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'[EE] Predicting keep alive voltages for incandesce'
2006\06\27@184523 by

I've come upon this interesting paragraph on piclist.com (don't I already
know what is written there? Well, no, in fact now that there is about 3GB of
text on the site, I find more and more often that I have no clue were stuff
came from. Rather fun actually!) and I was curious about the unanswered part
of it: How does one know how much voltage to put through the lamp to heat
the filament without producing light?

"One feature of incandescent bulbs that you can exploit is that when off, an
incandescent filament is very low resistance- much lower than when it is
glowing. This allows lamps to be used to protect against short circuits.
Also, you can trickle a small (fractions of a milliamp) current through the
bulb- if there is current flow, the bulb is good, none, the bulb is burnt
out. This method can only tell you the health of the bulb when the bulb is
off. Make the current small enough not to light the bulb. The reduced
resistance of a cold bulb results in larger current draw when it is first
powered; called "in-rush current". The filament color temperature (Kelvin)
at which the lamp is designed to operate determines the inrush current. A
300'K lamp will have an in rush current about the same as its operating
current, but a 2,500'K lamp will draw about 12 times the current when cold.
Most lamps will heat to normal current ratings in approximately 30-100
milliseconds. The in rush current can be reduced by using a "keep alive"
voltage to pre-warm the filament while not producing light."

So again, how do you know what the filament resistance / heat / current will

---
jamesnewtonpiclist.com  1-619-652-0593 phone
http://www.piclist.com/member/JMN-EFP-786
PIC/PICList FAQ: http://www.piclist.com

On 6/27/06, James Newton, Host <jamesnewtonpiclist.com> wrote:
{Quote hidden}

I have some experience doing this in a 12V, 100W application.  I think
the most practical thing is to make a graph of steady state current vs
voltage.

Around the bulb turn-on voltage, the response (brightness/current) is
*very* slow.  It first appeared to be hysteresis, but further study
showed the brightness to settle down with about a thirty second time
constant.   (This was meant for a closed-loop servo system, and it
drove me nuts.  If I did it again, it would be LEDs all the way.)

Anyway, I guess there's a tradeoff between inrush and idle current.

Regards,
Mark
markrages@gmail
--
You think that it is a secret, but it never has been one.
James Newton, Host wrote:

> How does one know how much voltage to put through the lamp to heat
> the filament without producing light?

I think you need to specify that better. As I understand it, the filament
sends out radiation the moment it gets heated, no matter how little. That
radiation is in the infrared range, and the hotter it gets, the higher the
spectrum it sends out. So there's probably not really a point where it
starts producing light, it just becomes more and more.

You're getting probably closer by watching power, rather than current or
voltage.

> So again, how do you know what the filament resistance / heat / current will
> be given a steady voltage?

Generated heat + emitted light = electric power. To know how much heat and
how much light, you'd need to know the temperature of the filament. This is
probably non-trivial, given that you don't know that much about the
filament. If you know the material, you maybe can estimate the temperature
from the increase in resistance from the cold resistance. The temperature
then gives you an estimate of the emitted spectrum, and that gives you the
percentage of energy emitted as light. This relationship probably can be
found described in some manufacturer literature, I'd hope.

As easier methods there are thumb-rule percentage of the rated power, or a
thumb-rule resistance increase from the cold resistance. The thumb rules
maybe created using the above...

Gerhard

A warm up current is sometimes maintained through incandescent lamps by
theatrical dimmers to reduce the inrush current and to smaller rise time
to bring the lamp to full (it's already part way there when it's "off"). I
think that when any current runs through the filament, you get light, it's
just a matter of how much (and what color, perhaps more infrared than
visible). We've set up dimmers where the lights looked like they were out,
but the customer calls back and says the lights are "ghosting." The light
becomes visible in the dark as your eyes adjust to the dark.

Long ago, a vacuum tube similar to an incandescent lamp was used as a
constant current source. These were "ballast tubes."

Radios in the 1930s used an incandescent lamp as a volume expander. The
lamp was across the audio at some point in the audio chain. Loud
programming would increase the lamp resistance, allowing the audio to get
louder. Quiet programming would decrease the lamp resistance, making the
audio quieter.

HP used incandescent lamps in their first audio generators to stabalize
the feedback gain. Too high an output level (headed towards clipping)
would increase the lamp resistance, decreasing the oscillator loop gain.

