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Transistor Circuit Troubleshooting #1

Simple Regulated Power Supply Troubleshooting. The voltage for TP1, TP2 and Vout was recorded over a load range of 0 to 100 ohms for three faulty simulated regulated power supply units below. From the recorded data determined the cause of failure.


In above circuit all resistors are in ohms. Unregulated power source is 15.0 V. The 2.0 ohm resistance represents the internal resistance of power source. The Zener diode provides a reference voltage of 7.4 V.  Rload will be varied during testing from 0 to 100 ohms. Regulation is specified as indicated by curve below. Regulation is achieved in the following manner:  Zener Diode maintains transistor base voltage at 7.4 volts; therefore, if Vout drops Base Emitter forward bias increases causing current and thus Vout to increase. The reverse occurs when Vout drifts upward.


Good Unit Test Results
Vout shown below
Input Resistance = Rload


Possible faults in Units Below are:

A. "2.0" ohm resistor resistance increased a lot.

B. Zener diode shorted.

C. Zener diode Open.

D. Transistor is Open between Collector and Emitter.

E. Capacitor leaky.

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Failed Unit 1 Test Results

Answers Bottom of Page

Failed Unit 2 Test Results

Failed Unit 3 Test Results


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Answers Below




Answers Below




Failed Unit 1 Answer - Transistor is Open between Collector and Emmiter. 


Failed Unit 2 Answer - Capacitor leaky. (Resistance dropped to about 135 ohms.


Failed Unit 3 Answer - "2.0" ohm resistor resistance increased to about 35 ohms.


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