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Incandescent lamp AC vs DC

D

D Yuniskis

Jan 1, 1970
0
Hi,

I need to hack together a power supply for an "illuminator".
Bulb is rated 6.5V 2.75A.

Easy solution is a DC "brick".

However, I think the original power supply was probably just
a transformer (i.e., driven with AC, no regulation (?)).

This got me thinking as to whether there *is* a difference
driving incandescent lamps with AC vs. DC -- i.e., what would
the mains frequency have to be in order for there to *be* a
difference?

And, how would that difference manifest itself, otherwise.
 
P

petrus bitbyter

Jan 1, 1970
0
D Yuniskis said:
Hi,

I need to hack together a power supply for an "illuminator".
Bulb is rated 6.5V 2.75A.

Easy solution is a DC "brick".

However, I think the original power supply was probably just
a transformer (i.e., driven with AC, no regulation (?)).

This got me thinking as to whether there *is* a difference
driving incandescent lamps with AC vs. DC -- i.e., what would
the mains frequency have to be in order for there to *be* a
difference?

And, how would that difference manifest itself, otherwise.

For an ordinary bulb a transformer with an output of 6.5V/3A will do. As
6.5V may not be available, 6.3V will also do. Mains frequency does not
matter.

petrus bitbyter
 
S

Sylvia Else

Jan 1, 1970
0
Hi,

I need to hack together a power supply for an "illuminator".
Bulb is rated 6.5V 2.75A.

Easy solution is a DC "brick".

However, I think the original power supply was probably just
a transformer (i.e., driven with AC, no regulation (?)).

This got me thinking as to whether there *is* a difference
driving incandescent lamps with AC vs. DC -- i.e., what would
the mains frequency have to be in order for there to *be* a
difference?

And, how would that difference manifest itself, otherwise.

Skin effect at higher frequencies would reduce the current, and
therefore the brightness.

Incandescent lights strobe slightly when driven by AC, and that's
certainly detectable, even if it's not obvious in most situations.

Sylvia.
 
S

Spehro Pefhany

Jan 1, 1970
0
Hi,

I need to hack together a power supply for an "illuminator".
Bulb is rated 6.5V 2.75A.

Easy solution is a DC "brick".

However, I think the original power supply was probably just
a transformer (i.e., driven with AC, no regulation (?)).

This got me thinking as to whether there *is* a difference
driving incandescent lamps with AC vs. DC -- i.e., what would
the mains frequency have to be in order for there to *be* a
difference?

And, how would that difference manifest itself, otherwise.

Tungsten incandescent lamps typically last longer on AC.

The effect is called "filament notching" (aka DC notching). It's more
of a problem with very low power long-life bulbs, IIRC, as opposed to
something like a car tail light bulb.

http://www.htl.co.jp/english/p_pro1_info.html


Best regards,
Spehro Pefhany
 
D

D Yuniskis

Jan 1, 1970
0
petrus said:
For an ordinary bulb a transformer with an output of 6.5V/3A will do. As
6.5V may not be available, 6.3V will also do. Mains frequency does not
matter.

You've missed the question:
"what would the mains frequency HAVE TO BE in order for
there to *be* a difference"

Surely you don't mean to suggest that exciting the filament at
500Kc would be no different than DC?
 
D

D Yuniskis

Jan 1, 1970
0
Hi Sylvia,

Sylvia said:
Skin effect at higher frequencies would reduce the current, and
therefore the brightness.

Yes, though presumably at *much* higher frequencies.
Incandescent lights strobe slightly when driven by AC, and that's
certainly detectable, even if it's not obvious in most situations.

Ah, crap! I forgot about that (I tend to perceive flicker
more than "The Average Joe" -- or "Sylvia" for that matter! :> )
And, if I tried to control the brightness of an AC driven
lamp using a typical "dimmer", I would be inviting this
problem. :(
 
D

D Yuniskis

Jan 1, 1970
0
Hi Phil,

Phil said:
There will be electromigration issues at some level--using AC tends to
reduce the rate of void formation, but I don't know how big an effect
that is.

Apparently a significant effect -- if you chase down the
URL Spehro provided. I had thought AC introduced mechanical
stress on the filament (heating and cooling rapidly to
effectively "vibrate").
 
D

D Yuniskis

Jan 1, 1970
0
Hi Tim,

Tim said:
You don't say what your input is, but you allude to a transformer, so I
assume you're running it off the mains.

I think other (hysterically speaking) power supplies have
been little more than a tapped transformer. But, i don't
have that so I am looking at options (hence the post).
Four issues that I can think of with AC vs. DC:

1: DC never reverses -- if there are aging effects that are exacerbated
or alleviated by the current always flowing one way, the lamp will age
more or less with DC.

2: At low enough line frequencies the filament will be significantly
heating and cooling each cycle, and will die an early death of thermal
fatigue. I suspect you'd have to be in the single-digit Hz for this to
happen, but it would strongly depend on the bulb.

