

But using filaments designed for higher operating voltages
at lower voltages increases longevity (at the expense of efficiency)
so how do we know they aren't just having "custom bulbs"
manufactured for this purpose? I.e., the "customer" is large
enough to warrant same...

 

[D Yuniskis]

Hi Sylvia,

I would think that just means that starting is when the lamp is most
susceptible to failure due to the accumulated damage.

Hmmm... I wonder if synchronizing the turn on with zero-crossings
has any *measurable* effect (or if it is so far down in the
noise as to be warrantless)

 

[Don Klipstein]

Hi Don,



So, your point would be that AC drive tends to disturb where this
spot would want to otherwise develop? But, can that be relied upon?
Or, are manufacturing variations sufficient to render this moot?

Actually, I was not considering AC-vs-DC since most filaments running
on AC appear to me to have temperature sufficiently steady to not
incur significant thermal cycling stress, which I was arguing against
degree of existence from cold starting anyway.

However, now that you bring my mind to this, use of AC as opposed to DC
(back to what I said a few days ago or whatever) could prevent one of
these fatal thin spots from forming at the positive end of the filament
(I hope I said positive end before). And likely mainly in
vacuum-containing incandescent lamps as opposed to gas-filled ones,
but of current rating high enough for the filament temperature to be
fairly steady throughout each AC half-cycle.
Then again, I expect the difference to be minor, merely measurable.

But, presumably, "gains *some*" (or the series R practice would be
foolish)?

My impression is that the slower start (dims the filament for several
percent of each "flash" from slowing of risetime) and mild dimming for a
significant fraction of each "flash" achieves somewhat significant life
extension. I do think that doing this instead of merely dimming the lamp
slightly (even with a series resistor) is foolish but done due to strong
belief in cold starts being more damaging than they usually actually are.

Are you sure they really power off (completely)? E.g., that
perhaps they don't keep the filament "warm" though not visible?

That would make a major difference in later part of the fall time,
and I don't see this happening. I also don't see sign of leakage through
LED replacements of the incandescents in Philadelphia.

(I don't know if operating the lamp at such reduced power
would make a difference -- I have a friend who does that
sort of thing, I should inquire)

I have heard of this being done in some stage lighting setups. I am
aware of halogen lamps sometimes being different - by sometimes having
filament notching at the ends of the filament producing thin spots that
are not excessively hot in steady operation but that do overshoot in
temperature during a cold start.

Of course, bulb replacement has to be the driving force behind
the rise in popularity of LED traffic lights.

Although it has to be a major force, especially in older major cities
where the lamps may have to be replaced by unionionized municipal
employees, I hear LED traffic signals being sold to the taxpayers mainly
from energy savings. And the energy savings can be substantial, since
LEDs normally specialize in producing light of one color or another,
while red and green incandescent traffic signals need something like 70%
of their photometric output to be blocked by a colored filter. Another
reason for energy savings is that incandescent traffic signal lamps tend
to be significantly less efficient than 750 and 1000 hour household lamps
due to being designed to last typically 8,000 hours, and also due to
having a filament of more-vibration-resistant design that has greater
heat consuction loss. A 92 or 116 watt incandescent traffic signal lamp
is typically replaced by an LED one that consumes 11-16 watts or so.

Savings of 80 watts at 50% duty cycle, at even 8 cents per KWH, easily
amounts to $28-$30 per year per lamp.

But using filaments designed for higher operating voltages
at lower voltages increases longevity (at the expense of efficiency)
so how do we know they aren't just having "custom bulbs"
manufactured for this purpose? I.e., the "customer" is large
enough to warrant same...

I used to trashpick from the dumpsters of the contractor that relamped
Philadelphia's streetlights and alley lights. The alley lights used 92
watt traffic signal lamps.

I have peeked into opened-up traffic signals in Philadelphia a few times
before, and found the color of the filaments strongly indicating that they
are either 120V or 130V traffic signal lamps. The life expectancy I am
seeing appears to me more consistent with 130V. And my "day job" is
delivery biker in Philadelphia, I went to school in Philadelphia and I
spent close to half my life living in Philadelphia.

- Don Klipstein ([email protected])