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Oscillator Circuits
49 questions By Tony R. Kuphaldt
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Question 31 of 49
This electric fence-charging circuit, which is designed to produce short, high-voltage pulses on its output, has failed. Now, it produces no output voltage at all:
A technician does some troubleshooting and determines that the transistor is defective. She replaces the transistor, and the circuit begins to work again, its rhythmic output pulses indicated by the neon lamp.
But after producing only a few pulses, the circuit stops working. Puzzled, the technician troubleshoots it again and finds that the transistor has failed (again). Both the original and the replacement transistor were of the correct part number for this circuit, so the failure is not due to an incorrect component being used. Something is causing the transistor to fail prematurely. What do you suppose it is?
Reveal answerI strongly suspect a bad diode. Explain why a defective diode would cause the transistor to fail prematurely, and specifically what type of diode failure (open or shorted) would be necessary to cause the transistor to fail in this manner.
Notes:There are many things in this circuit that could prevent it from generating output voltage pulses, but a failed diode (subsequently causing the transistor to fail) is the only problem I can think of which would allow the circuit to briefly function properly after replacing the transistor, and yet fail once more after only a few pulses. Students will likely suggest other possibilities, so be prepared to explore the consequences of each, determining whether or not the suggested failure(s) would account for all observed effects.
While your students are giving their reasoning for the diode as a cause of the problem, take some time and analyze the operation of the circuit with them. How does this circuit use positive feedback to support oscillations? How could the output pulse rate be altered? What is the function of each and every component in the circuit?
This circuit provides not only an opportunity to analyze a particular type of amplifier, but it also provides a good review of capacitor, transformer, diode, and transistor theory.
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Question 32 of 49
Spring- and weight-driven clock mechanisms always use a pendulum as an integral part of their workings. What function does a pendulum serve in a clock? What would a mechanical clock mechanism do if the pendulum were removed?
Describe what the electrical equivalent of a mechanical pendulum is, and what purpose it might serve in an oscillator circuit.
Reveal answerThe pendulum in a mechanical clock serves to regulate the frequency of the clock’s ticking. The electrical equivalent of a pendulum is a tank circuit.
Notes:Ask your students to brainstorm possible applications for electrical oscillator circuits, and why frequency regulation might be an important feature.
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Question 33 of 49
Two technicians are arguing over the function of a component in this oscillator circuit. Capacitor C1 has failed, and they are debating over the proper value of its replacement.
One technician argues that the value of capacitor C1 helps set the oscillation frequency of the circuit, and that the value of the replacement capacitor therefore must be precisely matched to the value of the original. The other technician thinks its value is not critical at all, arguing that all it does is help to provide a stable DC power supply voltage. What do you think?
Also, describe the purpose of this circuit: what is it?
Reveal answerThis circuit is a simple “CW” radio transmitter, used to broadcast information using Morse code.
The second technician is closer to the truth than the first, with regard to the capacitor. C1 is not part of the oscillator’s resonant network, and so does not set the oscillation frequency. However, if the replacement capacitor’s value is too far from the original’s value, this circuit will not start and stop oscillating as “crisply” as it did before, when the code key switch is repeatedly actuated.
Notes:Ask your students how they can tell that C1 is not part of the oscillator’s resonant network.
