RC Time Constant

Charging and discharging of a 10µF capacitor with variable time constant.

What it shows:

The growth and decay of current in an RC circuit with a time constant chosen so that the charge and discharge is visible in real time.

How it works:

By choosing the values of resistance and capacitance, a time constant can be selected with a value in seconds. The time constant τ is given by

τ = RC

To obtain useful values, we chose three resistors 100K, 200K and 400K in series with a 10µF capacitor, giving time constants of 1, 2 and 4 seconds respectively.

The circuit is mounted on a 75cm × 90cm plywood board. Actually, it is wired behind the board, with a schematic circuit diagram marked out on the front (layout in figure 1). The 12V battery and the capacitor are mounted on the front of the board, with the resistors mounted behind (resistor symbols are drawn in their place on the front of the board). A short clip lead is used to patch the desired resistor into the circuit.

figure 1. RC Circuit layout

RC time constant

Setting it up:

The growth and decay curves are displayed on a storage oscilloscope, which is connected across the capacitor. Use a 10X probe, since the impedance of the scope is comparable to the resistors in the circuit and the capacitor will drain even when the switch is open.


Although we use a 12V battery, any voltage will do; you'll just need to adjust the sensitivity. The curves look just like those in a text book! It is best to practice beforehand engaging the knife switch the moment a new sweep begins.