TABLE OF CONTENTS On the top of page 137, positive feedback in a relay system was seen to lead to a "latched" condition, where the relay continues to feed operating signals back to itself and therefore continue in one position, until the current is somehow interrupted by some other action. It should be noted, however, that when a relay armature operates, it hits the "normally open" contact, which is a hard barrier to any further motion. It might bounce back a little bit, but it can not "overswing." Therefore, positive feedback leads to stable latching and no further motion.
On page 138, it was seen that negative feedback can cause "oscillation" of the relay armature up and down. However, at the top of page 136, it was pointed out that feeding back the signal to stop operating (that is, negative feedback) took several milliseconds to occur. With a carefully timed delay (see the text just under Fig. 12.7), the up and down motion becomes complete, flipping all the way in both directions. The delay was provided by a capacitor plus the resistance of the coil, making an RC circuit with a time constant, ? (page 95).
Transistors are so fast that their natural delay times are in the billionths of a second, so we ordinarily use capacitive delays to produce oscillations with somewhat longer time constants. (For example, to produce an musical note that we can hear, the delay should be roughly...
