On 6/27/06, James Newton, Host <jamesnewton@piclist.com> 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?
>
> "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
> be given a steady voltage?

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
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