I don't know how to gauge that effect -- other than to trash
some bulbs and determine it empirically. My concern is that
they are smallish bulbs (roughly the size of a "tail light"
in an automobile) but use a fair pit of power (e.g., 20W).

Lifetime is typically only 100 PoH -- so I don't want to
make a foolish choice in powering them and find myself
burning through bulbs at an alarming rate (they are $5-$10
each and a PITA to replace/install)
3: As mentioned, at some high frequency you'd get high frequency effects
such as skin effect, followed by the inductance of the leads to the
filament, etc.

4: Some filaments are resonant, if you caught this resonant frequency
you'd induce accelerated mechanical fatigue.

But I doubt it's an issue at 50 and 60Hz, and perhaps not even 400Hz
aircraft line frequency.

Dunno. The low lifetime leaves me suspicious as to what
may be going on with the bulb's design :-/
 
J

Jon Kirwan

Jan 1, 1970
0
Hi Sylvia,



Yes, though presumably at *much* higher frequencies.


Ah, crap! I forgot about that (I tend to perceive flicker
more than "The Average Joe" -- or "Sylvia" for that matter! :> )
And, if I tried to control the brightness of an AC driven
lamp using a typical "dimmer", I would be inviting this
problem. :(

I've used a pyrometer to observe incandescent lamps operating
on 60Hz power. I would see something on the order of a 3%
change in flux at 120Hz. It was a casual playing around at
the time, though, and my detector system was filtered for
950nm center and a bandwidth of about 120nm, memory serving.
Anyway, I gathered from this rough experiment that the
filament in a typical 60W light bulb remains fairly hot
(doesn't cool that quickly) but does have some observable
variation when using an instrument to look more closely.

Jon
 
J

Jon Kirwan

Jan 1, 1970
0
Hi Phil,



Apparently a significant effect -- if you chase down the
URL Spehro provided. I had thought AC introduced mechanical
stress on the filament (heating and cooling rapidly to
effectively "vibrate").

I would imagine this to be worse at lower frequencies where
the wire could actually cool down and heat up enough. At
some point with higher frequencies, this 'vibration' wouldn't
be physically possible.

Jon
 
G

GregS

Jan 1, 1970
0
Hi,

I need to hack together a power supply for an "illuminator".
Bulb is rated 6.5V 2.75A.

Easy solution is a DC "brick".

However, I think the original power supply was probably just
a transformer (i.e., driven with AC, no regulation (?)).

This got me thinking as to whether there *is* a difference
driving incandescent lamps with AC vs. DC -- i.e., what would
the mains frequency have to be in order for there to *be* a
difference?

And, how would that difference manifest itself, otherwise.


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
with photodiods or cameras, don't need 120 Hz.
There is a fix using 3 phase to DC convereter. It sort of works with incandescents.

greg
 
P

Paul Keinanen

Jan 1, 1970
0
I've used a pyrometer to observe incandescent lamps operating
on 60Hz power. I would see something on the order of a 3%
change in flux at 120Hz. It was a casual playing around at
the time, though, and my detector system was filtered for
950nm center and a bandwidth of about 120nm, memory serving.
Anyway, I gathered from this rough experiment that the
filament in a typical 60W light bulb remains fairly hot
(doesn't cool that quickly) but does have some observable
variation when using an instrument to look more closely.

Apparently this was a 110 V lamp ?

The 6.5 V filament is much thicker and would have a very large thermal
inertia, so one would expect that the intensity variation to be far
less.
 
J

Jon Kirwan

Jan 1, 1970
0
Apparently this was a 110 V lamp ?

Yes. Sorry not to have included that comment in the above.
The 6.5 V filament is much thicker and would have a very large thermal
inertia, so one would expect that the intensity variation to be far
less.

I didn't try those at the time. Just commenting about what I
did try.

Jon
 
D

Don Klipstein

Jan 1, 1970
0
I need to hack together a power supply for an "illuminator".
Bulb is rated 6.5V 2.75A.

Easy solution is a DC "brick".

However, I think the original power supply was probably just
a transformer (i.e., driven with AC, no regulation (?)).

This got me thinking as to whether there *is* a difference
driving incandescent lamps with AC vs. DC -- i.e., what would
the mains frequency have to be in order for there to *be* a
difference?

And, how would that difference manifest itself, otherwise.

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

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.

- Don Klipstein ([email protected])
 
D

Don Klipstein

Jan 1, 1970
0
Skin effect at higher frequencies would reduce the current, and
therefore the brightness.

For most filaments, the frequency would have to get to the megahertz
ballpark, at least hundreds of KHz, maybe a few megahertz, to have skin
effect significantly dim the filaments. AWG 30 copper (diameter approx.
..01 inch or approx. .25 mm), at room temperature, has resistance not a
whole lot more at 100 KHz than at DC. Tungsten has more resistance than
copper, and at typical filament operating temperature has roughly 15 times
as much resistance as at room temperature.

