Current-Voltage Characteristics

A current-voltage (I-V) characteristic graph shows how the current through a component varies with the potential difference across it.

I-V graphs for common components

Fixed resistor (ohmic conductor): A straight line through the origin. Current is directly proportional to p.d. Resistance is constant.

Filament lamp: A curve that flattens at higher voltages. As current increases, the filament heats up, its resistance increases, and the current rises more slowly. The graph is symmetrical (works the same in both directions).

Diode: Almost no current flows in reverse bias (negative voltage). In forward bias, very little current flows until the threshold voltage (about 0.6 V for silicon). Above this, current increases rapidly. Resistance is very high in reverse and very low in forward bias above the threshold.

LED (light-emitting diode): Similar shape to a diode but with a higher threshold voltage (typically 1.5 to 3 V depending on colour). It emits light when forward-biased above the threshold.

Measuring I-V characteristics

1. Set up the component with a variable power supply (or variable resistor), an ammeter in series, and a voltmeter in parallel.
2. Vary the p.d. from zero upward, recording current at each voltage.
3. Reverse the connections to obtain the negative voltage region.
4. Plot current (y-axis) against voltage (x-axis).

Common errors and how to correct them

Drawing a straight line for a filament lamp. The filament lamp graph curves because resistance increases with temperature. It is not ohmic.

Showing current flowing through a diode in reverse bias. In reverse bias, the current through a diode is effectively zero (for IGCSE purposes). Only a negligible leakage current flows.

How to apply this in an exam

Sketch the correct shape for each component. For the filament lamp, show the curve flattening. For the diode, show zero current in reverse and a sharp rise above the threshold in forward bias. Label the threshold voltage for diodes and LEDs.