I've used incandescent lamps as current limiters in an electric vehicle
battery charger.  A near constant voltage source is applied to the battery
string through an incandescent lamp. As the battery voltage increases
during charging, the lamp resistance decreases, keeping the current from
falling as much as it would without the lamp.

Useful devices!

Harold

--
FCC Rules Updated Daily at http://www.hallikainen.com - Advertising
opportunities available!

> I have some experience doing this in a 12V, 100W application.
>  I think the most practical thing is to make a graph of
> steady state current vs voltage.
>
> Regards,
> Mark

You wouldn't happen to still have that graph would you?

---
jamesnewtonpiclist.com  1-619-652-0593 phone
http://www.piclist.com/member/JMN-EFP-786
PIC/PICList FAQ: http://www.piclist.com

> Generated heat + emitted light = electric power. To know how
> much heat and how much light, you'd need to know the
> temperature of the filament. This is probably non-trivial,
> given that you don't know that much about the filament. If
> you know the material, you maybe can estimate the temperature
> from the increase in resistance from the cold resistance. The
> temperature then gives you an estimate of the emitted
> spectrum, and that gives you the percentage of energy emitted
> as light. This relationship probably can be found described
> in some manufacturer literature, I'd hope.

Yeah, that's what I figured, but I'll be darned if I can find it in any of
the literature from at least three different mfgrs.

> As easier methods there are thumb-rule percentage of the
> rated power, or a thumb-rule resistance increase from the
> cold resistance. The thumb rules maybe created using the above...
>
> Gerhard

I'd be happy with those thumb-rules... You have any idea what they are? Or
do I have to create them myself?

---
jamesnewtonpiclist.com  1-619-652-0593 phone
http://www.piclist.com/member/JMN-EFP-786
PIC/PICList FAQ: http://www.piclist.com

BCCs:  Some possibly useful stats re incandescent filament visibility
at reduced voltages in 'table' at end.

>> I have some experience doing this in a 12V, 100W application.
>>  I think the most practical thing is to make a graph of
>> steady state current vs voltage.

> You wouldn't happen to still have that graph would you?

Would be very easy to generate for a given bulb.
And it will vary with bulb and circumstances.

Bulb temperature will vary with power dissipated.
Power dissipated is function of current squared and resistance.
Resistance is a function of temperature.
Current at given voltage will vary with temperature.
Radiation loss is probably reasonably predictable based on temperature
alone.
Some second order effects will occur due to convection and conduction
which will depend to some extent on bulb envelope dimensions - both
distance from filament and envelope shape.

Oh dear :-) !!!

I'd suspect that taking a variable power supply and the bulb of
interest would produce more useful results in a few minutes than you'd
get from pages of tables.

I'd also guess that 1% of nominal voltage would produce no output and
that 10% of voltage may produce something.

Oh, alright then ...
Mumble ...

T-10 12V 5W nominal.
Small auto dash etc bulb.

V    mA

12   340    nominal
11   330    much the same
10   310    dimming a bit
9     280    and a bit dimmer and yellower
8    270    yellowing
7    250    more
6    230    visibly down
5    200
4    180    very notably down but still very visible as lit bulb
3    150    dim - visible easily at 20 feet in workshop lighting
2    120    visible at 6 feet in workshop lighting
1v5 100    visible on inspection in lit room, clearly visible in dark
1v2    90    On close inspection in room light. Clear in dark
1    80    Not in room light. Whisper of light in dark.

Even at 1v there is no visible surge at turn on - current peaks to 300
mA + before stabilising.

SO
WITH THIS BULB
Voltage of 10% nominal gives filament which is perceptibly on if
carefully inspected in room light. Daylight probably not so. Notable
filament glow in dark.

5% of nominal is reasonably well off.

Note how bulb is a reasonably good current source for up to 20%
reduction in voltage from nominal.

Note that Ibulb is almost 30% of operating for V of 10% of operating.

Current limiting / feed seems a better way to control offness.
10% of on current would ensure substantial visible offness for this
bulb.
20% probably would too.
Even 30% is hardly perceptible.

