+V (V) M5.2h Result The current increases with voltage.
Conclusion The results allow us to conclude that the current rises sharply once the junction voltage has been reached. It is therefore non-ohmic. We use the following formula to calculate resistance at any point:
V = IR
V = voltage (V), I = current (A), R = resistance (W)
Errors
The following errors should be considered when conducting this experiment:
• difficulty in determining voltage and current from analogue meters
5.2d(ii) To investigate the variation of current (I) with pd (V) for a semiconductor diode (reverse bias) Outline of experiment
In this experiment, we will measure voltage and current simultaneously. A variable resistor is used to act as a potential divider. By using this variable resistor, the amount of voltage applied to the diode can be varied.
As the voltage changes, the current will also change, showing the relationship of V to I. LEAVING CERTIFICATE PHYSICS 447
NOTE
Make sure not to exceed maximum specified safe current limit for the diode being used.
NOTE
When the diode is in forward bias, use a milliammeter. The forward-bias section of the graph is given here.
• varying resistance due to thermal variations • taking too few readings to draw a smooth curve.
Precautions
• Use digital meters. • Avoid working near open windows or radiators. • 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 as current passes through it?
