Use of multimeter (Activity)

Use of multimeter

Aim

Use of multimeter to see the unidirectional flow of current in case of a diode and an LED and check whether a given electronic component (e.g., diode) is in working order.

Apparatus and Material Required

• A multimeter,
• a diode,
• a transistor (whose base diagram is known),
• plug key,
• a battery,
• resistance box,
• connecting wires and
• sand paper.

Principle

Diode:

A diode is a two-terminal semiconductor device that allows current to flow in only one direction. It is formed by a p-n junction, where the p-region contains holes (positive charge carriers) and the n-region contains electrons (negative charge carriers). When a voltage is applied:

• Forward Bias: The positive terminal of the voltage source is connected to the p-side (anode), and the negative terminal is connected to the n-side (cathode). This reduces the depletion region and allows current to flow. The multimeter displays a small forward voltage drop (typically 0.5–0.8 V for silicon diodes).
• Reverse Bias: The positive terminal is connected to the n-side and the negative terminal to the p-side. This widens the depletion region, preventing current flow. A functional diode shows no current or “OL” (open loop) on the multimeter in this condition.

2. Transistor:

A transistor is a three-terminal semiconductor device (emitter, base, and collector) that can amplify or switch electronic signals. It consists of two p-n junctions in either NPN or PNP configuration:

• NPN Transistor: Current flows from collector to emitter when a small current is applied to the base. The base-emitter and base-collector junctions are forward biased in testing.
• PNP Transistor: Current flows from emitter to collector when the base has a small current. The junctions are forward biased in the reverse polarity compared to NPN.
• Diode Behavior: The base-emitter and base-collector junctions of a transistor behave like diodes. By testing these junctions, the type (NPN or PNP) and functionality of the transistor can be determined.
• Identifying Terminals: The emitter-to-base voltage is slightly higher than the collector-to-base voltage due to the doping levels. The behavior of the junctions helps identify the emitter, base, and collector.

A digital multimeter, set to diode mode, is used to measure voltage drops across these junctions and verify the components’ working order.

Procedure

1. Testing a Diode:
   Set up the Multimeter:
   Turn the multimeter to the diode testing mode (often marked with a diode symbol).
   Forward Bias Test:
   Connect the positive (red) probe to the anode and the negative (black) probe to the cathode. A reading of approximately 0.5–0.8 V indicates the forward voltage drop for a working diode.
   Reverse Bias Test:
   Reverse the probes (red to cathode, black to anode). No reading or “OL” (overload) indicates the diode is blocking current, meaning it is functioning properly.
   
   B. Identifying Emitter, Base, and Collector of a Transistor:
   Set up the Multimeter:
   Switch the multimeter to diode testing mode.
   Base Identification:
   For an NPN transistor, place the red probe on the base and the black probe on the collector and emitter terminals. Readings should be similar (~0.6–0.7 V) for a functioning transistor.
   For a PNP transistor, reverse the probes (black on base, red on collector/emitter). Similar readings should appear.
   Emitter and Collector Identification:
   The emitter-to-base voltage is slightly higher than the collector-to-base voltage. Test by swapping probes between base, collector, and emitter.
   
   C. Distinguishing NPN and PNP Transistors:
   Identify the base terminal as described above.
   Note the polarity of the probes giving forward bias readings:
   NPN Transistor: Red probe on base, black on collector/emitter.
   PNP Transistor: Black probe on base, red on collector/emitter.
   
   D. Checking Transistor Functionality:
   In diode mode, measure the forward voltage between base-emitter and base-collector.Both should show ~0.6–0.7 V for a functioning transistor.
   Reverse the probes; no readings should appear in reverse bias if the transistor is operational.

Observations

Component	Test Type	Multimeter Reading	Functionality Conclusion
Diode	Forward Bias	~0.5–0.8 V	Working
	Reverse Bias	No Reading (“OL”)	Working
Transistor	Base Identification	~0.6–0.7 V	Base Identified
	Emitter and Collector	\(V_{emitter} > V_{collector}\)	Correct Identification
	NPN or PNP Determination	Based on Probe Polarity	Type Identified

Precautions

1. While obtaining resistance of any component, clean its leads properly.
2. Use the selector switch of the multimeter carefully for various
   measuring modes.

Conclusion

The diode’s unidirectional current flow was verified, confirming it is in working order.The emitter, base, and collector terminals of the transistor were successfully identified using the multimeter. The transistor’s type (NPN or PNP) and functionality were determined.

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Reference: NCERT Lab Manual

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