RM

At 03:45 PM 6/27/2006 -0700, you wrote:
>I've come upon this interesting paragraph on piclist.com (don't I already
>know what is written there? Well, no, in fact now that there is about 3GB of
>text on the site, I find more and more often that I have no clue were stuff
>came from. Rather fun actually!) and I was curious about the unanswered part
>of it: How does one know how much voltage to put through the lamp to heat
>the filament without producing light?

Hi, James:-

I like using math, but I think in this case the easiest and fastest way would
be to plug it into a variac and test it empirically!  A light dimmer could be
used too, but only with a true-RMS  meter.

- for a given lamp construction, the resistance varies over a 10-15:1 range
depending on filament temperature

- the light output in the visible spectrum, total light output, and peak
emission wavelength, and the visible color vary with the filament
temperature.

A filament at 400°C will look black to the eye, but will be emitting a fair
bit of IR.

- there are at least two quite different constructions for line voltage lamps-
vacuum used for low wattage bulbs and gas filled used for higher wattages.
As you might expect, the vacuum 'filled' bulbs increase in resistance
faster,
since it's only being cooled by radiation (4th power of temperature, IIRC)
vs. convection (ignoring loss through the filament support structure).
(the effect from the gas is strong enough to be exploited in vacuum gauges)

Here are a few data points for gas filled:
Voltage    Resistance (+/- maybe 3%)
0%         8%
10%       27%
20%       43%
30%       55%
40%       65%
50%       72%
60%       80%
70%       85%
80%       90%
90%       95%
100%      100%

There are plenty of data and formulas on *light output* vs. RMS voltage, but
usually only in a range near the nominal voltage.

BTW, there's a classic circuit for low distortion Wien bridge oscillators
which is
amplitude-stabilized by the nonlinear resistance of an incandescent pilot
lamp bulb
in the feedback path. The original form was used by Bill Hewlett in HP's
first product
(made in the famous Palo Alto garage)-- the HP200A and bought by (among
others) Disney for
Fantasia- one of their first big orders for 8 pieces at \$54 each. I have
it's descendant
the  HP204D for low-distortion audio testing in conjunction with an HP
distortion analyzer.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speffinterlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
->>Test equipment, parts OLED displys http://search.ebay.com/_W0QQsassZspeff

Russell McMahon wrote:
{Quote hidden}

A few years ago, there was a push on to keep incandescent bulbs ON but
at a very low current, to prevent
the sudden ON pulse that "causes the filament to weaken and then burn
out early". But I was told by someone
later that while that was true to some extent, the main reason is that
the air seal is slowly allowing more and more air
molecules to enter the bulb, and that is what causes the early demise.
Seems that a better seal COULD be made, but
then they'd sell fewer bulbs...

--Bob
> I've come upon this interesting paragraph on piclist.com
> (don't I already know what is written there? Well, no, in
> fact now that there is about 3GB of text on the site, I find
> more and more often that I have no clue were stuff came from.
> part of it: How does one know how much voltage to put through
> the lamp to heat the filament without producing light?
>
> "One feature of incandescent bulbs that you can exploit is
> that when off, an incandescent filament is very low
> resistance- much lower than when it is glowing. This allows
> lamps to be used to protect against short circuits.

I've seen this in a few places.  Car lights was one, a blown bulb was
detected by a lack of current flow when switched on.  Presumably a short
circuit would be fould be lack of current flow caused by  blown fuse.
The stop lights were pre-heated by a small amount of current trickled
thru them.  Quite low, a couple of volts of so.  The theory is that
reducing the current onrush at switch on stops them blowing, and the
light itself comes on faster, so safety is improved (much like LEDs).

I also fixed a set of blinky chaser lights at a fish 'n' chip shop once.
There were 3 strings of lights, driven by 3 relays that were triggered
in sequence.  The system had 2 voltage sources, mains and a drop-down
transformer.  When ON, the relay fed mains (240v) to its string of
lights.  When OFF, it got the lower voltage from the transformer.
Simple, worked well.  So old I suspect it was originally ran the warning
lights on Noah's Ark.

The tranformer had failed (it also powered the relay circuit.  I suspect
it was a 24v unit, but I found a 30v one and put it in.  I played with a
variac and the bulb didn't seem to light up at that low a voltage.
Wouldn't matter if it did, as any light it produced would be washed out
by its neighbour.

Electric stoves are like this too, the element on low power can't be
seen to be glowing with the lights on, but can in the dark.

