# AC circuits analysis in PSpice : tutorial 6

In this tutorial, I will help you do the analysis of an AC circuit. An AC circuit is the one in which the source of the circuit is of alternating current rather than DC (Direct current). At the start of the tutorial a brief introduction of AC (alternating current) is provided after that a simple AC circuit is implemented in PSPICE and the results were analyzed precisely. At the end of the tutorial you are provided with an exercise to do it by yourself, and in the next tutorials I will assume that you have done those exercises and I will not explain the concept regarding them

### Introduction to AC circuits analysis

A simple AC circuit is a circuit in which alternating current source is used. In AC circuit the source oscillates continuously as in the case of a sine wave. A simple AC voltage source is shown in the figure below,

Figure 1: Voltage source

On complete cycle of an AC voltage source consists of two half cycles, one being the positive half cycle and the other id the negative half cycle. The polarities in both the half cycles will be different from each other. The same ohms’ law as discussed in the previous tutorial applies on the AC voltage also, the presence of a resistor changes the value of the current accordingly with the resistor if the voltage is kept constant.

### Example of AC circuits Analysis in PSpice

• Lets’ design a simple AC circuit i.e. a circuit with AC source as a supply. Open the PSPICE design manager on your PC by typing design manager in the search bar. From the design manager click on the run schematic button to open a new blank schematic as shown in the figure below,

Figure 2: Open new schematic

• After opening the new schematic before jumping into designing first save the schematic by clicking on the file button at the top left corner and then selecting save as so that we can access it anytime in the future. Refer to the figure below,

Figure 3: Saving schematic

• Click on the get new part icon at the top bar of the schematic window in order to search for the components that are needed for circuit designing.

Figure 4: Get new part

• In the get new part window, type ‘r’ it will display a list of resistors available in PSPICE. From that list select a simple resistor as shown in the figure below,

Figure 5: Inserting a resistor

• Again open the get new part window and in the pat name block type ‘C’, select the capacitor from the list given and then click on place & close as shown in the figure below

Figure 6: Capacitor placement

• Again open the get new part window and in the pat name block type Vac, select the supply from the list given and then click on place & close as shown in the figure below

Figure 7: Place input sin

• Next step is to place a ground, do the same again and in the part name type Gnd and select the ground and then click on place & close as shown in the figure below,

Figure 8: Placing ground

• The placed components in the schematic window are shown in the figure below,

Figure 9: Placed components

• Click on the draw wire icon at the top bar of the schematic window in order to connect the already placed components for circuit designing.

Figure 10: Drawing wire

• Connect all the components to complete the circuit diagram as shown in the figure below,

Figure 11: Complete circuit diagram

• Next step is to set the attributes of the input AC supply. Double click on the AC supply you connected in the circuit previously and set the magnitude of the voltage of the supply to 10V and the DC voltage to 0 as shown in the figure below,

Figure 12: AC attributes

• Set the value of the capacitor according to the requirement of your circuit by double clicking on the component and changing the value to 3u and then clicking save attr as I set the value here to 3uF, as shown in the figure below,

Figure 13: Capacitor values

• On the top of the schematic window, click on the Voltage/Level Marker button as shown in the figure below,

Figure 14: Voltage marker

• Place it at the capacitor node as shown in the figure below,

Figure 15: Placed voltage marker

• Next step is to adjust the properties of the simulations in order to produce the graph of the voltage at the marker. Click on analysis and then click on Setup as shown in the figure below,

Figure 16: Simulation setup

• A widow will appear, click on the AC Sweep block on the window and adjust the properties of the window according to your requirement, refer to the figure below

Figure 17: AC sweep properties

• Change the start and end frequency in the property window according the portion of the graph you want to see. If we are interested in checking the voltage on a specific wire in spite of checking it at a node, double click on the wire and inn the window that appear as a result, type the name of the wire you want to label it with, as shown in the figure below,

Figure 18: Labeling the wire

• Now comes the simulation part, click on the analysis at the top bar of the schematic window and then click on simulate as shown in the figure below,

Figure 19: Simulation

• The output voltage at the capacitor node is displayed in the figure below,

Figure 20: Output voltage at the capacitor node

• The decreasing sweep at the capacitor node here implies the discharging of the capacitor. As in case of an AC circuit the input signal source consists of two half part or cycles. One is the positive half cycle of the AC signal and the other is the negative half cycle. During the positive half cycle, the direction of the current i.e. the polarity of the voltage will be in accordance with polarity of the capacitor and the capacitor will start charging during the positive half cycle, whereas during the negative half cycle due do the inversion the polarity of the voltage source, the polarity of the capacitor will be opposite to that of the source, hence taking into account the charging and discharging concept of a capacitor, the capacitor will start discharging the stored charge ( during the positive half cycle) to the circuit. The exponentially decreasing curve in the above graph implies the discharging phenomenon of the capacitor.
• Connect another voltage marker at the input of the AC source to the see the AC sweep of the source along with that of the capacitor as shown in the figure below,

Figure 21: Marker at input

• The output after connecting the voltage marker at the input too is shown in the figure below,

Figure 22: Output displayed along with input

• The green line at the top represents the input AC sweep of the AC source connect. The straight line represents that the input source is constant (neither increasing nor decreasing with time) unlike that of the capacitor voltage.

Exercise:

• Do the AC analysis of the circuit containing an inductor at the place of capacitor.

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