RLC Impedance Help

By — McGraw-Hill Professional
Updated on Sep 9, 2011


We’ve seen how inductive and capacitive reactance can be represented along a line perpendicular to resistance. In this section we’ll put all three of these quantities— R, X L , and X C —together, forming a complete working definition of impedance .

The Rx Half-plane

Recall the quarter-planes for resistance R and inductive reactance X L from the preceding sections. This is the same as the upper-right quadrant of the complex-number plane. Similarly, the quarter-plane for resistance R and capacitive reactance X C is the same as the lower-right quadrant of the complex-number plane. Resistances are represented by nonnegative real numbers. Reactances, whether they are inductive (positive) or capacitive (negative), correspond to imaginary numbers.

There is no such thing, strictly speaking, as negative resistance. That is to say, one cannot have anything better than a perfect conductor. In some cases, a source of dc, such as a battery, can be treated as a negative resistance; in other cases, a device can behave as if its resistance were negative under certain changing conditions. Generally, however, in the RX (resistance-reactance) half-plane , the resistance value is nonnegative (Fig. 15-11).

More About Alternating Current RLC Impedance Vector Representation Of Impedance

Fig. 15-11 . The RX impedance half-plane showing some points for specific complex-number impedances.

Reactance In General

Now you should get a better idea of why capacitive reactance X C is considered negative. In a sense, it is an extension of inductive reactance X L into the realm of negatives, in a way that generally cannot occur with resistance. Capacitors act like “negative inductors.” Interesting things happen when capacitors and inductors are combined.

Reactance can vary from extremely large negative values, through zero, to extremely large positive values. Engineers and physicists always quantify reactances as imaginary numbers. In the mathematical model of impedance, capacitances and inductances manifest themselves perpendicularly to resistance. Thus ac reactance occupies a different and independent dimension from dc resistance. The general symbol for reactance is X ; this encompasses both inductive reactance X L and capacitive reactance X C .

Vector Representation Of Impedance

Any impedance Z can be represented by a complex number R + jX , where R can be any nonnegative real number and X can be any real number. Such numbers can be plotted as points in the RX half-plane or as vectors with their end points at the origin (0 + j 0). Such vectors are called impedance vectors .

Imagine how an impedance vector changes as either R or X or both are varied. If X remains constant, an increase in R causes the vector to get longer. If R remains constant and X L gets larger, the vector also grows longer. If R stays the same as X C gets larger (negatively), the vector grows longer again. Think of point representing R + jX moving around in the plane, and imagine where the corresponding points on the resistance and reactance axes lie. These points can be found by drawing straight lines from the point R + jX to the R and X axes so that the lines intersect the axes at right angles. This is shown in Fig. 15-11 for several different points.

Now think of the points for R and X moving toward the right and left or up and down on their axes. Imagine what happens to the point R + jX and the corresponding vector from 0 + j 0 to R + jX in various scenarios. This is how impedance changes as the resistance and reactance in a circuit are varied.

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