13 Jul 2026

Testing Diodes and Transistors Using a Multimeter

practical ug-iv electronics multimeter diode transistor

Experimental arrangement

Series-limited DC test circuit with the ammeter in the device path and the voltmeter connected across the junction under test.

Aim

To identify semiconductor devices and test the condition of diodes and transistor junctions using a digital multimeter.

Apparatus

Digital multimeter, silicon diode, LED, NPN transistor, PNP transistor, and connecting leads.

Theory

A diode conducts strongly in forward bias and offers high resistance in reverse bias. A transistor contains two PN junctions. For an NPN transistor, the base-emitter and base-collector junctions conduct when the positive probe is connected to the base; for a PNP transistor the polarity is reversed.

Observations

Device/test	Forward reading	Reverse reading	Inference
Silicon diode	$0.68\,\text{V}$	OL	Good diode
LED	$1.86\,\text{V}$	OL	Good LED
NPN, base-emitter	$0.69\,\text{V}$	OL	Junction good
NPN, base-collector	$0.66\,\text{V}$	OL	Junction good
NPN, collector-emitter	OL	OL	No short circuit

Result

The tested silicon diode, LED, and NPN transistor are in working condition. The transistor is identified as NPN from the polarity of its two conducting junctions.

Precautions

1. Remove the device from the circuit before testing it.
2. Do not test a charged capacitor on the resistance range.
3. Use the diode-test range for junction-voltage readings.
4. Do not exceed the current limit of the multimeter.

Viva Questions

1. What does OL indicate? It indicates that the measured resistance is beyond the selected range or the junction is reverse biased.
2. Why does an LED show a larger forward voltage? Its semiconductor material has a larger effective band gap.
3. How are transistor terminals identified? By testing the two junctions and comparing their conducting polarities.
4. Why must power be removed? External circuit paths can give false readings and may damage the meter.