Page 29 - Understanding Automotive Electronics

2735 | CH 1 Page 16 Tuesday, March 10, 1998 10:52 AM

1 AUTOMOTIVE FUNDAMENTALS

point c) because of the resonant circuit formed between the coil and
capacitor.
It will be shown in Chapter 7 that in electronic ignition systems the
breaker points are replaced by a solid-state switch (in the form of a transistor).
In Chapter 3 it will be shown that a transistor in saturation is equivalent to a
closed switch, and a cutoff transistor is equivalent to an open switch. It is
further explained in Chapter 7 that the transistor state (i.e., saturation or
cutoff) is controlled electronically in order to set dwell and spark timing.
A multisurfaced cam, The mechanism for opening and closing the breaker points of a
mounted on the distrib- conventional distributor is illustrated in Figure 1.12. A cam having a number
utor shaft, is used to of lobes equal to the number of cylinders is mounted on the distributor shaft.
open and close the As this cam rotates, it alternately opens and closes the breaker points. The
breaker points. movable arm of the breaker points has an insulated rubbing block that is
pressed against the cam by a spring. When the rubbing block is aligned with a
flat surface on the cam, the points are closed, as shown in Figure 1.12a. As the
cam rotates, the rubbing block is moved by the lobe (high point) on the cam
as shown in Figure 1.12b. At this time, the breaker points open and spark
occurs.

Figure 1.12
Breaker Point
Operation

FPO

16 UNDERSTANDING AUTOMOTIVE ELECTRONICS