Lab Assignment
Background
In electronics, a low pass filter is utilized to remove high-frequency noise from a signal. One example would be static present in a radio or cell phone transmission. A simplest low-pass filter is constructed from a resistor (R) and capacitor (C) in series. Refer to Figure 1 below.
![1789_Test System - Low Pass Filter with Meter.jpg](https://secure.tutorsglobe.com/CMSImages/1789_Test%20System%20-%20Low%20Pass%20Filter%20with%20Meter.jpg)
Figure 1: Test System - Low Pass Filter with Meter
In this exercise, we will examine what happens when we "turn on" this circuit. Before we begin our analysis (initially), the switch which connects the voltage source (VDC) to the rest of the circuit is open. We will examine what happens at t = 0, when this switch closes and current begins to flow.
The equations below describe what happens to the voltage across the capacitor (vCap) and current flowing through the capacitor (iLoop) following t = 0, the moment the switch is closed.
iLoop = (VDC/ R) exp (-t/RC) (1)
vCap = VDC - iLoop R
Note the definitions which are repeated below:
iLoop = loop current flowing through capacitor
VDC = the source voltage
R = resistance
C = capacitance
t = time
vCap = capacitor voltage
Problems
1. Using Excel, calculate the values of iLoop and vCap every millisecond (ms) starting from t = 0 until t = 50ms. Use the following sets of values: R = 1 Ohms, C = 10 mF, VDC = 1 V. Remember, 50 ms is equal to 0.050 s and 10 mF is equal to 0.010 F.
2. Plot iLoop and vCap versus time. Remember, that iLoop is in amps and vCap is in volts.
3. In order to measure the voltage across the capacitor, it is necessary to use a meter with a 3ms delay, which means that M(t) = vCap(t-3ms). Calculate and graph, along with iLoop and vCap, this measurement (M). Remember, that M is also in volts like the value is measures.
4. Add a polynomial trendline to the measurement (M) points. Try polynomials of orders 2 through 6 and select the one that works best. Add the equation of the trendline.
5. Using the graph in your assignment and you ability to increase / decrease VDC, R, and C, comment on the effect of these variables on the recovery of vCap.
Remember, you can change the variables and watch the graph change immediately.
a. How do VDC, R, and C affect the final value of vCap?
b. How do VDC, R, and C affect its rate of recovery (how quickly the voltage grows)?