Discrete Semiconductor Devices and Circuits

Regulated Power Sources

28 questions By Tony R. Kuphaldt

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Question 16 of 28

Calculate the approximate output voltage of this regulated power supply circuit and the amount of current through the zener diode under no-load conditions:

Reveal answer

V_out ≈ -8.6 V I_zener ≈ 6.71 mA

Notes:
Calculating the output voltage is a much simpler task than calculating the zener diode’s current! Have your students explain their problem-solving techniques for this circuit.
Question 17 of 28

Calculate the approximate output voltage of this regulated power supply circuit, the amount of current through the zener diode, and the (unregulated) voltage across the 1000 μF capacitor, all under no-load conditions:

Reveal answer

V_out ≈ 6.2 V I_zener ≈ 25.6 mA V_capacitor = 32.5 V

Notes:
Calculating the output voltage is a much simpler task than calculating the zener diode’s current! Have your students explain their problem-solving techniques for this circuit.
Question 18 of 28

Predict how all component voltages and currents in this circuit will be affected as a result of the following faults. Consider each fault independently (i.e. one at a time, no multiple faults):

Resistor R₁ fails open:

Transistor Q₁ fails open, collector to emitter:

Transistor Q₁ fails shorted, collector to emitter:

Transistor Q₂ fails open, collector to emitter:

Transistor Q₂ fails shorted, collector to emitter:

Load fails shorted:

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

Reveal answer

Resistor R₁ fails open: No current through Q₁, no current through Q₂, no current through load, no voltage dropped across Q₁, full voltage dropped across collector-emitter of Q₂.

Transistor Q₁ fails open, collector to emitter: Approximately the same current through R₁, increased current through Q₂ base, increased current through Q₂ collector (load current),and less voltage dropped between collector-emitter of Q₂.

Transistor Q₁ fails shorted, collector to emitter: Increased current through R₁, almost zero current through all terminals of Q₂ and the load, full voltage dropped across collector-emitter of Q₂.

Transistor Q₂ fails open, collector to emitter: No change in current for R₁ or Q₁, zero load current, full voltage dropped across collector-emitter of Q₂.

Transistor Q₂ fails shorted, collector to emitter: Current through R₁ will most likely increase, zero voltage dropped across collector-emitter of Q₂, increased current through Q₂ and load.

Load fails shorted: No change in any current (in reality, load current will slightly increase), increased voltage drop across collector-emitter of Q₂, possible overheating of Q₂.

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.