ENSC 220
ELECTRIC CIRCUITS I

Lab #3: RL and RC Circuits

OBJECTIVE:

  • You will wind (and test) your own inductor, which you will also use in the AM radio project in a subsequent lab.
  • You will also investigate the properties of capacitors by making a simple capacitor and by designing and testing filters and a simple capacitive switch.

    PREPARATION:

  • Read Lab Handbook 2.3.4 (Capacitors) and 2.3.5 (Inductors)
  • Read Appendix III (pp. 227) on standard capacitor values.

    EQUIPMENT:

  • Basic lab tools and breadboard
  • Dual DC power supply
  • Digital Multimeter (DMM), Fluke 8010A
  • Function Generator, Wavetek 182A or equivalent
  • Oscilloscope, Tektronix 2235 or similar
  • LRC bridge
  • Operational amplifier, TL072
  • Light emitting diode
  • Resistors
  • Capacitors
  • Connnecting wire
  • Wire and core for inductor
  • Emery paper
  • Tape
  • Materials for capacitor

    NOTES:

  • When making circuit changes, turn down the current on the power supply and turn off the power supply. Turn it on again only after checking the circuit carefully.
  • Check the multimeter setup before hooking it into the circuit. Improper setup can give incorrect readings and/or damage the meter.
  • Lay out your circuit neatly and logically on your breadboard to facilitate subsequent changes or troubleshooting.
  • When operating the op amp from a single supply, note that the other power supply input on the op amp must be grounded.
  • Be sure that DC offset on the function generator is set at "0".
  • Use maximum power supply voltages of +/- 12 volts.
  • After sanding the insulating varnish off the taps on your inductor, you may need to tin them to be sure that all insulation is gone. Note that this cannot be done with the continuously variable inductor, because turns could be shorted by solder.
  • Use tape to secure taps and ends of windings on your inductor.

    METHOD:

  • INDUCTOR:
    1. Construction the inductor.

    The inductance L of an air-core inductor is approximated by:
    L = (d^2 n^2)/(18d + 40l)
    where:
    L = inductance (measured in uH)
    d = coil diameter (inches)
    l = coil length (inches)
    n = number of turns.

    You will be supplied with #26 AWG enamelled wire (0.0170" diameter) and a 5" piece of nominal 3/4" (actual O.D.1-1/16") plastic pipe to construct your inductor (you should not need any more than 30' of wire). You will probably want to experiment with inductances in the range of 50 - 200 uH when you finally build your AM radio. You can build a "tapped" inductor by inserting a twisted wire loop "tap" at several sites along the inductor length:

    Alternatively, you can build a "variable" inductor by winding the inductor, then use a piece of emery cloth (or fine sand paper) and lightly sand off the enamel wire coating. Then run a metal "slider" across the exposed windings to set up a variable tap:

    2. Use the LRC bridge to measure your total inductance and that of each tap. Compare to calculation.

    3. Also devise your own method to measure the total inductance (e.g., based on a step-response technique) using the terminal characteristic for an inductor: v = L di/dt . Compare to calculation.

  • CAPACITOR:
    1. Construct a capacitor. You can make a capacitor with two metal plates (cross sectional area A) separated by a dielectric of thickness d. Capacitance is determined by the formula, C = eA/d (where e is a constant determined by the material between the plates - the dielectric). Construct a capacitor using any material you choose (plates: tin cans, coins, tin foil, etc., dielectric: air, plastic film, paper, etc.).

    2. Measure the capacitance of your capacitor using the LRC bridge.

    3. Also devise your own method (e.g., based on a step-response technique) using the terminal characteristic for a capacitor: i = C dv/dt.

    4. HIGH PASS FILTER:

  • Construct a simple RC high pass filter circuit, using your capacitor.
  • With input level constant, sweep the input over a wide range of frequency (sine wave), and plot the output level response.
  • Repeat above using a capacitor of, say, 330 to 1000 nF instead of your capacitor.
  • Repeat using a capacitor of, say, 15 to 100 pF instead of your capacitor.

    5. LOW PASS FILTER:

  • Construct a simple RC low pass filter circuit, using your capacitor.
  • With input level constant, sweep the input over a wide range of frequency (sine wave), and plot the output level response.
  • Repeat above using a capacitor of, say, 330 to 1000 nF instead of your capacitor.
  • Repeat using a capacitor of, say, 15 to 100 pF instead of your capacitor.

    6. CAPACITIVE SWITCH:
    Use your capacitor (along with an op amp(s) circuit) to produce a simple capacitive switch. The switch should be designed to operate as follows:

  • Pressing on the capacitive element should alter its capacitance. Explain how.
  • Assuming a sinusoidal input, devise a circuit whose output amplitude depends on C. Experiment with different frequencies from 10 kHz - 1 MHz, and possibly with the R and C in your circuit, to obtain a maximum change in output level when your capacitor is pressed. If possible, try to get at least a 1.5x to 2x level change.
  • Measure the change in this output using a simple comparator circuit.
  • Process the output of this comparator so that you get a DC signal that goes up and down as you press on your capacitor. This can be done by choosing and setting an appropriate reference voltage on the comparator.
  • The comparator output should "switch" on an LED through a current limiting resistor as shown below.

    The resistor RL inserted between the comparator output and the LED is used to limit the current through the LED. You can try several resistors to adjust the intensity to your liking. The LED conducts current in one direction only (in the direction of the arrow) so you may have to reverse the LED connections if it does not light the first time. If you want to read more about diodes (the device you are using is a Light Emitting Diode - LED) then you can refer to Chapter 6.

    Your lab write-up should investigate a number of considerations (design, analysis, measurements, observations, testing, calibration, etc.) for all these experiments.

    Last modified: hu Aug 13 20:03:59 PDT 1998.