RC circuit lab: time constant on a scope
In this RC circuit lab a signal generator drives a resistor and a capacitor, and a two-channel oscilloscope shows the input beside the capacitor voltage, so the charging curve is something students watch and measure.
Objective and parts
By the end, students can
- Recognize the exponential charging curve of a capacitor and read a time constant from it.
- Calculate the time constant RC and the corner frequency 1 / (2 pi RC).
- Describe what an RC low-pass filter does to slow and fast signals.
Parts, already on the bench
- A signal generator, 2 V at 1 kHz
- One 1.6 k resistor
- One 100 nF capacitor
- A two-channel oscilloscope: channel 1 on the input, channel 2 on the capacitor
Nothing to hand out or collect: the circuit opens built and wired.
What students do
Each step is short enough to read out. Students work in the simulator beside this page.
- Calculate RC for 1.6 k and 100 nF before you start.
- Open the lab and press Run. Two sine waves appear, the capacitor voltage smaller and later than the input.
- Read the two amplitudes. At 1 kHz the output is about 0.7 of the input: 1.41 V from 2 V.
- Set the generator to 100 Hz, then to 10 kHz, and note the output each time.
- Go back to 1 kHz and switch the generator to a square wave. The output now climbs and falls along a curve.
- On the curve, find how long the output takes to cover 63 percent of the step. That time is RC.
Questions for students
Hand these out as they are. The expected answers are under the list, for you.
- What is the time constant of this circuit?
- What is the corner frequency, and what did the scope show there?
- What happened at 100 Hz and at 10 kHz?
- Why are the corners of the square wave rounded on the capacitor?
- The capacitor is changed to 1 uF. What happens to the time constant and the corner?
Answer key for the teacher
- 1600 ohms times 100 nF is 160 microseconds.
- About 995 Hz. At the corner the output is 0.707 of the input and lags it by 45 degrees.
- At 100 Hz the two traces lie almost on top of each other. At 10 kHz the output is about one tenth of the input. Slow signals pass; fast ones are cut down.
- The capacitor can only charge as fast as the resistor lets current in, so its voltage follows an exponential curve toward the new level.
- The time constant becomes ten times longer, 1.6 ms, and the corner drops by ten, to about 100 Hz.
For students who finish early
- Swap the resistor and capacitor to make a high-pass filter and repeat the three frequencies.
- Choose R and C for a corner at 5 kHz, build it and check it on the scope.
Give this lab to every student
Assign to my class puts this circuit on each student’s class page as a starter. They change their own copy and hand it in, and the work comes back to you in one list.
The class tools are free for 30 days for one class of up to 20 students, with no card. Opening and running the lab needs no account and stays free.