Using a CRO as a voltmeter

Class practical

A CRO (cathode ray oscilloscope) can be used to measure potential differences, and to see how they vary. This makes a good introduction to the oscilloscope. But, today, one would use a digital voltmeter for any serious measurements.

Apparatus and Materials

For each student group

• 1.5 V cells, connected in series, 3
• Low voltage supply
• Voltmeter, 0–5 V
• Leads, 4 mm, 2

• Oscilloscope

Health & Safety and Technical Notes

It may be advisable to set up the students' oscilloscopes with the gain at 1 V/div, the time-base off, and the AC-DC switch on DC. Adjust the focus and brightness controls to give a clear spot at the centre of the screen. (Too bright a spot will damage the screen.)

Procedure

1. Connect the input of the oscilloscope across one of the cells, as shown. Adjust the gain so that the deflection is one division on the graticule. Connect the voltmeter across the cell, in parallel with the oscilloscope.
2. Now connect across two cells, and across three. How does the deflection change?
3. Reverse the leads to the oscilloscope to show the deflection across one, two and three cells in the opposite direction. The CRO acts as a voltmeter.
4. Disconnect the cells. Set the output of the power supply to 2 V, and connect it to the input of the oscilloscope. Then change the output first to 4 V and then to 6 V.
5. Switch on the time-base (1 ms/div) to show the trace spread out. Does the power supply provide a steady voltage?

Teaching Notes

• A CRO is a good voltmeter with an almost infinite input resistance (or impedance, the term used when dealing with alternating potential differences). However, the digital voltmeter is often simpler to use and adequate unless you need to see the waveform.
• When the power supply is connected to the CRO, students might be disappointed. Unless it is a well-smoothed supply (and few school supplies are) then the trace will not be a straight line. The trace might be very bumpy, showing that the power supply has fully rectified potential difference, but no attempt has been made to smooth it. Other supplies might displace the trace from the zero of the y-axis, but then there will be a sine-wave ripple on the top of it. The supply has been partially smoothed.

This experiment was safety-tested in October 2006

A video showing how to use an oscilloscope: