Basic System ComponentsBasic System ComponentsEGR 345 Lab 4b – Electrical ComponentsLee C. GroenewegOctober 14, 1998Purpose:To review the concept of capacitance and inductance and to use LabVIEW to measure the voltage across a capacitor.Theory:The voltage across a capacitor is a time varying voltage. The capacitance can be found using the following equation:(1)where C = capacitance, I = current, T =change in time, V(t) = voltage at time tEquipment:Computer with a DAQ card running LabVIEW (PSE Dell 123)1 – capacitor rated for 470 F1 – C.A.D.E.T. Circuit trainer (to build the circuit on)1 – Digital Multimeter to measure resistances and voltages2 – resistors of value 7.5 k eachMiscellaneous leads and connectorsProcedure:)()())()((tVTtVTITtVTtVIVdtdICThe first procedure of the lab was to measure the values of the resistors to be usedin the circuit. The value for the resistor Rc in the circuit =7.56 k, and the value for the resistor Rd in the circuit = 7.56 k. Figure 1 is a schematic of the circuit built for this experiment.Figure 1. Circuit built to measure capacitance using LabVIEW.The second procedure required us to find the value of the capacitor. To find the value of the capacitor we had to plot the voltage across the capacitor over time. A graph plot of the voltage over time is attached as Appendix A. To determine the value of the capacitance; we had to find the voltage at two points on the graph plot and the time between the points. These values were entered into equation 1 tofind a value for the capacitance. The value for the current, I, was determined by measuring the voltage (Vr) across the resistor (Rc) and dividing it by the resistance, (I=Vr/Rc). The value was then to be compared to the value on the capacitor. This value was found to be –1.849F.Summary:The experimental value for the capacitor was found to be much less than it was rated for. This could be due to the capacitor not being fully charged when the reading wastaken. That is, the voltage discharged was not very large, giving a low number for the capacitance. The equations for capacitance, inductance and the calculations for this experiment are attached as Appendix B. This lab served as a reminder of the circuit analysis theory learned in a previous course. Italso incorporated the use of computer software to obtain results rather than using the traditional standard oscilloscope. This increased our knowledge about various tools for engineering students to solve problems. Using LabVIEW in this lab helped to teach us about the “new” tools available to us to solve “old”
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