Quick Answer: The Digital PN Junction Diode Characteristics Trainer is a bench unit for measuring how the current through a PN junction diode changes with applied voltage, read on digital meters in forward and reverse bias. Students use the readings to plot the diode’s V-I curve, the first semiconductor practical in most physics and electronics courses.
The Physics Behind the Curve
A PN junction forms a depletion layer with a built-in potential barrier. Forward bias lowers the barrier, and current grows roughly exponentially as the voltage approaches the knee, about 0.6 to 0.7 V for silicon and 0.2 to 0.3 V for germanium. Reverse bias raises the barrier, and only a tiny reverse saturation current carried by minority carriers flows.
The ideal diode equation, I = Is(e^(V/nVT) − 1), describes the forward branch, where VT is about 26 mV at room temperature. Plotting the natural logarithm of current against voltage for readings above the knee gives a straight line whose slope yields the ideality factor n, an extension often set in degree-level practicals.
Taking Good Readings
Forward currents reach milliamperes, while a silicon diode’s reverse current is far smaller, so the two branches are measured on different current ranges and plotted on different scales. Readings are packed closely around the knee, where the curve bends sharply, and spaced out elsewhere.
Meter placement matters. With the voltmeter connected directly across the diode it draws a little current that the ammeter also counts; with the ammeter inside the voltmeter connection, its own voltage drop is added to the reading. A digital voltmeter’s high input resistance keeps the first error small, and numeric displays remove pointer-reading errors.
Specifications
| Item | PN junction diode characteristics trainer with digital meters |
| Measured quantities | Diode voltage and current in forward and reverse bias |
| Results | V-I curve, knee voltage, static and dynamic resistance, reverse current |
| Typical forward knee | About 0.6 to 0.7 V for silicon diodes (general value) |
| Diode type, meter ranges and supply | Confirm at enquiry |
Where the Trainer Is Used
- Forward and reverse V-I characteristics in school, college and polytechnic semiconductor practicals
- Finding the knee (cut-in) voltage, and comparing silicon and germanium diodes where both are available
- Calculating static resistance V/I and dynamic resistance ΔV/ΔI at several points on the forward curve
- Introducing rectification by showing that the diode conducts readily in one direction only
Care & Handling
- Set the supply to zero before switching on, then raise it slowly while watching the current
- Keep the series resistor in circuit and stay within the diode’s forward current rating
- Do not push the reverse voltage towards breakdown, which a rectifier diode is not designed to survive
- Switch off before changing the bias connections, and keep the unit dry and covered in storage
Why Choose LabEquip
Almost every semiconductor course runs this practical, so departments often buy several units to give each pair of students its own set-up. LabEquip lists the trainer in its electrical and electronics range, along with a separate PN junction diode characteristics trainer listing and the digital Zener diode study system for the follow-on breakdown experiment.
Frequently Asked Questions
What is the knee voltage of a PN junction diode?
It is the forward voltage at which current starts to rise steeply, roughly 0.6 to 0.7 V for silicon and 0.2 to 0.3 V for germanium. It is found by extending the straight, steep part of the forward curve back to the voltage axis.
Why is the reverse current so small?
In reverse bias only minority carriers, which are few, can cross the junction. Their number is set by temperature rather than by the applied voltage, so the reverse current stays small and nearly constant until breakdown.
Why do the forward and reverse branches use different scales?
Forward currents run to milliamperes, while reverse currents in a silicon diode can be around a million times smaller. On a single scale the reverse branch would look like a flat line along the axis, so each quadrant is plotted with its own current scale.
What is the difference between static and dynamic resistance?
Static resistance is V divided by I at one point on the curve. Dynamic resistance is the slope ΔV/ΔI between two nearby points and describes the response to small changes, which matters in rectifier and signal circuits. Both fall as forward current rises.
Does temperature affect the readings?
Yes. For a silicon diode at fixed current, the forward voltage falls by roughly 2 mV for each degree Celsius rise, and the reverse current roughly doubles for every 10 degree rise. Let the unit settle and avoid touching the diode while measuring.
How is this trainer different from a Zener diode study system?
This trainer studies an ordinary PN junction diode, mainly its forward conduction and small reverse current, without driving it into breakdown. A Zener system uses a diode designed to work in reverse breakdown, so its main results are the breakdown voltage and voltage regulation.
Last Updated: October 2026
