Convert an AC voltage into a nearly constant DC voltage

Reimanuel Cruz Santiago

Often we use AC voltage to use electronic devices that need to function a nearly constant DC voltage.  So, we want to convert an AC voltage into a nearly constant DC voltage to be used as a power supply for electronic circuits.

As studied in FISI3172 the AC voltage is sinusoidal but we need to convert it into a nearly constant like DC voltage so, we need to redesign a new circuit that can give me an a control of the voltage to convert it nearly constant.  One option is to add in the circuit  a diode, the diode is a very important device that has two terminals, the anode and the cathode the voltage across the diode is referenced positive at the anode and negative at the cathode. Similarly, the diode current is referenced positive from the anode to cathode.  The characteristic of the Diode is if  the voltage applied to the diode is positive, relative a large amounts of current flow for small voltages named this condition forward bias but for the other hand, for moderate negative values of voltage the current that pass for the diode is very small or cero called this condition revers-bias region. This property is very useful for example in automobile, diodes allow current from the alternator to charge the battery when engine is running but when engine stops, the diodes prevent the battery from discharging through the alternator.  The diodes consist of a junction between two types of semiconducting materials with one side of the junction with impurities create n-type material in wish large numbers of electrons move freely and in the other side of the junction different impurities are employed to create a positively charged particles known as holes.  If a voltage is applied with positive polarity on the n-side the barrier is enchanced and the charge carriers cannot cross the junction.  On the other hand, if a voltage is applied with positive polarity on the p-side the barrier is reduced and large currents cross the junction.

Figure 1 Example of a Diode

To rectifier the circuit for charging a battery we can use a half-wave rectifier that when we connect to a source voltage if the voltage is positive, the diode is in the forward-bias region and when the voltage is negative the diode is in reverse biased and no current flow.

Figure 2 Example of a half-wave rectifier

Finally, a circuit that can be used to convert an AC voltage into a nearl constat DC voltage to be used as a power supply for electronic circuits is the Half-wave rectifier but with a capacitor.  It function in the form that when the AC source reaches a positive peak, the capacitor is charged to the peak voltage and when the source voltage drops below the voltage stored on the capacitor, the diode is reverse biased and no current flow through the diode and the capacitor continues to supply current to the load, slowly discharging until the next positive peak of the ac input.  As show in the Figure 3, current flows through the diode in pulses that recharge the capacitor.

Figure 3 Half-wave rectifiers with capacitor

Reference:

Electrical Engineering Principles and application 5th Hamley