Electronics Repair Manuals

Figure 2-6 shows various circuit symbols used in schematic diagrams to represent inductors. Figures 2-6A and 2-6B represent alternate, but equivalent, forms of lhe same thing; i.e., a fixed value, air-core inductor ("coil" in the vernacular). The other forms of inductor symbol shown in Fig. 2-6 are based on Fig, 2-6A but are just as valid if the "open-loop" form of Fig- 2-6B is used instead.

The form shown in Fig, 2-6C is a tapped fixed-value air-core inductor. By providing a tap on the coil, different values of fixed inductance are achieved. The inductance from one end of the coil to the tap is a fraction of the inductance available across the entire coil. By providing one or more taps, several different fixed values of inductance can be selected. Radio receivers and transmitters sometimes use the tap method, along with a bandswitch, to select different tuning ranges or 'bands."

Variable inductors are shown in Figs. 2-6D and 2-6E. Both forms are used in schematic diagrams, although in some countries Fig. 2-6D implies a form of construction whereby a wiper or sliding electrical contact rides on the uninsulated turns of the coil. Figure 2-6E implies a construction where variable Inductance is achieved by moving a magnetic core inside of the coil.

Figure 2-6F indicates a fixed value (or tapped, if desired) inductor with a powdered iron, ferrite or nonferrous (e.g., brass) core. The core will increase (ferrite or powdered iron) or decrease (brass) the inductance value relative to the same number of turns on an air core coil.

Forms of inductor symbol:

(A) fixed (open loop style); C [ (B) fixed (closed loop style):

(C) tapped; (I)) variable (style 1); (E) variable (style 2); (F) powered iron or ferrite Svvwtyv $lug-tuned core inductor.

Impedance (Z) is the total opposition to the flow of ac in a circuit and as sttch it is analogous to resistance in do circuits. Impedance is made up of a resistance component (If) and a component called reactance (X) Like resistance, reactance is measured in ohms. If the reactance is produced by an inductor, then it is called ¿ri-ductive reactance (X0 and if by a capacitor it is called capacitive reactance (Xv). Inductive reactance is a function of the inductance and the frequency of the ac source:

Xu - 2irFL

where

XL = inductive reactance in ohms (H) F - ac frequency in hertz (Hz) L - inductance in henrys (H).

In a purely resistive ac circuit (Fig, 2-7A), the current (J) and voltage (V) are said to be in phase with each other (i.e.t they rise and fall at exactly the same times in the ac cycle). In vector notation (Fig. 2-7B), the current and voltage vectors are along the same axis, which is an indication of the zero-degree phase difference between the two.

I and V

I and V

Time

2-7 (A) Current and voltage iii phase with each other (sine wave with vector notation); (B) vector relationship.

In an ac circuit that contains only an inductor (Fig. 2-8A), and is excited by a sine-wave ac source, the change in current is opposed by the inductance. As a result, the current in an inductive circuit lags behind the voltage by 90°, This is shown vec-toriaUy in Fig. 2-8B, and as a pair of sine waves in Fig. 2-8C.

The ac circuit that contains a resistance and an inductance (Fig. 2-9B) shows a phase shift (0), shown veetorially in Fig. 2-9A, oiher than the difference seen in purely inductive circuits. The phase shift is proportional to the voltage across the inductor and the current flowing through it. The intpendance of tills circuit is found by the Pythagorean rule described earlier, also called the root of the sum of squares method (see Fig. 2-!J)-

The coils used in radio receivers come in a variety of different forms and t ypes, but all radios (except the very crudest untuned crystal sets) will fwve at least one coil.

Now let us turn our attention to the other member of the LC tuned circuit, the capacitor.

2-8 (A) Inductor ac circuit; (B) vector relationships; (CJ sine-wave representation.

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