I-V characteristics

Investigating the I-V characteristics of a variety of circuit components.

During this practical, students will investigate how different circuit components affect the current and potential difference of a circuit. They will do this using a lamp, a resistor and a diode. Their use of the circuit equipment and their ability to assemble and draw circuits using diagrams is also assessed during this investigation. Students will plot a graph of current against potential difference.

Equipment (per participant)

• Safety glasses
• Bench power supply
• Connecting leads (black)
• Connecting leads (red)
• Digital voltmeter
• Digital ammeter
• Digital milliammeter
• Mounted lamp 12V

• BEK resistor (100Ω)
• BEK diode
• BEK resistor (10Ω – labelled ‘P’)
• BEK rheostat

Method

Activity 1: The characteristics of a filament lamp.

Students should connect the circuit as per the diagram provided by the examination board. The ammeter must be connected in series between the filament lamp and the power supply. The voltmeter must be connected in parallel with the lamp.
Students then take the readings from the ammeter and voltmeter and record them in a suitable table.
They then adjust the rheostat a little and take the new readings on the meters.
This process is repeated several times.
The power supply is switched off, the positive and negative terminals on the power supply are reversed and the power supply is switched back on. Students should find that the readings on the meters should now be giving negative values.
Students then repeat the procedure so that they have several negative readings.
These values are then plotted onto a graph and a line of best fit can be drawn giving the characteristic of the filament lamp.

Activity 2: The characteristics of a resistor.

In their circuit, ensuring the power supply is switched off, students must now replace the filament lamp with a 100Ω resistor.
They must switch the terminal connections on the power supply so that they are in their original position connected to the power supply (so positive readings are seen on the meters).
The power supply is switched on and the meter readings are recorded in their table.
As in the previous task, students then adjust the rheostat a little and record the readings.
The process is again repeated so they have several positive readings.
The power supply is switched off, the positive and negative terminals on the power supply are again reversed and the power supply is switched back on. Students should find that the readings on the meters should now be giving negative values.
Students repeat the procedure, so they have several negative readings.
These values are then plotted onto a graph and a line of best fit can be drawn giving the characteristic of the resistor.

Activity 3: The characteristic of a diode.

In their circuit, ensuring the power supply is switched off, students must now replace the resistor with a diode.
They must switch the terminal connections on the power supply so that they are in their original position connected to the power supply (so positive readings are seen on the meters) and turn the voltage dial to less than 5V.
Students also need to replace the ammeter with a milliammeter and connect the extra 10Ω resistor (labelled ‘P’) in series with the milliammeter.
The readings on the voltmeter and milliammeter are taken and recorded in their table. Students should make sure they take at least one reading whilst the voltage is at 0.6-0.7V
As in the previous tasks, students then adjust the rheostat a little and record the readings. They repeat this procedure, so they have several positive readings.
The power supply is switched off, the positive and negative terminals on the power supply are again reversed and the power supply is switched back on. Students should find that the readings on the meters should now be giving negative values.
Students repeat the procedure, so they have several negative readings.
These values are then plotted onto a graph and the plots are joined up giving the characteristic of the diode.

Technician tips

The 10Ω resistor should be labelled ‘P’ to indicate it is to be used as a protective resistor when using the diode in the circuit.
If using a power supply with a limit lock, the power supplies could be locked to 6V to ensure circuit components are not damaged by accidentally putting a higher voltage through them.
Students can either use the rheostat to vary the potential difference, or they can test a range of voltages by varying the voltage on the power supply.