Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds
 

Using a CRO to show rectification by a diode

Demonstration

The CRO (cathode ray oscilloscope) gives a visual display of the rectifying action of a diode.

Apparatus and materials

Oscilloscope

Diode (see technical note)

Power supply, low-voltage, AC

Crocodile clips, 2

Leads, 4 mm, 3

Health & Safety and Technical notes


A cathode-ray tube requires voltages classified as 'hazardous live'. The casing nearly always has ventilation holes, some of which may give access to these voltages. Classes should be warned not to poke anything through the holes.

A video showing the use of an oscilloscope is freely available at the National STEM Centre eLibrary.  

1N4001 diodes are very cheap and suitable. 

 

Procedure


a Set the power supply to 12 V AC.  

Apparatus set-up

b Connect the power supply, as shown, to the oscilloscope to display the waveform. The gain control should be set to 2 V/cm and the time-base to 10 ms/cm (10 ms/div).  

connect diode
c Switch off the power supply. Connect the diode into the circuit as shown above. Switch on and observe the waveform. 

d If you have a double beam CRO, then you can show the original signal as well as the rectified one. 
 
e Repeat with the diode reversed.


Teaching notes


1 One way round, the diode will remove the positive peaks of the trace. The other way round, the negative troughs will be removed. 

2 Full-wave rectification by a diode bridge, made from 4 diodes as shown below, could also be shown. 

Connecting four diodes
 
Think about the way to arrange the four diodes so that in the course of a cycle of the AC supply current will go through in each half cycle and make humps in the same direction. The sketch does not show you which way each diode must point. You need to decide that. 
 
3 If you are using a real CRO (rather than a datalogger), you could mention that the CRO itself is using an electron beam to display the changing voltage at its input. 
 
4 You could discuss the miniaturization that is possible by building integrated circuits onto a wafer of semiconductor. Students may have heard of Moore’s law, in which Intel’s co-founder Gordon Moore proposed the trend that the number of components on an integrated circuit would approximately double every two years. It has held from 1972 to at least 2006. 
 
This experiment was safety-checked in January 2007
 

Related experiments


I/V characteristic of a semiconductor diode