Electron
Electricity and Magnetism

Using a CRO to show rectification by a diode

Practical Activity for 14-16 PRACTICAL PHYISCS

Demonstration

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

Apparatus and Materials

  • Diode (see technical note)
  • Power supply, low-voltage, AC
  • Crocodile clips, 2
  • Leads, 4 mm, 3
  • Oscilloscope


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.

Read our standard health & safety guidance


1N4001 diodes are very cheap and suitable.

Procedure

  1. Set the power supply to 12 V AC.
  2. 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).
  3. Switch off the power supply. Connect the diode into the circuit as shown above. Switch on and observe the waveform.
  4. If you have a double beam CRO, then you can show the original signal as well as the rectified one.
  5. Repeat with the diode reversed.

Teaching Notes

  • One way round, the diode will remove the positive peaks of the trace. The other way round, the negative troughs will be removed.
  • Full-wave rectification by a diode bridge, made from 4 diodes as shown below, could also be shown.
  • 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.
  • 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.
  • 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-tested in January 2007

  • A video showing how to use an oscilloscope:

Electron
is a constituent in our description of Beta Decay
is a type of Lepton
can exhibit Wave-Particle Duality
is a constituent of the Plum Pudding Model
has the quantity Charge
ONLINE COMMUNITY FORUM

Have a Physics Teaching Question?

Want to ask it in a safe, friendly, knowledgeable environment? TalkPhysics is an online community for anyone involved in the teaching of pre-19 physics.

Visit TalkPhysics