FAULT FINDING ON A SINGLE STAGE BJT AMPLIFIER


This example is a fault finding exercise on a single stage BJT amplifier with the circuit shown. This exercise combines elements of both education and training in the area of maintenance support.


The key features of the first example are outlined in the summary below. The practical work is integrated with the theoretical background, to both reinforce the theoretical knowledge of the student, and deepen that knowledge through it's application to a practical problem. The student work starts with connecting the power to the circuit and measuring voltages on it.


DC TESTING INSTRUCTIONS

The circuit is set up as above, with the 0V and +10V of the power supply connected as shown. The common of the test meter is also connected to the 0V line.
The lead from the "voltage input" terminal of the test meter is used to check the voltages at various points in the circuit.


A list of possible voltage readings are then presented to the student to select the nearest to those actually measured. The style of this exercise is tutorial, where questions are put to the student as the work progresses, and immediate feedback is given after the student has selected an answer. Here the student was asked to deduce what fault could have produced the voltages that they measured.


STUDENT FAULT IDENTIFICATION

Please choose the option below which is the closest match to your deduction.

  • C2 short circuit
  • Base to emitter of Tr1 short circuit
  • C3 short circuit
  • C3 open circuit
  • More information required to make diagnosis

Each of the options in the list has a link to it's own feedback page, giving the right answer and the reasons for it. Between the student answer and the feedback, however, is a questionnaire to try and establish what led the student to the answer, and hence what knowledge was used, if any. The conditions measured in the circuit, by the student, and the deduction, are repeated at the top of the questionnaire. The box labelled "student ID" is the number allocated to an individual student, at a particular establishment, to identify the answers sent back to the edutronic web site.


FAULT QUESTIONNAIRE 1

Please enter your student I.D. number:

Which of the following statements are TRUE for measured voltages of E=1.13V, B=1.74V and C=10.0V, AND C2 is a short circuit.
[You may select as many answers as you think are TRUE]

  1. The voltage across R4 suggests collector current is present.
  2. The same voltage at each end of R3 means no current is flowing.
  3. TR1 base-emitter junction is not conducting.
  4. TR1 base-collector junction is short circuit.
  5. Voltage at R1/R2 junction is changed by TR1 fault.


The circuit was contructed initially on veroboard with faults inserted by a set of four switches. The wiring from the switches to the parts of the circuit affected was hidden under a second blank veroboard. This was to prevent the student simply following the wires, rather than finding the fault. The feedback is sent by a program running on the server which processes the answers sent back and saves them on disk. The date and time at which the answers were processed is logged, so the use of the feedback to correct the previous answers is not possible. A second attempt would be saved, but with a later time stamp, allowing it to be eliminated if required. The processed questionnaires can be collected and emailed back to the tutor at the student's eatablishment, or printed out and posted instead. This material can be used for assessment, provided conditions of computer use are monitored at the student end.
The boards can be made up locally using the circuit provided, with details of the fault switch connections available by email via contact page. Alternatively complete working PCBs are available from edutronic.co.uk at a reasonable cost.

Student faultfinding Amplifier Board

Return to Example Materials Menu