Thanks for the several replies. I see that I left out some important info,
as usual.
The tube being protected is a TH347, and is expensive, $3500 new, $1400
rebuilt. I have lost one tube to turn on surges so would like to make sure the
replacement is the last one!
The filament is rated for 6.0v, 34a so the current limiting has to be done
in the transformer primary where the current is about 1.9 amps. The
manufacturer cautions that the filament current must be limited to 75 amps on
the first
cycle. I would like to limit the current to about 30 amps peak during the
initial turn on period. There is a series connected 50 watt rheostat for
voltage
adjustment after everything is up to temperature. There is also a relay in
the circuit that shorts out the NTC thermistor after about 10 seconds. The
shorting is done for several reasons; 1> to have a way for it to cool off and
be
ready for a recycle, 2>to allow it to run cold nearly all the time for long
life, 3> to eliminate the voltage drop across the device and 4> to help
reduce voltage variations due to variation in cooling air circulating around
the
thermistor.
The fact that the cold filament resistance is only about .0176 ohms makes
the problem a bit more complex. The filament transformer is a 6.3vac, 50 amp
unit from Dahl (34 amp would be a special build, costs more).
Considering that primary current is about 1/18 of the secondary current, I
am looking for 30/18 amps there or about 1.666 amps. Assuming that the cold
filament resistance is .0176 ohms, then in a perfect transformer, the
reflected
resistance will be 18^2 X .0176 or 5.7 ohms. The voltage at the primary
would need to be 1.666 X 5.7 or about 9.5v at turn on. If only a resistor is
used
then it must drop the remainder of the line voltage. In the case of 117vac
the resistor would need to drop 107.5 volts. The resistor value would be 107.5
/ 1.666 or about 65 ohms.
After a few milliseconds the filament resistance will have increased. I
found a reference about Tungsten resistance increasing at the rate of about .5%
per degree C. Starting at room temperature the tube needs to reach about 2000
degrees C so the question is how far on the way to 2000 degrees C will the 65
ohm resistor allow?
I wrote a simple Basic program to iterate the filament resistance vs
voltage/current and it shows the circuit to stabilize at 27.2a filament
current,
1.12a primary current, .041 ohm filament resistance and 325 degrees C. So far
so
good. BUT, if the resistor is shorted at this point the filament current will
jump to about 6/.041 or 146 amps! This is not good!
This is where the NTC thermistor was added to the mix. Actually, there are
two of them in series, 120 ohms each so the starting current is very low and
increases as the thermistors heat up.
I don;t have a scope (and it would need to be a storage scope as things are
changing quickly and I already know that the first cycle is not the one
producing the most current). So the next approach was the one ohm primary
resistor
with diode and 100 mFd storage capacitor. At turn on the capacitor is
charged to the peak value across the one ohm resistor (less diode drop). My DVM
has
a 10megohm input impedance so there is plenty of time to measure the
capacitor voltage before the cap discharges.
I am moderately confident of this approach but was hoping for some help with
the mathematics involved with modeling the problem. I know that the
thermistor resistance is dropping based on its temperature but I don't know how
to
convert the amperes flowing into degrees C and how long it takes for all this
to happen. A similar thing is happening in the filament structure but with the
current limiting action this seems a lot more predictable as the temperature
should be closely related to the filament volts X amps flowing at a given
time sample. The Basic program will give a fair accounting of the filament
action.
Another slant on this is that the thermistors start out with maximum power
dissipation per ac cycle and taper off to a low value per cycle; the filament
starts out with minimum dissipation per cycle and ends up at maximum. I am
trying to confirm that all is well at the crossover point.
Voltage regulation and adjustment is a whole different subject that gets
dealt with after that relay shorts out the limiter circuit discussed above.
Marv, W6FR has given some excellent input on a regulator circuit that will be
very helpful when the current limiting problem is resolved.
Any help, suggestions or comments are appreciated.
Thanks/7,
Gerald K5GW
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