This experiment is based on the provided document (Exercise 1.7.40) and outlines the procedure for quickly testing the condition of a transistor (identifying open or shorted junctions) using a Digital Multimeter (DMM) in its diode testing/range setting.
Objectives
To be able to identify opens and shorted junctions within a transistor.
To be able to test the condition of a transistor (serviceable/defective) using an ohmmeter or multimeter.
| Tools/Equipment/Instruments | Materials/Components |
| Trainee's tool kit - 1 Set | Transistor assorted types - 10 Nos |
| Digital multimeter with crocodile clip probes - 1 Set | Transistor data book - as required |
Procedure: Testing the Transistor
The DMM's diode testing function measures the voltage drop across a junction. For a functioning transistor, the Base-Emitter (B-E) and Base-Collector (B-C) junctions should behave like diodes: they should show a low resistance/voltage drop in the forward-biased direction and an open circuit (high resistance/"OL") in the reverse-biased direction.
Task 1: Preparation
Pick one of the labeled transistors from the given lot.
Identify the transistor type (NPN or PNP), pin configuration (Base, Emitter, Collector), and record this data.
Connect the crocodile clip probe to the DMM.
Set the DMM selector switch to the Diode testing/range.
Task 2: Testing Base-Emitter (B-E) and Base-Collector (B-C) Junctions
The test involves measuring the junction resistance/voltage drop in both the forward-biased and reverse-biased directions.
| Step | Action | Expected Reading (Forward Bias) | Expected Reading (Reverse Bias) |
| 4 & 6 (Forward) | Connect the positive probe to the Base and the negative probe to the Emitter (E) (Step 4), then to the Collector (C) (Step 6). | NPN: 0.45V to 0.9V (low resistance/voltage drop). PNP: 0.45V to 0.9V (low resistance/voltage drop). | See Reverse Bias (Steps 5 & 7). |
| 5 & 7 (Reverse) | Reverse the polarity of the connections in Steps 4 and 6 (i.e., connect the negative probe to the Base and the positive probe to the Emitter/Collector). | See Forward Bias (Steps 4 & 6). | NPN/PNP: "OL" (Over Limit/Open Circuit/High Resistance). |
*Note:
For an NPN transistor, the Base is connected to the P-type semiconductor. Forward bias is $\text{Positive } \to \text{Base}$, $\text{Negative} \to \text{E/C}$.
For a PNP transistor, the Base is connected to the N-type semiconductor. Forward bias is $\text{Negative} \to \text{Base}$, $\text{Positive} \to \text{E/C}$.*
Task 3: Testing Collector-Emitter (C-E) Terminals
Connect the positive probe to the Emitter terminal and the negative probe to the Collector (C) terminal (as shown in Fig 1, lower right diagram in the image).
Repeat step 8 with reversed polarities of the DMM.
Expected Reading (C-E and E-C): The DMM should typically read "OL" in both directions since this path consists of two series-opposing junctions inside a typical bipolar junction transistor (BJT) when the Base is floating.
Task 4: Conclusion
Carry out Steps 4 to 9 for all the remaining labeled transistors.
Compare the resistance values recorded in forward and reverse directions between B-E, B-C, and E-C terminals.
Analysis and Conclusion
The condition of the tested transistor is determined by comparing the recorded resistance/voltage drop values to the expected diode behavior:
| Condition | Base-Emitter & Base-Collector Junctions (Forward/Reverse) | Collector-Emitter Terminals (Both Directions) | Conclusion |
| Good/Serviceable | Shows a low resistance reading (0.45V to 0.9V) in one direction (forward bias) and "OL" in the reverse direction. | Reads "OL" in both directions (or very high resistance). | Good/Serviceable Transistor |
| Defective (Shorted) | Shows the same low resistance value in both directions (e.g., 0.00V or a very low Ohm value). | Shows a low resistance value in both directions. | Shorted Junction/Terminals |
| Defective (Open) | Reads "OL" in both directions. | Reads "OL" in both directions. | Open Junction |
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