I've always wanted to set up an experiment to test bulb life.  3 bulbs,
on permanently one, one flashing, and one flashing but with pre-heating.
Set the flash cycle to a few seconds to allow the second one to cool
down.  I guess I already know the answer, but it would be interesting to
see if there were any major differences.

Tony

>Radios in the 1930s used an incandescent lamp as a volume expander.

I have also seen them used as a compandor - the particular example I
remember was in a series of articles that Electronics Australia ran many
years ago, where they built an electronic organ. A suitable 6.3V 300mA dial
lamp was used from the speaker output to drive a spring reverberation unit.
As the output volume got greater the lamp glowed brighter, so a lower
proportion of the output signal was fed to the reverb channel.

>HP used incandescent lamps in their first audio generators
>to stabalize the feedback gain. Too high an output level
>(headed towards clipping) would increase the lamp resistance,
>decreasing the oscillator loop gain.

I believe the later glass-vacuum encapsulated thermistors give a faster
response and flatter output level as frequency is changed.

>I've used incandescent lamps as current limiters in an electric
>vehicle battery charger.  A near constant voltage source is
>applied to the battery string through an incandescent lamp.

I currently have an old car battery on float charge like this. Also the
technique was used in power supplies for kids toys, such as model railway
controllers - the lamp doing double duty as an output short circuit
indicator, as well as being the current limiter.

James Newton, Host wrote:
> A 300'K lamp will have an in
> rush current about the same as its operating current,

Well yeah, considering that 300K is only 27C or 81F.  It shouldn't be much
of a revelation that when you don't change the temperature much from ambient
the resistance doesn't change much either.  In a lot of places you'd have to
cool the fillament to reach 300K.

> but a 2,500'K
> lamp will draw about 12 times the current when cold.
>
> ...
>
> So again, how do you know what the filament resistance / heat / current
> will be given a steady voltage?

The equations get pretty complicated because black body radiation is
significant at one end of the range and convection the dominant effect at
the other end.  In practise, the visible light from a bulb drops off very
quickly as the voltage goes down from the operating point.  I remember
playing with a variable transformer and a car headlight back in grade school
to see what happened when I lowered the voltage and at what point I could
just see the fillament glowing in a dark room.  If I remember right, that
was around 2/3 to 1/2 the rated voltage.

A few years ago I was working on a flight simulator project that needed to
control lots of lamps from a computer.  We used a PWM scheme, but found that
when trying to turn a bulb on full from cold, the current limit would kick
in and prevent the bulb from ever getting warm enough to increase the
resistance to bring the current below the trip point.  (It's an interesting
aside that incandescent bulbs exhibit hystersis and are bi-stable if current
controlled.  I've mused about building some device based on that
characteristic, but have never done so.)  We could fix it in software by
starting the PWM slowly with pulses short enough so that the current limit
didn't respond, but often enough to still raise the temperature.  This way
we could bootstrap the temperature to where we could apply full PWM duty
cycle without excess current.  However, this took too long to be acceptable
in some circumstances, so in the end we did this at system startup and kept
the fillaments partially warm all the time but well below visible
temperature.  If I remember right, we used about 10% of rated voltage and
applied a faster ramp of about 100mS to full brightness when requested.  It
also had the nice side effect of extending bulb life.

******************************************************************
Embed Inc, Littleton Massachusetts, (978) 742-9014.  #1 PIC
consultant in 2004 program year.  http://www.embedinc.com/products
Russell McMahon wrote:
> SO
> WITH THIS BULB

Exactly.

The one I remember playing with was the high beam fillament of a headlight,
and it had rather different characteristics if I remember right (although I
may not).  It might make sense that the big fat fillament I was using falls
off much more quickly than the relatively much more delicate fillament you
tested.  Of course I may just be plain wrong remembering events from 35
years ago.

I have some old auto headlights with high beam only fillaments left, but not
where I am right now.  Actually this is something James should do on his
bulb.  It should only take a few minutes with a variable power supply and
multimeter.

******************************************************************
Embed Inc, Littleton Massachusetts, (978) 742-9014.  #1 PIC
consultant in 2004 program year.  http://www.embedinc.com/products
James Newton, Host wrote:

>> This relationship probably can be found described in some manufacturer
>> literature, I'd hope.
>
> Yeah, that's what I figured, but I'll be darned if I can find it in any
> of the literature from at least three different mfgrs.

