RC, LR, and LC Circuit Labs
Peggy Bertrand
Oak Ridge High School, Oak Ridge, TN
RC Circuit Lab
RC Circuit Lab Equipment – low tech
· Capacitors (100 mF to 1000 mF)
· Resistors (15,000 W or higher)
· D-Cells
· Knife switch (single-pull double-throw)
· Connecting wires or connectors
· Voltmeter or digital multi-meter
RC Circuit Lab Equipment – high tech
· Add a PASCO Science Workshop 750, 500, Spark or Vernier LabQuest
RC Circuit Lab Instructions to Students
· Build an RC circuit that can undergo charge and discharge cycles with a flip of the switch. Use capacitors of between 100 mF and 1000 mF, and resistors of between 10,000 W and 100,000 W.
· Collect voltage data over the resistor and over the capacitor for the charge and the discharge cycles and produce graphs of these data.
· Compare the calculated (predicted) and graphically determined time constants for your circuit.
· Repeat for a second circuit with a different capacitive time constant.
· Your report must include:
o Your graphs.
o Circuit diagram.
o Graphically determined capacitive time constants.
o Predicted capacitive time constants.
o Comparison of time constant values.
RC Circuit Diagram and Sample Student Results
The graph above was obtained using a 15,000 W resistor and a 1000 mF capacitor. Calculated time constant is 15 s; graphically obtained time constant is 14 s. Sampling was done with a Science Workshop 750, although with the long time constant shown, electronic data acquisition is not necessary for this laboratory.
LR Circuit Demonstration/Lab
Because of the expensive equipment required to get good quantitative results for LR circuits, it may be necessary to do this lab as a demonstration.
LR Circuit Demonstration/Lab Equipment
Because of the short inductive time constant in LR circuits, it is helpful to have the following:
· PASCO Science Workshop 750 ($679)
· PASCO Power Amplifier CI-6552A ($375)
· PASCO LRC Circuit Board CI-6512 ($118)
Alternatively, try the following:
· Vernier LabQuest ($329)
· Square Wave Signal Generator (BK Precision 3003 for $208)
· Inductors (as large an inductance as you can find or make)
· Connecting wire or connectors
LR Circuit Diagram and Sample Results Using Square Wave Generator
The graphs above show one section of the output voltage and the resulting current in a circuit containing an 8.3 mH inductor and 16.5 W of resistance. The time constant is calculated to be 5 x 10-4 s. Graphically, it appears to be 4 x 10-4 s. A PASCO Science Workshop system was used to collect voltage data, and to create the positive square wave signal. Sampling rate was 5000 Hz.
Alternative Student-Built LR Circuit Diagram and Results
The graphs above show data collected from a circuit containing an 8.3 mH inductor and 16.5 W of resistance. A high quality switch was used to open and close the circuit, and a PASCO Science Workshop 750 sampled the voltage at 5000 Hz. These data illustrate qualitatively how an inductor affects current in an RL circuit, but quantitative analysis of the time constant has only order of magnitude accuracy.
LC Circuit Lab
LC Circuit Lab Equipment
· Capacitors (100 mF to 300 mF)
· Inductors (as large as possible)
· DC Power Supply or 9-Volt batteries
· Knife switch (single-pull double-throw)
· Connecting wires or connectors
· Pasco Science Workshop 750 (or Vernier LabQuest)
LC Circuit Lab Instructions to Students
• Build an LC circuit with an inductor, capacitor, DC power supply, and a single-pull double-throw switch.
• The switch should be used to charge the capacitor using the DC voltage source when it is in one position, and to discharge the capacitor through the inductor when it is in the other position.
• There is resistance in your inductor that will damp the oscillations, so don’t add any additional resistance! (You are really building an LRC Circuit)
• Rapidly sample the voltage across the capacitor to obtain a damped oscillator curve.
• Compare the period you obtain to the predicted period.
LC Circuit Lab Circuit and Sample Student Results
The experiment above was run with run with a 0.270 H inductor and a 680 mF capacitor using a PASCO ScienceWorkshop 750 running at 5000 Hz. The period of oscillation obtained from the graph was 0.084 s, which is close to the theoretical value of 0.083 s. The resistance in the inductor causes damping of the oscillation.