RC Time Constant
RC circuit - capacitance - DC circuits
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 300K in series
with a 10µF capacitor, giving time constants of 1, 2 and 4 seconds respectively.
The circuit is mounted on a 75cm x 90cm plywood board. Actually, it is wired
behind the board, with a textbook style circuit diagram marked out on the front
(layout in figure 1). The 45V battery and the capacitor are mounted on the
front of the board, but the real resistors are behind because they're too small
to see, and their symbols drawn in their places.
Figure 1. RC Circuit layout
Setting it up:
The growth and decay curves are displayed on a storage oscilloscopetot,
which is connected across the capacitor. Use a x10 probe, otherwise the impedance
of the scope is comparable to the resistors in the circuit. The oscilloscope
display projection is described in Tools of the Trade.
Comments:
Although we use a 45V 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.
Rating***