Using a CRO to show an AC waveform
This experiment enables students to become familiar with the various controls on the CRO (cathode ray oscilloscope).
Apparatus and materials
For each student group
Power supply, low-voltage, AC
Leads, 4 mm, 2
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.
a As a demonstration, the oscilloscope is set with the gain at volt/cm switched to 1, the time-base control to 1 ms/cm and the AC-DC switch to AC. The AC terminals of the low-voltage power unit (2 volt AC) or the 2-volt terminals of the transformer are connected to the Input and Earth terminals of the oscilloscope. The variable control on the time-base is adjusted until four or five cycles of the waveform appear on the screen. The pattern traced on the screen should remain fixed in position.
b As a class experiment, students connect the 2-volt AC supply in the same way to the input terminals on the class oscilloscope. The gain should be set at 1 V/cm, the time-base on range 2 ms/cm and the AC-DC switch on DC. Students should adjust the variable control on the time-base to see the pattern opening up and closing.
1 It may be best to show this experiment first as a demonstration and then let the students achieve the same on their oscilloscopes.
2 This experiment could be combined with the making of model transformers. The secondary coil can be connected to the y-terminals, and the trace noted as each extra coil is added to the secondary.
This experiment was safety-checked in March 2007