Page 207 - The Mechatronics Handbook

0066_Frame_C11 Page 13 Wednesday, January 9, 2002 4:14 PM

FIGURE 11.16 Practical voltage source.

FIGURE 11.17 Practical current source.
its terminal:

v S t()
it() = ----------- (11.21)
R
This circuit suggests that the ideal voltage source is required to provide an infinite amount of current to
the load, in the limit as the load resistance approaches zero.
Figure 11.16 depicts a model for a practical voltage source; this is composed of an ideal voltage source,
v S , in series with a resistance, r S . The resistance r S in effect poses a limit to the maximum current the
voltage source can provide:

i S max = v S (11.22)
----
r S
It should be apparent that a desirable feature of an ideal voltage source is a very small internal resistance,
so that the current requirements of an arbitrary load may be satisfied.
A similar modification of the ideal current source model is useful to describe the behavior of a practical
current source. The circuit illustrated in Fig. 11.17 depicts a simple representation of a practical current
source, consisting of an ideal source in parallel with a resistor. Note that as the load resistance approaches
infinity (i.e., an open circuit), the output voltage of the current source approaches its limit,
v S max = i S r S (11.23)

A good current source should be able to approximate the behavior of an ideal current source. Therefore,
a desirable characteristic for the internal resistance of a current source is that it be as large as possible.

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