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rgf(at)dcn.davis.ca.us
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 Posted: Sat Mar 07, 2009 8:27 am Post subject: ACS's Lightsaver gadget |
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Aircraft Spruce offers this gadget
(http://www.aircraftspruce.com/catalog/elpages/lightsaver.php) to eliminate
the large current inrush to a cold tungsten filament, thus supposedly
greatly increasing lamp life. Sounds reasonable, but wonder what the
experts think. And I'm curious why it has to be mounted on a heat-sink
surface...does it consume lots of energy while the lamp is on?
"WHY INSTALL A LIGHTSAVER
Typical landing and taxi lights in aircraft utilize a lamp that has a
tungsten filament. Tungsten, which is a special metal, produces relatively
high efficiency white light when electrical current flows through it.
Tungsten wire is wound between two electrodes within the lamp. Unfortunately
the metal wire has an undesirable characteristic. The metal filament has an
extremely low electrical resistance when the temperature of the tungsten
filament is at ambient (25C). Once the lamp is turned on, the filament
resistance changes from close to 0 ohms (dead short) to 1.5 ohms. An
increase of approximately 100 times. The dead short condition causes a
massive inrush of current for several mille seconds. A typical 100 watt 12
volt lamp which when on draws 8 to 9 amps may see 800-1000 amps for that
split second depending on the charged condition of the aircrafts battery.
This causes severe stress to the lamp filament, which over repeated on/off
cycles results in a fracture (open) in the filament. This is why the failure
of the lamp typically occurs at turn on.
HOW A LIGHTSAVER WORKS
The Lightsaver prevents this massive inrush allowing the filament to draw
the rated current even at turn on with no change in typical light output
performance. The filament life increases up to 10 times due to reducing the
stress induced by inrush current. This is something the lamp manufacturers
don't want you to know. After all they are in the business of manufacturing
replacement lamps for your aircraft.
HOW IS THE LIGHTSAVER INSTALLED INTO MY AIRCRAFT?
The installation is simple. The Lightsaver is placed in series with the lamp
on either the positive or negative side of the electrical interconnection.
The Lightsaver must be mounted to a heat-conducting surface such as the
firewall of the aircraft. Mounting is accomplished using screws and nuts to
secure it to the firewall."
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rv-9a-online(at)telus.net
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| Posted: Sat Mar 07, 2009 9:06 am Post subject: ACS's Lightsaver gadget |
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These devices look like standard power resistors available from Digikey and
others.
What I use are inrush current limiters (also from Digikey) that have a
negative temperature coefficient so that they have a high resistance when
cold, but a low resistance when warm.
See
http://search.digikey.com/scripts/dksearch/dksus.dll?Detail&name=KC023L-ND
These are the ones I used for my 75W halogen lamps.
The only side-effect may be that a wig-wag flasher may need to be modified
to slow down the flashing rate to ensure maximum lamp brightness during
operation.
I believe that they are better than resistors in this application.
Cheers
Vern Little
---
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fvalarm(at)rapidnet.net
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| Posted: Sat Mar 07, 2009 9:48 am Post subject: ACS's Lightsaver gadget |
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Can I assume these current limiters will also enable the switch that
controls the lights to last longer by reducing the arc on switch closure?
Or is the arc mostly on switch opening, where the current limiter will have
no benefit?
Bevan
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retasker(at)optonline.net
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| Posted: Sat Mar 07, 2009 11:12 am Post subject: ACS's Lightsaver gadget |
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Mostly the latter. However, that is not to say that they won't benefit
the switch by limiting the current surge on turn-on.
B Tomm wrote:
[quote]
Can I assume these current limiters will also enable the switch that
controls the lights to last longer by reducing the arc on switch closure?
Or is the arc mostly on switch opening, where the current limiter will have
no benefit?
Bevan
--
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rv-9a-online(at)telus.net
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| Posted: Sat Mar 07, 2009 12:16 pm Post subject: ACS's Lightsaver gadget |
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If you look at switch ratings, when they are connected to a lamp load, the
rating is greatly reduced. This is because of the large inrush currents
when filaments are cold. The inrush current limiters will help limit this
current, boosting switch life.
