Mains, rectification and display

The object of this article is to provide a safe, easy to reproduce and understand guide to creating voltage-time displays of “mains” and rectified mains voltages for in class demonstration purposes.

Safety

The use of mains voltages in class situations needs careful consideration and, where bare connections are present, redesigning. Risks are significantly reduced if lower voltages are used. A step down transformer will, to all intents and purposes, give a scaled representation of mains.
Most schools and colleges will have a “Westminster” power supply. This is a low voltage , high current device which has an output isolated from the mains (that is there is no physical electrical connection between the mains side and the output side). It also has the advantage of having a built in rectifier circuit.

Caveats on choice of transformer

Do not use this method for supplies that have one of their outputs tied to earth (for example a signal generator).
Some Westminster power supplies may have additional circuitry which may affect the output waveforms.- The old black Unilab supplies work well, but others should be tested first.
Only safe, tested units should be used. Demountable transformers are not recommended unless made safe for class use.

Settings for the oscilloscope

You are looking for mains frequency waveform that, when using the suggested power supply, will vary from about +2V to -2V (+3V to -3V -see later). Your oscilloscope needs to be set to display this.
The timebase of 5ms/division, and the gain to 1V/division are suggested. The channel should be set to “d.c.” and all controls to the calibrated position.
With the input set to ground, put the trace at the centre of the screen. It can then be put back to “dc”. The oscilloscope can be set to “auto” trigger, and the trigger level adjusted until a stable trace is displayed.

Things to show

Taking the input from the a.c. output of the power supply (yellow terminals) a sinusoidal display should be visible.
Moving to take one a.c. output and a d.c. output half wave rectification can be seen. It is suggested that the positive (red) terminal is used. Finally taking output from both d.c. terminals full wave rectification can be seen.
Typical images should look like:

AC waveform on a CRO
Half-wave rectification on a CRO (storage scope not used so trace disappears due to image timing)
Full wave rectification (again no storage mode used)

Doing calculations with the trace

Period/Frequency

Assuming that your oscilloscope is not widely off calibration, you should be able to make a few calculations on the traces on screen. If you have set your time base to 5ms/division, one complete cycle of the waveform should take about 4 units before it is repeated. Remember you can use the “X-position” controls to adjust the trace to a convenient starting point for counting. The period (T) of the wave is therefore: 4 divisions x 5ms/division = 20ms as T= 1/frequency frequency (f) = 1/T =1/0.020 s = 50 s^-1 or 50Hz

Voltage

The oscilloscope may display voltages that are a lot higher than you are expecting. There are two reasons for this. Firstly the voltages should be treated as nominal on any device unless their precision is made explicit. Secondly, but more importantly the voltage quoted may be a “RMS” value. RMS is an abbreviation for “root mean square” this is the equivalent power of a d.c. supply. It is smaller than the peak voltage.
Move the trace, using the X-position control so that the peak is along the central vertical (this often has 0.2 division guides). From the display read where the trace crosses the axis. If you have not moved the Y-position controls this is the peak voltage. You can check by putting the input to the oscilloscope to ground and adjusting if required. A 2V RMS supply should have a peak at about 2.8V.
Peak Voltage = RMS Voltage x √2
For a 2V RMS supply this means:
Peak Voltage = 2V x √2
=2 x 1.41421..
=2.8V
Peak to peak voltages should be double this (peak maximum to peak minimum).

Mains, rectification and display

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