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I–V characteristics – resistor, filament lamp, diode and LED

PhysicsElectricity & CircuitsAges 16–17

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A virtual version of the I–V characteristics practical: a variable d.c. supply, an ammeter in series and a voltmeter in parallel with the component. Set the voltage in steps, read two digital meters (with small random errors), record a table, plot I against V in both directions by reversing the supply, and work out R = V/I at each point. Compare a fixed resistor (straight line through the origin), a filament lamp (a curve that flattens as the filament heats up) and a silicon diode and red LED (which conduct only forwards, above about 0.6 V and 1.8 V, with a protective resistor).

Lesson: Current, potential difference and resistance: I–V characteristics

What it shows

An I–V characteristic shows how the current through a component depends on the potential difference across it. A fixed resistor at constant temperature gives a straight line through the origin, so it obeys Ohm's law. In a filament lamp the current heats the tungsten wire, its ions vibrate more and the resistance rises, so the graph curves and flattens. A diode conducts in one direction only: almost no current flows in reverse, while in the forward direction the current rises sharply above about 0.6 V for silicon and about 1.8 V for a red LED.

How to use

Choose a Component, set Supply E with the slider or the − / + buttons, and read the Voltmeter and Ammeter. Press Record reading to add a point to the table and graph, then Open switch S, change E and repeat. Use Reverse supply for negative values. For the diode and LED tick Zoom in around the origin, and tick Show reference curve to compare. Tap a point to work out R = V/I.

Parameters you can change

  • Component Fixed resistor, Filament lamp (6 V, 0.3 A), Silicon diode, Red LED
  • Resistance R 10–100 Ω
  • Protective resistor (diode, LED) None, 100 Ω, 330 Ω
  • Initial supply voltage 0–12 V
  • Show reference curve

Questions to explore

  1. Why does the resistance of a filament lamp increase as the potential difference across it increases?
  2. What does the I–V graph of a diode show in reverse bias and in forward bias above 0.7 V?
  3. Why should you open the switch between readings and put a protective resistor in series with an LED?

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