I have been experimenting with some NTC thermistors to use in a filament
transformer 120vac primary to limit the secondary (tube filament) current flow.
The unit showing the most promise, model CL90, is rated at 120 ohms cold,
dropping to about 1.2 ohms hot.
Can anyone here help with the method to calculate the resistance change vs.
time in one of these devices?
At turn on, the tube filament resistance increases about 10 X with time as
the thermistor decreases about 100 X with time. What I really want to determine
is the peak filament current while all the resistance changes are taking
place.
As a first step, I have inserted a 1 ohm resistor in series with the primary
and added a diode and filter C that charges to the peak voltage across the
resistor. If Ohms law works here, at turn on, it seems that the peak current
in Amps would equal the charged voltage in Volts.
It would be reassuring if the measured data agreed with some calculations.
Anyone been there/done that?
Thanks/73,
Gerald K5GW
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