Before I started writing Part II, I did a bit of arm-twisting when John got back from Vietnam, hoping he’d do some of the heavy lifting for me and make a series of measurements that would illustrate the points I alluded to in the first part of the Filament Supply article. John, as usual, measured things to a fare-the-well, and gave me more things to write about. With a big hat-tip to John A, all of the figures and test data in this article come from Research Report #002, DC Filament Supply Test.
The usual DC power supply for DHT filaments is similar to a typical transistor-amp power supply, with a high-current transformer driving a four-diode bridge, and the bridge connected to a large-value first cap. (Transistor amps typically have a plus and minus supply, while a filament supply has only one rail, but that doesn’t change the basic behavior of the supply.)
It should be kept in mind the first cap is not a filter; to act as a filter, it would have be preceded by a known resistance or an inductance. As it is, the diode bridge is severely nonlinear, so cannot be analyzed in classic RC or LC filter terms. A more accurate characterization of the first cap is a charge/discharge element; the rectifier bridge is only turned on for the brief moment when the incoming AC voltage is higher than the stored voltage of the DC side of the circuit. At all other times - and certainly through the zero-crossing region - the rectifiers are completely shut off and the circuit is powered from stored energy in the first cap.
What interesting is the consequence of the DCR of the large electrolytic cap, the rectifier bridge, the power transformer, and the power line going into the circuit. The DCR is only thing limiting the current flow; in principle, with zero DCR, the current flow into the first cap would be infinite! Obviously, the DCR isn’t infinite, but it isn’t a controlled variable, either. As the power transformer gets bigger, the diode drop goes lower, and the stray DCR of the first cap goes lower, more peak charging current flows into the first cap, and the charging interval gets shorter.
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