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A photograph of the outside and inside of a commercially available
logic probe is provided in Fig. 8.90(c). Note the increased
complexity of system because of the variety of functions that the probe
can perform.
8.14 COMPUTER ANALYSIS
PSpice
The bridge network of Fig. 8.70 will now be analyzed using PSpice to
ensure that it is in the balanced state. The only component that has not
been introduced in earlier chapters is the dc current source. It can be
obtained by first selecting the Place a part key and then the SOURCE
library. Scrolling the Part List will result in the option IDC. A left
click of IDC followed by OK will result in a dc current source whose
direction is toward the bottom of the screen. One left click of the mouse
(to make it red—active) followed by a right click of the mouse will
result in a listing having a Mirror Vertically option. Selecting that
option will flip the source and give it the direction of Fig. 8.70.
The remaining parts of the PSpice analysis are pretty straightforward,
with the results of Fig. 8.91 matching those obtained in the analysis
of Fig. 8.70. The voltage across the current source is 8 V positive to
ground, and the voltage at either end of the bridge arm is 2.667 V. The
voltage across R5 is obviously 0 V for the level of accuracy displayed,
and the current is of such a small magnitude compared to the other current
levels of the network that it can essentially be considered 0 A. Note
also for the balanced bridge that the current through R1 equals that of
R3, and the current through R2 equals that of R4. FIG. 8.91
Applying PSpice to the bridge network of Fig. 8.70.
COMPUTER ANALYSIS ⏐⏐⏐ 307