3.4 Investigating the current - voltage characteristics of a diode
 

The current through a diode flows in one direction only - see graph 3.

The resistance in the opposite direction is very high - hence effectively a 'one way' system.

Circuit 43 shows how you can investigate the current - potential difference characteristics of a diode.

The voltmeter is wired in parallel with the thermistor, the p.d. V is measured in volts (V).

The variable resistor allows you to vary the p.d. and current flow.

The ammeter, wired in series, gives you the current I reading in amps (A).

A diode has a very high resistance in the reverse direction.

There is also a threshold p.d. (e.g. 1.4 V) before any current flows at all - look at the graph carefully - there is a short horizontal portion before the current rises from zero and eventually becomes linear.

Therefore you get a top right portion of the graph 3 compared to graphs 1 an 2 above.

This is because when you do the experiment using the circuit described above, on reversing the connections, you will find no current flows as you vary the p.d.

Its a non-linear graph.

If the gradient is changing, then the resistance is changing.

When the current (A) is NOT proportional to the p.d (V), the diode is described as a non-ohmic conductor (doesn't obey Ohm's Law!).

The phrase non-linear component may be used.

The graph (3) is constructed on a crosswire axis. The top right half is your first set of results, you then reverse the terminals on the power supply and repeat the experiment giving the bottom left part of the graph.

Since the current only flows one way through a diode, it can be used to convert an ac current into a dc current

Be able to describe the circuit and explain the method of how to investigate the electrical resistance of diode.

Be able to describe and explain the current-voltage graph for electrical behaviour of a diode.

Be able to do any calculations of results for the current-voltage graph for the diode, a non-ohmic conductor.

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