This article explains the operation of the full wave rectifier shown in figure one. The full wave rectifier converts alternating current to direct current. Direct current always flows in one direction. For an explanation of alternating current, here is the URL of my article ‘Basic Electrical Theory: Introduction To Alternating Current.’
A transformer consists of a primary winding and a secondary winding. In figure one the primary winding is connected to the AC voltage source. The secondary winding is connected to the four diodes. Each winding is essentially a coil.
A current build up in primary coil results in a magnetic field building around the primary coil. The magnetic field crosses the secondary coil causing current to flow in the secondary coil. When the current in the primary coil diminishes, the shrinking magnetic field across the secondary coil causes a current flow in the opposite direction. Hence the polarity of the voltage across the secondary coil changes every time the direction of current flow in the primary coil changes.
Here is a excerpt from my article ‘Introduction to Electronics: The Diode’
“A diode is an electronic component that only conducts in one direction. It is used extensively in electronic circuits. Figure One shows four diodes: D1, D2, D3 and D4.
A diode consists of two types of materials. A P-type and an N- type. In the P type material, some of the atoms have less electrons than protons. Hence, these atoms have a positive charge. The part of the diode that has P type material is called the anode. The anode is the triangle shaped part of the diode.
In the N-type material, some of the atoms have more electrons than protons. These atoms have a negative charge. The part of the diode that has N type material is called the cathode. The cathode is the straight line part of the diode
If a positive voltage exists at the anode and a less positive voltage exists at the cathode, the diode will conduct. When a diode is conducting, it is said to be forward biased. Typically, a forward biased diode’s anode is 0.6 volts higher than it’s cathode. If a negative voltage exists at the anode and a less negative voltage appears at the cathode, the diode will have a very high resistance and will not conduct. Under these conditions, the diode is said to be reversed biased. For practical purposes we consider a reverse biased diode to be an open circuit meaning that there is no electrical path through the diode and no current through the diode.”
In figure one, the four diodes in the full wave rectifier form a bridge. The bridge is always unbalanced because two diodes are always reverse biased during it’s operation.
The voltage waveform of the alternating current source is shown in figure 2a. You can find an explanation of alternating current in my article about alternating current at
Reference figure one for this explanation: When the instantaneous voltage at point c is positive, the current flows from point c through diode D1, point A, resistor R1, point B and diode D2 to point d. Diodes D1 and D2 are forward biased. Diodes D3 and D4 are reverse biased.
When the instantaneous voltage at point d is positive, the current flows from point d through diode D4, point A, resistor R1, point B and diode D3 to point c. Diodes D3 and D4 are forward biased. Diodes D1 and D2 are reverse biased.
Note that the current always flows from Point A through the resistor R1 to Point B. Hence the voltage waveform across R1 is the waveform shown in figure 2b.
Figure 3 shows the bridge circuit with a capacitor in parallel with R1. An explanation of the capacitor is provided in my article ‘Basic Electrical Theory: The Capacitor.’
The purpose of the capacitor is to hold the voltage across R1 at the peak voltage with the result being shown in the waveform in figure 4. Note that the DC voltage across R1 is not at a constant amplitude. The variations of the DC voltage across R1 are known as ripple. The ripple exists because the capacitor was unable to hold the voltage at it’s peak value after the instantaneous voltage of the waveform (figure 2b) across R1 started decreasing from it’s peak value. This is due to the capacitor discharging through R1. Ideally the voltage across R1 should remain at it’s peak value until the next peak of the waveform. The ripple is not desirable.
As you can see, this circuit does convert alternating current to direct current. However, the output is not well controlled. My next article explains the principles of the DC power supply which controls its outputs and prevents variations in the output voltage thereby eliminating the ripple.
I have a Bachelor of Science In Electrical Engineering and worked as an Electronics Technician.
Electronic Principles: Third Edition