Vern
---
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nuckolls.bob(at)aeroelect
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| Posted: Sat Mar 07, 2009 4:05 pm Post subject: ACS's Lightsaver gadget |
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At 10:25 AM 3/7/2009, you wrote:
| Quote: |
Aircraft Spruce offers this gadget
(http://www.aircraftspruce.com/catalog/elpages/lightsaver.php) to eliminate
the large current inrush to a cold tungsten filament, thus supposedly
greatly increasing lamp life. Sounds reasonable, but wonder what the
experts think. And I'm curious why it has to be mounted on a heat-sink
surface...does it consume lots of energy while the lamp is on?
|
| Quote: | "WHY INSTALL A LIGHTSAVER
Typical landing and taxi lights in aircraft utilize a lamp that has a
tungsten filament. Tungsten, which is a special metal, produces relatively
high efficiency white light when electrical current flows through it.
|
<snip>
| Quote: | HOW IS THE LIGHTSAVER INSTALLED INTO MY AIRCRAFT?
The installation is simple. The Lightsaver is placed in series with the lamp
on either the positive or negative side of the electrical interconnection.
The Lightsaver must be mounted to a heat-conducting surface such as the
firewall of the aircraft. Mounting is accomplished using screws and nuts to
secure it to the firewall."
|
This is a stretch of factual data into a pretty good bucket
of floobydust. Yes, the cold resistance on an incandescent
lamp is quite low compared to it's operating resistance.
At the same time, when you close the switch, the lamp doesn't
see 14 volts either. There is resistance associated with
ship's wiring that adds to the cold resistance of the lamp.
Here's a trace I took on a 55W halogen lamp some years ago.
http://www.aeroelectric.com/Pictures/Curves/Exemplar_Incandescent_Lamp_Inrush.gif
This was with SHORT leads on the workbench . . . and yes,
a 100w lamp would be a bit worse. However, I doubt that it
exceeds 100 Amps and no where near the 800 amps claimed in
the ad.
The thing about lamp life is a bit bogus too. Back in the
good ol' days when a 4509 tractor lamp was considered a
go landing light, they were indeed service life limited
by vibration and inrush cycling . . . but even then, how
many hours does a landing light burn per year as compared
to a tractor . . . or even headlights on your car?
Then halogens came to the cars . . . and some years later
to airplanes (aviation IS leading edge technology . . .
right? Yeah . . . right!). The inrush currents went up
slightly but the service life of the filaments went way up.
How often do you put new bulbs in your car these days?
I think my 2002 van has had one set of headlamps since
new.
Okay, how about this inrush mitigation if you're using
wig-wags? See:
http://www.aeroelectric.com/Pictures/Curves/Wig_Wag_Currents.jpg
Here's the same lamp as the first trace except flashed
at 500 mS intervals. Note that we do see the initial
inrush on first-power but the lamp doesn't have enough
time to cool off between flashes to repeat the inrush
on second and subsequent applications of power.
Further, I'm mystified by the picture of the device offered
by aircraft spruce. It appears to be the same package as
the RH series mil-spec power resistors. See:
http://www.alliedelec.com/Search/ProductView.aspx?IMG=895-0400
Of course, someone COULD package some device other
than a resistor in that housing but . . .
See:
http://search.digikey.com/scripts/DkSearch/dksus.dll?Cat=656273&keywords=inrush%20limiter
Run down the selections for current and hit on 8A.
Click on the KC023L-ND and you find a device that
can go in series with a 100W lamp that adds 1.3 ohms
of resistance to the lamp circuit when cold. Add
anther .2 ohms for lamp, perhaps .3 ohms for wiring
and we're up to 2.8 ohms. 14V/2.8 = 5A for the new
inrush. In operation, the KC023L resistance drops
to .04 ohms which means it tosses off about 300
millivolts of your lamp supply power.
