Winding the Transmit and Receive Coils

1. Obtain two segments of PVC (polyvinyl chloride) pipe.
2. Obtain two pieces of 26 AWG magnet wire, each long enough to result in 25-30 turns on the outside of the PVC segment as well as approximately 4" at the beginning and end of the coil. You may find useful the fact that the tiles on the floor are 12" square.
3. Every student must wind a coil. Observe the notch on one end of the PVC segment. Begin winding the coil from the notch. Take your time winding, making sure that the coil turns are on the surface of the PVC and snug against the previous coil turn. Try to wind the coil near the notched end rather than in the middle of the PVC segment. Leave approximately 4" of length of magnet wire at the beginning and end of the coil for connection. When you have completed the winding, place a piece of scotch tape along the length of the coil to hold the windings in place. Count (and record) the number of windings on each coil.
4. Obtain a match and burn 1/2" of enamel off both ends of the coil to expose the copper conductor. The flame only needs to contact the magnet wire for one second. There are stick matches in the lab. Use a small piece of 400 grain paper to scrape off the burned enamel from the ends of the coils.
5. Obtain a piece of masking tape, place it on the PVC tube, and write your initials on the tape.
6. Use the multimeter to measure the DC resistance of your coil.
7. The inductance of your coil will be in the vicinity of 10-30 H. Think about how you might measure the inductance. Hint: Suppose the inductance is 10 H and you placed the coil in series with a 100 resistor. Then, you applied a 300 kHz square-wave to the series combination. What would the voltage across the 100 resistor look like? Each student is required to measure the inductance of their coil. To measure the inductance of your coil, obtain a 50 resistor, measure its value, and connect it in series with your coil on either your project board or the bench-top board. Connect the function generator across the series combination. The equivalent circuit of this measurement scheme is shown in Figure .

  
Figure: Circuit for Measuring Coil Inductance

Note that the 50 internal resistance of the function generator has to be included in the analysis. Measure the inductance of your coil by considering the time constant of the RL circuit.

Let R be the total series resistance (100 in this case). A KVL around the loop produces the following first order differential equation in the current i(t):

The complementary (transient) solution to this differential equation is

which has a time constant . The time constant is the amount of time required for the exponential to drop to times its initial value.

The function generator defaults to an amplitude of 100 mV PP. Set the amplitude to a value large enough to produce a distinct exponential voltage across the 50 resistor.