Analog & Digital Electronics

ANALOG & DIGITAL ELECTRONICS

LAB 2: LR Time Constants (Feb. 15)

Equipment

Multimeter, breadboard, Oscilloscope, inductor and resistors.

Purpose

I want you get more practice with the lab equipment and especially the oscilloscopes. I also want you to get experience working with inductors.

Part I: LR Time Constants

Set up the circuit at the right using a 1kW resistor and a 10mH inductor. (Measure the actual inductor and resistor values.) The function generator (labeled G) should be on the square wave setting with a peak to peak voltage of Vo ≈ 10V. When the generator’s voltage goes from the lowest to the highest voltage, the voltage across the resistor should be /

where t = L/R, the LR time constant. This t should equal the time it takes the voltage across the resistor to go 63% of the way from the lowest voltage to the highest on the rising curve. Measure this time using the cursors. You will probably want the generator’s frequency (in Hz) to be around 1/(20t). Does your measured t match your measured values of R and L? (Note that your inductor will also have some resistance, and you should measure this with your DMM to make sure it is < your resistor.) I want to see your experiment at this time.

Part II: Frequency Response

Use the same circuit you had in part II above, but now use a sine wave input of about 10V peak to peak, or about 3 to 4V rms. Measure the output of the generator and the voltage across the resistor at the same time for frequencies of 30, 100, 300, 1000, 3000, 10,000, 30,000, 100,000, 300,000 and 106 Hz. Make a table showing the frequency and each voltage. (I recommend that you use the rms voltage.)

Plot the output voltage in dB {ratio of 20 Log (Vresistor/Vgen)} as a function of Log(Frequency). At what frequency is the output -3dB? (This frequency in w should = 1/t from part II.)