Analog Integrated Circuits

Open-Loop OpAmp Circuits

19 questions By Tony R. Kuphaldt

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Question 13 of 19

Predict how the operation of this thermostat circuit (where the cooling fan motor is supposed to turn on when the temperature gets too high) will be affected as a result of the following faults. Consider each fault independently (i.e. one at a time, no multiple faults):

Cable fails open:

Comparator U₁ fails with output saturated positive:

Resistor R₁ fails open:

Cable fails shorted:

Transistor Q₁ fails shorted (drain-to-source):

For each of these conditions, explain why the resulting effects will occur.

Reveal answer

Cable fails open: Fan turns on and never turns off.

Comparator U₁ fails with output saturated positive: Fan refuses to turn on.

Resistor R₁ fails open: Fan refuses to turn on.

Cable fails shorted: Fan refuses to turn on.

Transistor Q₁ fails shorted (drain-to-source): Fan turns on and never turns off.

Notes:
The purpose of this question is to approach the domain of circuit troubleshooting from a perspective of knowing what the fault is, rather than only knowing what the symptoms are. Although this is not necessarily a realistic perspective, it helps students build the foundational knowledge necessary to diagnose a faulted circuit from empirical data. Questions such as this should be followed (eventually) by other questions asking students to identify likely faults based on measurements.
Question 14 of 19

Predict how the operation of this sound-activated relay circuit will be affected as a result of the following faults. Consider each fault independently (i.e. one at a time, no multiple faults):

Zener diode D₁ fails open:

Resistor R₁ fails open:

Resistor R₂ fails open:

Microphone voice coil fails open:

Comparator U₁ fails with output saturated positive:

Diode D₃ fails shorted:

For each of these conditions, explain why the resulting effects will occur.

Reveal answer

Zener diode D₁ fails open: Volume threshold increases (relay may not energize at all).

Resistor R₁ fails open: Any sound at all will energize the relay.

Resistor R₂ fails open: Relay refuses to energize.

Microphone voice coil fails open: Relay energizes randomly, as comparator input is now sensitive to static electricity.

Comparator U₁ fails with output saturated positive: Relay immediately energizes, whether or not there is sound.

Diode D₃ fails shorted: Relay refuses to energize, SCR will likely be damaged due to overheating after a sound event.

Notes:
The purpose of this question is to approach the domain of circuit troubleshooting from a perspective of knowing what the fault is, rather than only knowing what the symptoms are. Although this is not necessarily a realistic perspective, it helps students build the foundational knowledge necessary to diagnose a faulted circuit from empirical data. Questions such as this should be followed (eventually) by other questions asking students to identify likely faults based on measurements.
Question 15 of 19

Trace the output waveform of this comparator circuit:

Reveal answer

Follow-up question: explain what the phrase duty cycle means with reference to a “square” or “pulse” waveform.

Notes:
During discussion, ask your students to explain how the output waveform of this comparator circuit comes to be, step by step. Ask them how they arrived at their solution, and if there is a way this AC/DC problem can be simplified to one that is DC only for easier analysis (determining what the output voltage will do for a certain set of input conditions).