Microammeter, voltmeter, potential divider (variable resistor), diode, d.c. power supply
d.c. power supply
microammeter
+ –
diode
potential divider
voltmeter
M5.2i Investigating the variation of current with voltage for a semiconductor diode (reverse bias)
Method
1. Set up the circuit as shown in M5.2i, making sure the microammeter is in series and the voltmeter is in parallel with both the diode and microammeter.
2. Use the variable resistor to apply a low voltage (~ 0.2 V) to the diode. 3. Record the voltage across and current through the diode for this setting. 4. Keep increasing the voltage by 1 V, recording I and V for each setting. (Use the variable resistor to vary the voltage applied.)
5. Record all values in the table and plot a graph of I against V. Voltage (V )(V) Current (I)(lA)
NOTE
When the diode is in reverse bias, use a microammeter. The reverse bias section of the graph is given opposite.
Result
A very small current exists as voltage is increased. Conclusion
The results allow us to conclude that the current would not seem to rise significantly, even as voltage is increased sharply. This shows that a diode in reverse bias has a leakage current only. This is the reason for using a microammeter to detect the small current.
Errors
The following errors should be considered when conducting this experiment:
• difficulty in determining voltage and current from analogue meters
448 FUSION
Characteristic graph –V (V)
reverse bias
–I (µA) M5.2j
• varying resistance due to thermal variations • taking too few readings to draw a smooth curve.
Precautions
• Use digital meters. • Keep the experiment away from draughts and radiators in the room.
• Move up in smaller voltage increments, which will give more readings with which to draw the graph.
Possible exam questions 1.
2.
How would you calculate resistance at certain points of your graph?
What does the graph tell you about a diode in reverse bias as current passes through it?
