Download Using an Oscilloscope to Observe Rectification

Using an Oscilloscope to Observe Rectification
The purpose of this experiment is to introduce the student to the oscilloscope as a
measurement instrument and to use it to observe and measure the operation of a
simple rectifier circuit.
Rectification.
Rectification is the name given to the process where an a.c. voltage source is
switched on and off repeatedly so that current flows only one way through a load. A
circuit that performs this function is called a rectifier circuit. Most rectifiers are built
using diodes. Rectifiers are used in order to convert a.c. voltages such as the mains
supply into d.c. voltages such as are required by most electronic circuits and some
motors.
Consider the circuit below:
The diode is forward biased when the a.c. voltage goes through a positive half cycle.
Under forward bias the diode conducts and allows current to flow in the load
resistor.
The diode is reverse biased when the a.c. voltage goes through a negative half cycle.
When reverse biased the diode does not conduct and hence no current flows in the
load resistor.
The Oscilloscope
An oscilloscope is a measurement instrument that displays a picture of the way that a
voltage changes over a period of time – i.e. it draws a graph that plots voltage
(vertically) against time (horizontally). The oscilloscope display screen is divided
into 1 cm squares. The controls on the oscilloscope are used to obtain a stable
display of the voltage waveform and to calibrate the vertical and horizontal axes of
the oscilloscope so that measurements may be made. Oscilloscope operation can be a
complicated process.
Procedure:
1. Connect the circuit together and connect the a.c. voltage source to the circuit
as shown.
2. Set the voltage amplitude to about 10 volts and the frequency to about 1000
Hz.
3. Connect the oscilloscope to the a.c. voltage source and set the controls to
display the waveform of the a.c. voltage Vin. Sketch this waveform to scale in
your lab notebook.
4. Now connect the oscilloscope to measure the voltage across the load resistor
and set the controls to display the waveform of this voltage VL. Sketch this
waveform to scale in your lab notebook.
5. Compare the two voltage waveforms and explain in your lab notebook how
the voltage across the resistor is produced from the original a.c. voltage.
6. Describe in your lab notebook how the voltage amplitude is measured using
the oscilloscope.