The Swinging Choke

Author: N.H. Crowhurst

Another kind of filter circuit employs the so-called "swinging" choke. All smoothing chokes employ iron cores with air gaps that prevent saturation. By properly choosing the size of the air gap, a special action is produced. At low load currents, the core is not saturated, but for higher current it progressively approaches saturation, which makes the circuit act as a capacitor-input filter. Capacitor-input filters produce higher output voltages; hence, the output at the filter can be made to rise with increased load current.

At small load currents, the inductance of the choke is sufficient to make the filter behave as a choke-input arrangement, and the output voltage is not more than 0.637 of the alternating peak voltage. As the current drain increases, the choke begins to saturate, and the rectifier starts pulse-feeding the capacitor at the output end of the choke. The circuit then begins to act as a capacitor-input filter and the output voltage rises.

Because the current is increasing at the same time, the output cannot possibly reach the peak value of the applied a-c because the drain effect willcause dips between the peaks, but the average voltage can rise with a carefully designed filter of this kind. This is useful because it will serve to offset the voltage drop in the supply circuit that always tends to reduce the output voltage with increased load current. If the rise produced by the swinging choke just offsets the losses produced by increased current through the rectifier, the power transformer, and possibly a further smoothing choke, the output voltage of this kind of filter will be almost perfectly constant as the load current is changed.

The swinging choke filter