Practical - 30 :- Identify the leads of the given assorted types of transistors and test It.

🎯 Objective

To identify the Base (B), Emitter (E), and Collector (C) leads of assorted, unmarked transistors and to determine the transistor's type (NPN or PNP) using a multimeter (ohmmeter function).

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🛠️ Apparatus Required

1. Assorted unmarked transistors (low-power, e.g., TO-92 package).

2. Digital or Analog Multimeter with Diode/Ohmmeter function.

3. Connecting wires/probes.

4. Component leads with alligator clips (optional, for convenience).

💡 Theory Recap (Using Multimeter/Ohmmeter)

A Bipolar Junction Transistor (BJT) consists of two back-to-back $PN$ junctions (Base-Emitter and Base-Collector). The ohmmeter function of a multimeter uses an internal battery to apply a small voltage.

1. Forward Bias: When the multimeter is connected to forward-bias a junction ($P$ to positive lead, $N$ to negative lead), it measures a LOW resistance (or a voltage drop of $\approx 0.6 \text{ V}$ in diode mode).

2. Reverse Bias: When the multimeter is connected to reverse-bias a junction ($P$ to negative lead, $N$ to positive lead), it measures a VERY HIGH resistance (or $\text{OL}$/Open Loop).

The key for identification is that the Base is the common terminal to both junctions.

• NPN Transistor: The Base is the P-type terminal. It will show a LOW resistance reading to both the Emitter ($N$) and the Collector ($N$) when the positive (+) meter lead is connected to the Base.

• PNP Transistor: The Base is the N-type terminal. It will show a LOW resistance reading to both the Emitter ($P$) and the Collector ($P$) when the negative (-) meter lead is connected to the Base.

• Emitter vs. Collector: In the active region, the Base-Emitter junction (forward-biased) typically shows a slightly LOWER forward resistance than the Base-Collector junction (also forward-biased) in BJT testing. This difference is often subtle, but sometimes useful.

⚙️ Procedure for Lead Identification and Type Determination

Step 1: Identify the Base (B) Lead and Transistor Type (NPN/PNP)

1. Set the multimeter to the Ohms (Omega) or Diode ( --> \ <--) range. Important: If using the Ohms range, verify the polarity of the meter leads using a voltmeter (typically, the Black lead is COMMON (-) and the Red lead is POSITIVE(+) for Ohms mode, but this can vary).

2. Take the unmarked transistor and label the leads L1, L2, and L3 for tracking.

Test No.	Connection	Expected Result	Observation & Conclusion
1.1	Place the Positive (Red) lead on L1. Touch the Negative (Black) lead to L2. Record the reading.	LOW Omega / 0.6  V (Forward) OR HIGH Omega OL (Reverse)	---
1.2	Keeping the Positive (Red) lead on L1. Touch the Negative (Black) lead to L3. Record the reading.	LOW Omega/ 0.6 V (Forward) OR HIGH Omega OL (Reverse)	If both 1.1 and 1.2 are LOW: $L1$ is the Base (P-type). The transistor is NPN.
1.3	If Step 1.2 failed, try placing the Negative (Black) lead on L1. Touch the Positive (Red) lead to L2 and L3 sequentially.	If both readings are LOW: L1 is the Base (N-type). The transistor is PNP.	---
1.4	Repeat the above process, using L2 as the common lead, then L3, until the Base lead is identified.	The lead that gives a LOW resistance/voltage reading to the other two leads (in the same multimeter lead configuration) is the Base.	---

Step 2: Distinguish Between Emitter (E) and Collector (C)

Once the Base lead is identified and the type (NPN or PNP) is known, perform a final comparison.

1. NPN Transistor (Base = P): The meter Positive (+) lead is on the Base.

   • Compare the LOW resistance/voltage reading between (Base-Emitter) and (Base-Collector).

   • The junction that registers a SLIGHTLY LOWER forward resistance (or voltage drop) is generally the Base-Emitter (B-E) junction. The other lead is the Collector.

2. PNP Transistor (Base = N): The meter Negative (-) lead is on the Base.

   • Compare the LOW resistance/voltage reading between (Base-Emitter) and (Base-Collector).

   • The junction that registers a SLIGHTLY LOWER forward resistance (or voltage drop) is generally the Base-Emitter (B-E) junction. The other lead is the Collector.

Note: If the difference is indistinguishable, you cannot reliably tell Emitter from Collector using only this static test. For practical circuit use, the device must be installed and tested under active conditions (or a dedicated transistor tester must be used).

Step 3: Test for Faulty Transistors

• Junction Test: If any junction (B-E or B-C) shows LOW Omega in both forward and reverse bias directions, the junction is short-circuited (faulty).

• Junction Test: If any junction shows VERY HIGH Omega / OL in both forward and reverse bias directions, the junction is open-circuited (faulty).

• C-E Test: The Collector-Emitter junction should always show HIGH Omega/ OL in both directions (since the base is open). A low resistance indicates a shorted transistor (faulty).

Transistor No.	Common Lead (L1, L2, or L3)	Polarity on Common Lead (+ or -)	Reading to Lead 1 (LOW/HIGH)	Reading to Lead 2 (LOW/HIGH)	Transistor Type (NPN/PNP)	Base (B) Lead	Emitter (E) Lead	Collector (C) Lead	Status (Good/Faulty)
T1
T2