Understand an basic resistor-capacitor (RC) timing circuit.

*Capacitance*
is a fundamental property of all bodies which describes their ability to
store electrical
charges. *Capacitors*
are two-terminal electronic components engineered for specific capacitance
properties. Capacitors are used for a variety of purposes in circuits,
including energy storage, power supply smoothing, signal filtering, and
timing.

Unlike resistors, the fundamental relation between voltage across a
capacitor and the current flowing through it is described by
a *differential equation* involving time. For this reason,
capacitor circuits always involve an element of temporal behavior.

In this example, a resistor and a capacitor form a very simple timing
circuit. When the momentary switch is closed, current flows through the
resistor and charges the capacitor. As current stores charge in the
capacitor, the OUT voltage rises. As the OUT voltage rises, the voltage
drop across the resistor is reduced and so the charging current flow is
reduced. This leads to a characteristic first-order exponential response
in which the voltage climbs rapidly at first and then slows to
asymptotically approach the source voltage, with a *time constant*
of t = R * C. With the given values (10K Ohms
and 100 microFarads), the time constant is 1 second, so the waveform
will easily be seen on an oscilloscope and can almost be seen on a
DMM.

Once the capacitor is charged and the switch is opened, it will hold charge and energy and the OUT voltage will ideally remain constant. Any real-life capacitor has a finite leakage, so in practice, the voltage will slowly decline back toward zero.

Capacitors come in both polarized and non-polarized varieties, depending
on the type of material in use. In general, larger capacitors
are *electrolytic* and are polarized, as in the diagram.

- Wire the circuit, being sure to observe the capacitor polarity. There are additional 100 uF parts in the Assorted bin, or you can use a larger size if needed.
- Observe the OUT voltage on an oscilloscope for three conditions: while the momentary switch is closed, with the switch open, and while shorting the capacitor with a jumper wire.

For a challenge, try adding a diode to create a
simple *envelope
detector*.

Modern touch screens measure the capacitance of a human finger touching the surface.

Very large capacitors can produce dangerously high currents and sparks when shorted. Extremely large “supercapacitors” have been recently developed for energy storage and are used in regenerative braking systems on electric vehicles to capture energy.

This is the barest scrape into the rich dynamic behavior possible with
capacitor circuits. There are an endless number of analog filter and
timing circuits using capacitors. A related component is
the *inductor* which also has complementary dynamic behavior
involving asymptotic current changes.