I didn't come up with anything either, on a short search.

But I found some other interesting stuff:

- http://www.zeit.de/stimmts/1999/199933_stimmts_gluehbir (German):
Since 1924, an international cartel (?) consisting of GE (USA),
Osram/Siemens (Germany) and Assoc. Elect. Ind. (UK) divided up the world
market between themselves and also defined the "durability point" at 1000h.
Making the filament stronger increases durability by decreasing the energy
efficiency. Chinese lamps seem to be designed for 5000 hours (not part of
the cartel :). Since energy cost is a major factor of the lifetime cost of
a lamp, it may not be beneficial (financially) to increase the lifetime
wouldn't publish their contract, so this is "leaked classified
information", at most :)

- www.wissen.swr.de/warum/gluehlampe/themenseiten/t2/s1.html#
(German):
A nice little toy virtual lab that associates durability, color temperature
and electric power. Doesn't seem to work with the low powers you want, and
doesn't give real numerical results (except for the durability).

- freespace.virgin.net/tom.baldwin/bulbguide.html:
Some general info about incandescent lamps. See
http://freespace.virgin.net/tom.baldwin/bulbguide.html#pay for an argument
about why making a bulb last longer may not "pay".

>> As easier methods there are thumb-rule percentage of the rated power, or
>> a thumb-rule resistance increase from the cold resistance. The thumb
>> rules maybe created using the above...
>
> I'd be happy with those thumb-rules... You have any idea what they are?
> Or do I have to create them myself?

I think you'd probably have to do something along the lines of what Russell
did.

I suspect that to some degree the results of what means "off" will vary
with the type of lamp, possibly with the rated power of the lamp (see the
report from Harold; I think that maybe very high power lamps glow with a
lower percentage of voltage or current than lower power lamps), and almost
certainly with the application (environment light).

Gerhard

>  ... I remember
> playing with a variable transformer and a car headlight back in
> to see what happened when I lowered the voltage and at what point I
> could
> just see the fillament glowing in a dark room.  If I remember right,
> that
> was around 2/3 to 1/2 the rated voltage.

More like about 5% to 10% - see my rough results in another recent
post.
A 1v 5W bulb had a barely visible filament glow at 1V. Current was
still an appreciable %age of full value but power, being VI product,
was well down.

Russell

At 10:26 AM 6/28/2006 -0300, you wrote:

>- http://www.zeit.de/stimmts/1999/199933_stimmts_gluehbir (German):
>Since 1924, an international cartel (?) consisting of GE (USA),
>Osram/Siemens (Germany) and Assoc. Elect. Ind. (UK) divided up the world
>market between themselves and also defined the "durability point" at 1000h.
>Making the filament stronger increases durability by decreasing the energy
>efficiency. Chinese lamps seem to be designed for 5000 hours (not part of
>the cartel :). Since energy cost is a major factor of the lifetime cost of
>a lamp, it may not be beneficial (financially) to increase the lifetime
>wouldn't publish their contract, so this is "leaked classified
>information", at most :)

Or maybe just an attempt to create a reasonable industry standard for
life and energy efficiency.

As you listed in your post, lifetime curves of voltage vs. hours and
efficiency vs. voltage are easily available. If don't mind a reddish
light and poor energy  efficiency (a 25-cent bulb will burn much more
energy than that over its lifetime), then you can dim the bulb or buy
a 130V bulb for 120V  use. Until I put compact fluorescents in our
garage door opener it had heavy duty (even worse energy efficiency due
to the heavy filament and supports) 130V bulbs to withstand the motor
vibration. As it's only on for a few minutes a day, energy efficiency
wasn't a big deal. At the other end of the spectrum (so to speak),
photoflood lamps have a blue-white light and high energy efficiency, but
last an average of only 6 hours. I have a lit bundle-of-branches
'tree' in my office (Josh K. has seen it) that has white Xmas-type lights
on it. I replaced one of of the (series string) bulbs with a (carefully
insulated) 1N4007 to knock the voltage down to 70% of line and get several

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speffinterlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
->>Test equipment, parts OLED displys http://search.ebay.com/_W0QQsassZspeff

> I replaced one of of the (series string) bulbs with a (carefully
> insulated) 1N4007 to knock the voltage down to 70% of line and get several
>

Which reminds me... Some projectors use 70V lamps. I assume they get the
70V by half wave rectifying the 120VAC line. But why not just use a 120V
lamp? I'm guessing that it MAY have something to do with being able to
have a shorter filament making the light more of a point source and easier
to focus. Is the guess correct?

