ENROLLMENT SYTEM
Learning Objectives:
At the end of this topic you will be able to;
describe, by drawing a graph, the effect of a capacitor on the output waveform from a rectifier; know that the size of the ripple voltage is dependent on the method of rectification, the values of capacitance and load resistance; describe, by drawing graphs, the effect of loading on the output waveform of a smoothed rectifier.
Capacitive smoothing.
In our work on power supplies so far we have determined how to calculate the peak value of the output voltage from an rms value, and how to use half-wave and full-wave rectification methods to change a.c. into variable value d.c. The next stage is to smooth out the pulses of variable d.c. into a more regular output that might be suitable for use in electronic circuits.
The component needed to assist us with this stage of the process is the capacitor. You should remember that from Topic 2.2.1. the capacitor is a device that can store electrical charge. The addition of a capacitor to our rectifier circuits considerably alters the output observed on an oscilloscope as discussed in the following sections. The capacitor used in these applications is usually an electrolytic type with a very large capacitance value, typically ≥1000μF.
We will first look at the case of the half-wave rectified circuit, initially with no load resistor attached. For simplicity, the transformer has been replaced with a low voltage a.c. source.
The resulting oscilloscope graph for this circuit is as follows:
From the graph you can see that the output voltage shown in red holds its value at the maximum voltage, once the charge has built up on the capacitor during the initial charging cycle. As there is no load resistor applied to the circuit none of this charge comes off the capacitor and the voltage is maintained at it’s maximum value. It may appear that we have created our smooth d.c. supply, however this is only a false hope. Let us see what
At the end of this topic you will be able to;
describe, by drawing a graph, the effect of a capacitor on the output waveform from a rectifier; know that the size of the ripple voltage is dependent on the method of rectification, the values of capacitance and load resistance; describe, by drawing graphs, the effect of loading on the output waveform of a smoothed rectifier.
Capacitive smoothing.
In our work on power supplies so far we have determined how to calculate the peak value of the output voltage from an rms value, and how to use half-wave and full-wave rectification methods to change a.c. into variable value d.c. The next stage is to smooth out the pulses of variable d.c. into a more regular output that might be suitable for use in electronic circuits.
The component needed to assist us with this stage of the process is the capacitor. You should remember that from Topic 2.2.1. the capacitor is a device that can store electrical charge. The addition of a capacitor to our rectifier circuits considerably alters the output observed on an oscilloscope as discussed in the following sections. The capacitor used in these applications is usually an electrolytic type with a very large capacitance value, typically ≥1000μF.
We will first look at the case of the half-wave rectified circuit, initially with no load resistor attached. For simplicity, the transformer has been replaced with a low voltage a.c. source.
The resulting oscilloscope graph for this circuit is as follows:
From the graph you can see that the output voltage shown in red holds its value at the maximum voltage, once the charge has built up on the capacitor during the initial charging cycle. As there is no load resistor applied to the circuit none of this charge comes off the capacitor and the voltage is maintained at it’s maximum value. It may appear that we have created our smooth d.c. supply, however this is only a false hope. Let us see what