e. The third step in the study of this circuit is to change the frequency of
the ac generator to a high frequency while maintaining the same voltage amplitude.
The current through resistor R1 will remain unchanged, but the currents in the
inductive and capacitive branches will be just the opposite of what they were in
the low-frequency condition.
The capacitive path will now have a low impedance
because of the low reactance of the capacitor at high frequencies.
The current
through resistor R2 will be high, causing a high-voltage drop across it.
The
voltage across capacitor C will be low due to its low reactance. The impedance of
the third path will be high because of the high reactance of the coil at high
frequencies.
Very little current will flow in this branch, therefore, causing a
low voltage across resistor R3. A high voltage will appear across the coil because
of its high reactance.
f. The effect of both ac and dc on the circuit components can be determined
by turning switches S1 and S2 on at the same time. The important effects will be
those caused by changing from low frequency to high frequency. The path through R1
will carry both dc and ac current equally, regardless of the frequency used. The
path through R2 will carry a small ac current at low frequencies and a large ac
current at high frequencies.
Direct current will be blocked from this branch
because of the insulating property of a capacitor. (When used in this manner, the
capacitor is often called a blocking capacitor.) Notice that the dc current through
R3 remains the same, but the ac current will change inversely to the change in
frequency. High-frequency current will be greatly reduced, or choked by the coil's
action in the branch containing R3. For this reason, a coil in series with a dc
circuit containing ac is often called a choke coil. Thus it can be seen that with
both ac and dc present in a circuit, the current flow of either may be permitted,
stopped, or restricted by the proper choice of circuit component values.
17.
IMPEDANCE
The total opposition to the flow of ac is called impedance, and is designated
by the letter Z. Impedance is the opposition to current flow caused by both the
resistance and the reactance in an ac circuit, and it is measured in ohms.
Resistance and reactance cannot be added together by simple addition to give the
impedance because there is a 90 angular difference between them at all times. The
resistance in the circuit absorbs electrical energy and converts it to heat,
whereas reactance stores electric energy temporarily in the form of a magnetic or
an electric field, and returns it later to the circuit whence the energy came.
a. Significance of Impedance.
Since the value of impedance will always be
greater than either the resistance or reactance taken as a separate value, the
total current flowing in the circuit will be limited by the value of impedance, not
by resistance or reactance alone.
Since the impedance is an overall circuit
characteristic, equipment manufacturers usually quote characteristics of their
equipment in terms of their input and output impedances.
b. Impedance Matching. Maximum power is transferred to a load only when the
impedance of the load equals that of the source. Regardless of the method used to
achieve impedance matching, the purpose is the same, namely, to alter the impedance
characteristics of the circuit or its load, or both, so that each matches the
characteristics of the other.
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