Power Diode Characteristics —
Laboratory Experiment
Course: Power Electronics / Electrical Machines Lab
Experiment No.: XX
Duration: 2–3 hours
Objective: To study and plot the V–I characteristics of a power diode in forward and reverse
bias and determine the diode parameters (forward resistance, knee current, reverse
saturation current and breakdown voltage).
Apparatus
Power diode (e.g., 1N5408 or similar power rectifier)
DC power supply (0–30 V) × 2 (or one variable supply and a fixed reverse source)
Function generator (optional for dynamic tests)
Ammeter (0–1 A, or suitable range)
Voltmeter (0–50 V)
Resistive load/series resistor (e.g., 1 Ω — 1 kΩ depending on test)
Breadboard or PCB and connecting leads
Digital oscilloscope (for transient/reverse-recovery if required)
Temperature sensor or thermometer (optional, for noting ambient temp)
Safety equipment (eye protection)
Theory (brief)
A diode allows current to flow primarily in one direction. The current–voltage
characteristics have two operating regions relevant for power diodes:
1. Forward bias: When anode is positive relative to cathode. The diode conducts after the
threshold (approximately 0.6–1.2 V for silicon power diodes) and shows a forward voltage
drop V_F. The relationship can be approximated by:
I_F = I_S (e^(V_D/ηV_T) - 1)
or at high currents: V_D = V_D0 + I_F × R_s
2. Reverse bias: When cathode is positive relative to anode. Ideally only a small leakage
current flows until breakdown voltage is reached.
Circuit Diagrams
1. Forward characteristic measurement — diode in series with supply and resistor,
ammeter in series, voltmeter across diode.
2. Reverse characteristic measurement — diode reversed with high-value series resistor,
measure leakage current.
3. Breakdown test (if allowed) — increase reverse voltage with large resistor until
breakdown, record V_BR.
Procedure
A. Forward V–I characteristic
1. Assemble forward-bias circuit.
2. Increase supply in steps and record V and I.
3. Plot data.
B. Reverse V–I characteristic
1. Assemble reverse-bias circuit with large resistor.
2. Increase supply in steps, record leakage.
C. Breakdown test
1. Carefully increase reverse voltage until breakdown, note value.
D. Optional — Transient / Reverse Recovery test using oscilloscope.
Observation Tables
Point V_supply (V) Series R (Ω) I_D (A) V_D (V) Notes
Point V_reverse (V) Series R (Ω) I_R (A) Notes
Calculations
1. Forward dynamic resistance: r_d = ΔV / ΔI
2. Series resistance: slope of V–I in high current region
3. Reverse leakage: from reverse bias data
4. Breakdown voltage: where reverse current sharply increases
Graphs to Plot
- Forward V–I curve (V vs I)
- Reverse I–V curve (I vs V)
- Zoomed forward region for knee and resistance.
Precautions
• Start with high series resistance
• Verify ranges before connecting
• Use large resistors for breakdown tests
• Observe polarity carefully
• Handle circuits safely
Viva Questions
1. Why does the forward voltage of a power diode increase with current?
2. What is reverse recovery time and why is it important in switching applications?
3. How would temperature affect diode characteristics?
4. How do you protect a diode during breakdown testing?
5. Explain difference between ideal diode equation and practical high-current behavior.
References
Refer to the datasheet of the specific diode used for comparison with measured results.
