Rensselaer Polytechnic Institute Introductory Physics II

PHYS 1200 Fall 2008

Activity 9 RL and RC circuits

In this experiment, you will observe the time dependence of voltage in driven RL and RC circuits.

Equipment:

Mobile Card (Oscilloscope function)

Mobile Card (Function generator)

Electronic Breadboard

0.1 microFarad capacitor

1 milliHenry inductor

1000 ohm resistor

Background:

Capacitor

When a capacitor is charged to voltage and then allowed to discharge across a resistor, the resulting voltage is (see for example Chapter 27.09 in Halliday, Resnick, Walker 8th ed.) In our circuit, we will charge through a diode, which allows current to flow one way but not the other. A positive voltage phase, the diode allows current to charge the capacitor. When the voltage is negative, the diode effectively isolates the capacitor from the source.

In the following circuit, the capacitor charges through the diode and discharges through the resistor.

1) Setting up your Mobile Card Oscilloscope

  • Set up your Mobile Card and enable your oscilloscope. Attach a red wire lead to the A1+ contact of the Card and a black wire to A1- contact two over from the A1+ contact.
  • Set Function Generator 1 to 100 Hz, 2V peak to peak, and Waveform: Square Wave.
  • Enable Channel 2 on the Oscilloscope
  • Channel 2 - on; Volts/div - 1V; Coupling - DC; Input - AWG1 (Function generator)
  • TRIGGER: Mode - Auto,; Source - Ch2;Slope - Falling.
  • Horizontal: Time/Div - 1 millisec; Mode - Y-T
  • You should have a nice single step in a square wave on the screen

2) Setting up the capacitor circuit.

You will want to construct a circuit like the one in the figure above. This is easier when you use an electronic breadboard like the one in your kits. The principle of the breadboard is simple. A wire can be pushed into each hole in the breadboard and is thus connected to common terminal strips inside the breadboard. In your breadboard each set of five holes is connected together. See the Appendix for details.

Here's a suggested layout.

In the figure, FG1 is the output from the function generator on the Mobile Card. The function generator outputs are labeled AWG1 and AWG2 and are approximately in the middle of the connector strip. Connect between AWG1 and GND

3) Capacitor measurements

  • Set up Channel 1 on the oscilloscope to measure A1 DIF. (Leave Channel 2 on so you can use it to control the sweep trigger.) Set the Volts/div to about 250 mV.
  • With the Horizontal Time/Div set at 2 ms you should see two cycles. The charging stage should rise rapidly and the decay stage should drop off slowly.
  • Sketch the Ch1 waveform on your paper.
  • Increase your sweep speed to about 200 s/div so you can see the slow decay across the screen. (You may find it useful to STOP collection so you don't have to chase a moving target.)
  • Enable cursors and move Ch 1 cursors to measure the maximum and minimum Ch 1 voltage on the screen.
  • Record the minimum and maximum voltages ______
  • Choose two points on the decay curve that are about 75% and 25% of the range from maximum to minimum. Use the CH 1 time and voltage cursors to measure the time and voltage of those points.
  • Record the time and voltage of those points. ______
    ______
  • Estimate the RC decay constant using: .

______

  • Is the value of RC consistent with the values for RC you were given at the start of this lab?

Section 4 Homework Exercise

When a resistor and inductor are wired in series, the inductor limits the rate of change of the current. For the decay portion of the cycle, with .

  • Rewire the circuit to reproduce the series LR circuit in Fig. 20.21 in Halliday, Resnick, Walker.

Here's a suggested layout.

  • Set the horizontal time base to 2 ms/div and Ch1 to 1 V/div. Sketch the waveform you see.
  • Expand the time base so you can see the decay curve. (About 1 to 2 s is ok.)
  • Stop the collection so you have a fixed screen and use the cursors as above to determine the L/R time constant.
  • Is this time constant consistent with your expectations from theory and the numbers you were given at the start of this document?

  • Appendices

Electronic Breadboards

Electronic breadboards have lots of simple connectors for electronic components. Holes in rows and columns are usually connected to one another so it's easy to connect components to each other. The interconnect layout for a typical breadboard is shown here. (A few typical devices are also shown to indicate how they might be inserted in the breadboard.)

The leads of most components can be pushed straight into the holes. ICs are inserted across the central gap with their notch or dot to the left.

The double top and bottom strips usually carry power or common signals - typically about 25 pins are connected in a row. The groups of five running vertically are called terminal strips and make it easy to connect devices together.

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