[AMPS] Swinging Choke Theory

[Peter Chadwick]

In the choke input filter, the current is only 'constant' with infinite
inductance - see Terman. For a finite inductance, there is a variation,
although the esign of the filter minimises that. In the 'tuned choke'
filter, the idea is that the parallel C and L look like a big inductance
when near (slightly HF of ) resonance, so keeping the current much more
nearly constant in the choke e.g on 50 Hz mains with full wave rectification
and 100Hz ripple, a 10Henry choke and a 0.25 microfarad shunt capacitor look
like a choke of 762.5 Henries. So a swinging choke will move further from
'resonance' (the definition of resonance is important here) as the current
goes up, but as the critical inductance comes down, the regulation may not
be too bad - depending on values, choke resistance etc. Not ideal, but
possibly workable.

One thing to avoid is the combination looking capacitive at minimum current;
as the choke inductance goes down with increasing current, you can hit
resonance with some very high voltages.

73

Peter G3RZP


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