Negative Feedback and Oscillation

In the circuit of Fig. 12.7, there is an action that feeds back and makes the operation of the relay weaker, rather than stronger. This action is simple and direct — it is the disconnection of power to the coil. In the first experiment, the capacitor is not attached. A high pitched buzz is heard, because the armature vibrates (oscillates) at a fairly high frequency. The negative feedback occurs too soon for the armature to get all the way down to the NO contacts. 

This type of simple circuit (without the neon bulb or capacitor) is commonly used to operate doorbells, with a bell clapper attached to the armature, which hits a metal bell. It was also used for the older model telephone ringers, before modern transistorized oscillators became available. Without the clapper and bell, it is still used a lot for various buzzer applications. (More modem buzzer designs use the piezoelectric diodes discussed in Chapters 14 and 15.) 

If the relay has a transparent case, sparks can be seen at the NC contacts, and this would eventually damage them, if the experiment were continued for a long time. The inductive kick of the coil is not allowing the current to stop very fast. 

Attaching the capacitor with clip leads as shown by the dashed lines, the sound changes to definite clicks, separated by a second or so. This is complete oscillation, which is more effective because the capacitor has delayed the current stoppage until there is enough time for completing the operation. If the capacitor is used, and if output wires (not shown) were attached between the NO contact and the negative wire, then the output signal would be a square wave.  

The lesson here is that negative feedback is more effective in causing oscillation if it is delayed somewhat, depending on the type of operation desired. (This will be seen again in Chapter 16, using transistors. positive 

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