Here's the ace-kicker. For a KC023L to do it's
job, it NEEDS TO BE ALLOWED TO GET HOT. That's
what causes its resistance to drop down to a handful
of milliohms. The device pictured in the ACS catalog
says it needs to be heat-sinked. Hmmmm . . . do
you suppose the product pictured is just a resistor?
Adding say .1 ohms fixed resistance to the circuit
would reduce that many hundreds of amps of claimed
inrush to something around 140 . . . assuming one
could ignore all the other resistances associated
with the wiring.
My first pass at this product's claims suggests that
first it is not offered with a full understanding
and explanation of the simple-ideas that control
incandescent lamp inrush but more important, it
ignores the relative ruggedness of modern halogen
lamps and the non-worry about using them in wig-wag
service.
If you really, Really, REALLY want to add inrush
limiting to your incandescent lamps, I'll suggest
you pick one from the Digikey listing cited above
for a whole lot less money. If you want my advice,
I was in a group of techno-wiennies that looked
into using these on the GP-180 at Lear. We did put
one in the nose gear taxi light fixture . . . and
discovered that you have to mount them so that
the DONT get heat-sinked . . . lest they blow
up. They need to warm up to work. They're a pain
in the arse to mount . . . not unlike polyfuses.
If the Lightsaver guys were offering a real, negative
tempco inrush limiter, it would be in some sort of
package that ALLOWED heating.
Bottom line . . . I'll suggest you save your money.
Return on investment for these things is poor
to nil.
Bob . . .
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nuckolls.bob(at)aeroelect
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| Posted: Sat Mar 07, 2009 4:20 pm Post subject: ACS's Lightsaver gadget |
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At 02:11 PM 3/7/2009, you wrote:
| Quote: |
<rv-9a-online(at)telus.net>
If you look at switch ratings, when they are connected to a lamp
load, the rating is greatly reduced. This is because of the large
inrush currents when filaments are cold. The inrush current
limiters will help limit this current, boosting switch life.
Vern
|
Great point! . . . I missed this thought in my earlier posting.
Consider that when you "close" a switch, the contacts bounce
many times. See:
http://www.aeroelectric.com/Pictures/Curves/switch_transition_S700-1.jpg
This bounce and transition trace on a Carling toggle
shows about 1.8 mS to travel from one condition to the
other. When I spread that bounce out to count the closures
there was about 6-7 as I recall. Relays can be even worse.
The heavier their contacts, the worse they bounce. See:
http://www.aeroelectric.com/Pictures/Curves/S704-1_Contact_Bounce_without_Diode.jpg
http://www.aeroelectric.com/Pictures/Curves/S704-1_Contact_Bounce_with_Diode.jpg
These two traces are for the contacts of the plastic 30A
relay (S704). These things bounce like Tigger at a
birthday party.
What's more, all the bouncing happens in that very
tiny interval of high inrush for warming up the incandescent
lamp filament. This means that an 10A switch controlling
an 8A lamp sees perhaps a dozen or more 40A "hits"
every time you turn on a cold lamp.
That's why lamp ratings on switches are so heavily
de-rated. If you want to put an inrush limiter on
you landing lights, do it for the switch . . . not
the lamp.
Thanks for the heads-up Vern.
Bob . . .
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steve(at)tomasara.com
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| Posted: Sun Mar 08, 2009 9:05 am Post subject: ACS's Lightsaver gadget |
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Interesting traces Bob,
What do you think would have the better reliability in the field for a
150W halogen landing light (without a surge suppressor), A S704-1 relay
with an ~16V transient suppressor for the catch diode, or a S701-1
switch connecting directly to the lamp?
I'm not sure how to trade off the bigger contacts, but they bounce
longer, dilemma...