Harold

--
FCC Rules Updated Daily at http://www.hallikainen.com - Advertising
opportunities available!

>-----Original Message-----
>From: piclist-bouncesmit.edu [piclist-bouncesmit.edu]
>Sent: 28 June 2006 16:29
>To: Microcontroller discussion list - Public.
>Subject: Re: [EE] Predicting keep alive voltages for incandescent lamps
>
>
>> I replaced one of of the (series string) bulbs with a (carefully
>> insulated) 1N4007 to knock the voltage down to 70% of line and get
>>
>
>
>Which reminds me... Some projectors use 70V lamps. I assume
>they get the 70V by half wave rectifying the 120VAC line. But
>why not just use a 120V lamp? I'm guessing that it MAY have
>something to do with being able to have a shorter filament
>making the light more of a point source and easier to focus.
>Is the guess correct?

That sounds reasonable. The only other explanation I can muster is that lower voltage lamp filaments are physicaly tougher as they are shorter and thicker, which is why standard house lamps don't last too long in inspection lights.

Regards

Mike

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At 08:29 AM 6/28/2006 -0700, you wrote:
> > I replaced one of of the (series string) bulbs with a (carefully
> > insulated) 1N4007 to knock the voltage down to 70% of line and get several
> >
>
>
>Which reminds me... Some projectors use 70V lamps. I assume they get the
>70V by half wave rectifying the 120VAC line.

That would give you ~85VAC. It also might be prohibited because of the
unpleasant input waveform. Maybe they regulate the voltage fed to those bulbs.

>But why not just use a 120V
>lamp? I'm guessing that it MAY have something to do with being able to
>have a shorter filament making the light more of a point source and easier
>to focus. Is the guess correct?

Don't know for sure, but it wouldn't be surprising if there was an optimum
tungsten filament voltage which was between 12V and 120V. Newer cars with
42V electrical systems may benefit from this, to the extent they still use
incandescent lamps.

>Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speffinterlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
->>Test equipment, parts OLED displys http://search.ebay.com/_W0QQsassZspeff

On 6/28/06, Spehro Pefhany <speffinterlog.com> wrote:
> At 08:29 AM 6/28/2006 -0700, you wrote:
> > > I replaced one of of the (series string) bulbs with a (carefully
> > > insulated) 1N4007 to knock the voltage down to 70% of line and get several
> > >
> >
> >
> >Which reminds me... Some projectors use 70V lamps. I assume they get the
> >70V by half wave rectifying the 120VAC line.
>
> That would give you ~85VAC. It also might be prohibited because of the
> unpleasant input waveform. Maybe they regulate the voltage fed to those bulbs.
>

Are you referring to film projectors?  They use a transformer for the
bulb voltage. I believe this is to keep noise out of the audio
preamplifier.  I found this out when I made an SCR dimmer for a
projector.

Regards,
Mark
markrages@gmail
--
You think that it is a secret, but it never has been one.
On 6/27/06, James Newton, Host <jamesnewtonpiclist.com> wrote:
>
> > I have some experience doing this in a 12V, 100W application.
> >  I think the most practical thing is to make a graph of
> > steady state current vs voltage.
> >
> > Regards,
> > Mark
>
> You wouldn't happen to still have that graph would you?

It was at a previous job.  Like others thave said, it wouldn't help you much.

It just takes a couple minutes to draw.  Easiest if you have a bench
power supply with built-in ammeter.

Regards,
Mark
markrages@gmail
--
You think that it is a secret, but it never has been one.
Regarding glass/wire seals,
I've been wondering where I could get the special wire they use for the
pass-through on a glass bulb?
I am interested in playing around with simple homemade vacuum tubes.
--
Martin K

Bob Axtell wrote:

{Quote hidden}

{Quote hidden}

Yes, it's film projectors. How would the lower voltage keep noise out of
the audio preamp? I DO remember seeing old 16mm projectors that used a
high frequency oscillator to run the sound exciter lamp. Never could
figure out why they didn't just use DC.

Harold

--
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