> Incandesent lamps draw a nearly constant current independent of voltage.
Not true.
> This is because their resistance changes with temperature. The hotter
> the filament gets, the higher their resistance gets.
Absolutly.
> The situation is
> different when you first turn it on though.
No, it is the same. And you explain it in your next statment.
> If you measure the
> resistance of a cold bulb, you will find it is nearly a short. And
> at the instant it is turned on there is high inrush current, but only
> for a very short time.
...while the filiment heats up and the resistance increases.
It is true that the resistance of a bulb increases with temperature but
the current DOES vary with the voltage. For a typical bulb, if you run
it at 60% its rated voltage the current will be about 75% it's current
at rated voltage. If you run at 140% Rated Voltage the current will be
about 125% rated current.
The inrush current at turn on is 10-11 times rated current. After 0.010
seconds the is 2-3 times rated current. After 0.070 seconds the bulb is
operating at rated current.
I hope this helps.
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