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RECTI 115
trolytic condenser, as is indicated by the fact that its terminals are marked + and — . 1 >i recti y above this is another circuit (more elaborately filtered) which contains L-l, L-2 and L-3; C-l, C-2 and C-3. R-l is a rheostat (compare Fig. 35 B) by which the voltage output of this circuit may be adjusted. R-3 is the terminal or "Bleeder" resistor filter. S-l is an electro-magnetic relay. A as current flows in this circuit (completing its path through the external load), S-l draws down the hinged bar shown just above it, out of contact with the arrow head. But if the external circuit is f< >r any reason opened, S-l no longer carries current, the bar springs back and contacts the arrowhead, and current then flows ( tracing positive to negative) from the left side of L-2 down through P-5. This is an automatic provision to protect the other circuits against any rise in voltage if the external load in series with S-l be switched out of use. The student should find it instructive to trace the two similar, but not identical, filter circuits at the right side of the diagram. He will also note a 24-volt output at the bottom of the diagram, which is not filtered.
(11) Filter circuits which are similar in principle it* not always in detail to those just explained are often, in practice, linked with voltage dividers (Fig. 11 E) — the direct current, after it has been rectified and filtered, is applied to a voltage divider from which a number of different d.c. voltages are drawn, according to the require
sments of the amplifying tube elements to be supplied. The combination of rectifier, filter and voltage divider is particularly common in sound amplifiers. Examples of it will be given later when amplifier circuits are considered in detail.
(12) Rectified d.c. is not always filtered. Direct current supplying the field windings of loud speakers often is not, inasmuch as those windings are themselves indue tances, hence do a good job of filtering their own input. Sometimes, however, condensers are associated with them, in which case they are. electrically speaking, filter inductors which serve simultaneously as the elect magnetic windings of loud speaker units. Occasionally