On Saturday, March 12, 2011 1:27:19 AM UTC-8, Existential Angst wrote:


Inyone have data (or a link), or feel like doing the 'speriment?

Oddly, this cannot usefully be done as a V-versus-I plot.

The resistance of a tungsten filament, measured cold, is very
different from the resistance when incandescent and hot.
And, that difference means that a hot filament, lowered
to 60V, may have different current than a cold filament
raised to 60V. Try rotating a dimmer clockwise until
the light comes on, then turn counterclockwise to turn
it off- the effect is clear.

So-called 'ballast tubes' are just incandescent lamps designed
so the current is near constant over a range of voltages.

whit3rd wrote:

On Saturday, March 12, 2011 1:27:19 AM UTC-8, Existential Angst wrote:

Inyone have data (or a link), or feel like doing the 'speriment?

Oddly, this cannot usefully be done as a V-versus-I plot.

The resistance of a tungsten filament, measured cold, is very
different from the resistance when incandescent and hot.
And, that difference means that a hot filament, lowered
to 60V, may have different current than a cold filament
raised to 60V. Try rotating a dimmer clockwise until
the light comes on, then turn counterclockwise to turn
it off- the effect is clear.

So-called 'ballast tubes' are just incandescent lamps designed
so the current is near constant over a range of voltages.


They aren't tungsten, like a lightbulb.


--
You can't fix stupid. You can't even put a Band-Aid™ on it, because it's
Teflon coated.

On Mar 12, 3:23*pm, whit3rd wrote:
On Saturday, March 12, 2011 1:27:19 AM UTC-8, Existential Angst wrote:
Inyone have data (or a link), or feel like doing the 'speriment?

Oddly, this cannot usefully be done as a V-versus-I plot.

The resistance of a tungsten filament, measured cold, is very
different from the resistance when incandescent and hot. *
And, that difference means that a hot filament, lowered
to 60V, may have different current than a cold filament
raised to 60V. * Try rotating a dimmer clockwise until
the light comes on, then turn counterclockwise to turn
it off- the effect is clear.

Yes, but it's from the dimmer, not the filament. I saw a thermal
settling time when I changed the Variac voltage and had to chase the
current setting I wanted.

The rule of thumb I've heard and confirmed was a 12X resistance change
from cold to hot. The current roughly doubles from 25V to 125V.

jsw

Years ago my boss became obsessed with the inrush current on an incandescent
filament and proceeded to scope current inrush through a 100W bulb for days,
taking pictures of the old storage scope (phosphor memory screen and 35mm
camera OMG!).

We found the best peak was about 10 times the quiescent full-on current. I
forget how long it took to fade to normal current. IIRC It was about 2-10
cycles depending on the mass of the filament and how you hit the start in
the cycle zero crossing.

-----------------

"Jim Wilkins" wrote in message
...
Yes, but it's from the dimmer, not the filament. I saw a thermal
settling time when I changed the Variac voltage and had to chase the
current setting I wanted.

The rule of thumb I've heard and confirmed was a 12X resistance change
from cold to hot. The current roughly doubles from 25V to 125V.

jsw

---------------

The resistance of a tungsten filament, measured cold, is very
different from the resistance when incandescent and hot.
And, that difference means that a hot filament, lowered
to 60V, may have different current than a cold filament
raised to 60V. Try rotating a dimmer clockwise until
the light comes on, then turn counterclockwise to turn
t off- the effect is clear.

On Mar 12, 10:04*pm, "Josepi" wrote:
Years ago my boss became obsessed with the inrush current on an incandescent
filament and proceeded to scope current inrush through a 100W bulb for days,
taking pictures of the old storage scope (phosphor memory screen and 35mm
camera OMG!).
...

Another tech did that for fuses. I didn't realize what he was doing
until he asked me to order another case of film. He was really
downfallen and discouraged after I showed him the I^2*T curves in the
Littelfuse catalog.


"Jim Wilkins" *wrote in message

...
Yes, but it's from the dimmer, not the filament. I saw a thermal
settling time when I changed the Variac voltage and had to chase the
current setting I wanted.

The rule of thumb I've heard and confirmed was a 12X resistance change
from cold to hot. The current roughly doubles from 25V to 125V.

jsw

---------------

The resistance of a tungsten filament, measured cold, is very
different from the resistance when incandescent and hot.
And, that difference means that a hot filament, lowered
to 60V, may have different current than a cold filament
raised to 60V. * Try rotating a dimmer clockwise until
the light comes on, then turn counterclockwise to turn
t off- the effect is clear.

You quoted what I saw but did you read and understand it? The voltage
corresponding to the setpoint current was the SAME whichever way I
approached it, after the temperature stabilized, which took perhaps
half a second. The lamp dimmer had considerable hysteresis but the
Variac didn't. I used the same equipment to slow down the laminate
trimmer earlier, and they behaved the same way.

I once salvaged a bunch of damaged Powerstats (The Superior Electric
equivalent) to make a few complete ones. The cores are rolls of steel
tape, like duct tape. There are no notches or air gaps. I tried to
make speaker crossovers from them but their response fell off to
useless around 400Hz.

We had built the Powerstats into a large machine for GE. The break
whistle sounded while it was on the forklift there and the union
driver didn't wait to lower it. When they returned it had tipped the
forklift forward and smashed in the front panels on the floor.

jsw