Skip to content

I–V characteristics

Curricula: GCSE (I–V characteristics), 2de, Sek I, A-level, IB B.5

Open the full lab ↗

Aim

Measure the current–voltage characteristics of a resistor, a filament lamp and a diode, and compare their shapes.

The bench

One 3 V source feeds three branches through the "+" (left) and "−" (right) buses; each branch has an ammeter and a voltmeter across the component. At the top is a 100 Ω resistor, in the middle a 6 V / 3 W lamp, at the bottom a diode with a 100 Ω protective resistor.

Procedure

  1. Vary the source voltage from 0 to 6 V in steps of 1 V.
  2. At each step write down the "voltage — current" pairs for all three components.
  3. Plot three \(I(U)\) graphs.
  4. Connect the diode the other way round: swap the plugs of the two cables on its terminals — anode and cathode — and repeat the measurements.
Expected result

At 3 V: the resistor — about 2.78 V and 28 mA; the lamp — about 2.74 V and 314 mA; the diode — about 0.61 V and 22 mA (a real silicon diode has slightly more at this current, about 0.7 V). The resistor's graph is a straight line. The lamp's curve bends: the filament heats up and its resistance rises — about 12 Ω at 6 V. The diode hardly conducts below 0.5 V; beyond that the current rises sharply, while the voltage across it stays at about 0.6–0.7 V. With the diode reversed, the diode current is zero.

Questions

  1. Why does the resistance of the lamp rise with voltage?
  2. Why is a resistor connected in series with the diode?
Answers
  1. The current heats the filament, and the resistance of tungsten grows with temperature: the higher the voltage, the hotter the filament. At 3 V the lamp has 2.74 V / 314 mA ≈ 8.7 Ω, at 6 V about 12 Ω, and a cold filament has only about a fifteenth of its resistance at full glow.
  2. Above about 0.6 V the diode current rises very steeply, and the diode itself hardly limits it. The 100 Ω resistor takes the rest of the voltage and sets the current, about 22 mA at 3 V; without it only the source's 0.5 Ω internal resistance, the ammeter and the cables would limit the current, which would reach amperes and destroy a real diode.