Steve Stearns
Boulder/Longmont, Colorado
CSA,EAA,IAC,AOPA,PE,ARRL,BARC (but ignorant none-the-less)
Restoring (since 1/07): N45FC O235 Longeze Cothern/Friling CF1 (~1000 Hrs)
Flying (since 9/86): N43732 A65 Taylorcraft BC12D
| Quote: | At 02:11 PM 3/7/2009, you wrote:
> ><rv-9a-online(at)telus.net>
> >
> >If you look at switch ratings, when they are connected to a lamp
> >load, the rating is greatly reduced. This is because of the large
> >inrush currents when filaments are cold. The inrush current
> >limiters will help limit this current, boosting switch life.
> >
> >Vern
>
Great point! . . . I missed this thought in my earlier posting.
Consider that when you "close" a switch, the contacts bounce
many times. See:
http://www.aeroelectric.com/Pictures/Curves/switch_transition_S700-1.jpg
This bounce and transition trace on a Carling toggle
shows about 1.8 mS to travel from one condition to the
other. When I spread that bounce out to count the closures
there was about 6-7 as I recall. Relays can be even worse.
The heavier their contacts, the worse they bounce. See:
http://www.aeroelectric.com/Pictures/Curves/S704-1_Contact_Bounce_without_Diode.jpg
http://www.aeroelectric.com/Pictures/Curves/S704-1_Contact_Bounce_with_Diode.jpg
These two traces are for the contacts of the plastic 30A
relay (S704). These things bounce like Tigger at a
birthday party.
What's more, all the bouncing happens in that very
tiny interval of high inrush for warming up the incandescent
lamp filament. This means that an 10A switch controlling
an 8A lamp sees perhaps a dozen or more 40A "hits"
every time you turn on a cold lamp.
That's why lamp ratings on switches are so heavily
de-rated. If you want to put an inrush limiter on
you landing lights, do it for the switch . . . not
the lamp.
Thanks for the heads-up Vern.
Bob . . .
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nuckolls.bob(at)aeroelect
Guest
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| Posted: Sun Mar 08, 2009 12:13 pm Post subject: ACS's Lightsaver gadget |
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At 01:03 PM 3/8/2009, you wrote:
| Quote: |
Interesting traces Bob,
What do you think would have the better reliability in the field for
a 150W halogen landing light (without a surge suppressor), A S704-1
relay with an ~16V transient suppressor for the catch diode, or a
S701-1 switch connecting directly to the lamp?
I'm not sure how to trade off the bigger contacts, but they bounce
longer, dilemma...
|
Gee . . . you noticed! It's a credit to your curiosity
combined with an awareness of the need for trade-offs.
I an my contemporaries were faced with thousands of
such questions over our careers . . . with input from
a host of special interests that included marketing,
purchasing, inventory management, manufacturing
engineers and other systems guys who regarded every
one else's specialty as witchcraft.
Going for lower parts count is always a good lick.
The 150W lamp presents a problem of sorts. It takes
right at 9.5A in normal operation. But given what
we know of a switch's ability to CARRY current after
the bouncing is over, I'd bet that an S701 combined
with a Cantherm MF72-3D25 inrush limiter (3 ohms
cold) will limit your inrush to under 5 amps and toss
off only 9 x .044 = 0.37 volts in operation.
See:
http://www.cantherm.com/products/thermistors/cantherm_mf72.pdf
and
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=317-1234-ND
This would be the lowest parts count solution I can
deduce and I think it has a fair shot at satisfactory
performance.
I'll do some thinking about practical ways to mount
the inrush limiter so that it is well supported,
well connected, but not thermally deprived of the
ability to warm up.
In the GP-180, one of the guys crimped flexible
leadwires to the part, wrapped it with a couple
of layers of fiberglas door gasket for a wood
burning stove and clamped the assembly into the
inside surface of the Grimes lamp fixture housing.
Kinda clumsy but it worked. Their first efforts
took too much heat out of the thing which caused
catastrophic stresses from power dissipation.
Bob . . .
----------------------------------------)
( . . . a long habit of not thinking )
( a thing wrong, gives it a superficial )
( appearance of being right . . . )
( )
( -Thomas Paine 1776- )
----------------------------------------
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