- Don Klipstein ([email protected])
 
D

Don Klipstein

Jan 1, 1970
0
I've used a pyrometer to observe incandescent lamps operating
on 60Hz power. I would see something on the order of a 3%
change in flux at 120Hz. It was a casual playing around at
the time, though, and my detector system was filtered for
950nm center and a bandwidth of about 120nm, memory serving.
Anyway, I gathered from this rough experiment that the
filament in a typical 60W light bulb remains fairly hot
(doesn't cool that quickly) but does have some observable
variation when using an instrument to look more closely.

950 nm is just a little on the short wavelength side of peak
spectral power distribution per unit bandwidth wavelength for
most tungsten incandescent lamps.

Amount of radiation per unit bandwidth in this case should be
slightly more than proportional to temperature to the 5th power.

(Total radiation is proportional to temperature to the 4th power,
and total bandwidth in wavelength terms is inversely proportional to
temperature, and the peak usually gets closer to 950 nm as temperature
increases.)

If the amount of radiation in a 120 nm band centered at 950 nm varied by
3% over an AC half cycle for that particular tungsten incandescent lamp,
then I expect the filament temperature for that particular lamp varied by
slightly less than .6%.

I also expect visible light output varied more. I seem to think that
a 120V 60W 1000 hour 845-890 lumen "A19" incandescent lamp has filament
temperature around proportional to voltage to the .39 power, maybe .4,
and light output proportional to voltage to the 3.4 power, maybe 3.5.

(based on, "extrapolated a bit from my knowledge", from what a 100 watt
120V 750 hour 1670-1750 lumen "A19" does, as best as I know as reported
in http://members.misty.com/don/incchart.html)

That means photometric output being roughly proportional to temperature
to the 8.7 power at 2870 K or so, very slightly more at the slightly
lower roughly 2800 K of the 60 W lamp that I described above.

Based on this, I would expect the 60W lamp that I described to have
photometric output varying roughly 1.5 times as much throughout an AC
half-cycle as such lamp's 950 nm output does.

- Don Klipstein ([email protected])
 
D

Don Klipstein

Jan 1, 1970
0
I was just thinking of the old discs you put in a lamp socket to make
them last. A diode I guess.

I have had experience with those "discs" of two types:

1: Diode: Power consumption reduced to ~58-60% of "normal", light
output reduced to ~26-29% of "normal". Life expectancy has good chance of
being increased by a factor of 50-90.

2: Thermistor: My experience is that those "soft-start" tungsten
incandescent lamps and claim to double their life expectancy.

I did one notable experiment where I found that when the "disc" was
fully warmed up, it had enough resistance remaining to dim the lamp to
an extent worth a goodly 50% life extension, maybe 55%. That means
10%, maybe 11% less light while combined power consumption of the lamp
and the "disc" is at most 2% less than that of the lamp alone.
I have to make and modify many lamps and
microscope lamps to convert the AC to DC for light noise problems. If
your working with photodiods or cameras, don't need 120 Hz.
There is a fix using 3 phase to DC convereter. It sort of works with
incandescents.

By any chance is this the thingy that I somewhat remember as an
unfiltered 6-diode rectifier for "Y"/"wye" 3-phase AC? (4.5% less average
voltage and similar less RMS voltage than with filter capacitor, 6.8-6.9%
less power consumption than with filter capacitor, ~13-15% less light
without filter capacitor than with, while life expectancy is increased
maybe 70-80%?)

- Don Klipstein ([email protected])
 
S

Sylvia Else

Jan 1, 1970
0
Hi Sylvia,



Yes, though presumably at *much* higher frequencies.


Ah, crap! I forgot about that (I tend to perceive flicker
more than "The Average Joe" -- or "Sylvia" for that matter! :> )
And, if I tried to control the brightness of an AC driven
lamp using a typical "dimmer", I would be inviting this
problem. :(

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.
 
D

D Yuniskis

Jan 1, 1970
0
Hi Sylvia,

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

No, I was trying to see what the pros and cons (from the
lamp's perspective) of AC vs DC drive would be.

E.g., DC is easiest to "get done" -- dig through box of
"bricks" looking for 20W @ ~6V, cut off existing "DC"
connector, attach appropriate connector, done!

And, DC is easiest to *dim* (since that *should* lengthen
life expectancy of the bulb).

But, before investing the time to do this, I wanted to be
sure there were no other issues that would make this an
unwise approach.

E.g., Spehro's posted URL suggests DC related failures.
I, OTOH, had always *assumed* AC presented more mechanical
stress to the filament.

The zinger is the "100 hr" life expectancy. Sheesh! Unpopped
popcorn lasts longer than that! :-/
 
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