RC Electronic Circuits:

An RC circuit is any circuit containing, in addition to a power supply, just resistors and capacitors. An example of some of these components on a circuit board is shown below. Please contain your enthusiasm.

Examples: A very simple RC circuit just has a resistor, a capacitor, and a voltage source in series:

Here's a more complicated RC circuit, with several resistors and capacitors:

The purpose of an RC circuit is too allow charge to be temporarily stored. This is particular useful if you still want your circuit to work, after you have turned off a switch. This is the principle behind why your car lights now stay on for a bit after you turn your car off.

When resistance is measured in Ohms and capacitance (C) is measured in farads, then the time constant of the circuit in seconds, is just RxC.

The current flowing in the circuit after the power supply is switched off is:

I = I_oe^-t/rc

Example:

In the circuit below, how much current will flow 2 seconds after the switch is closed?

Note the unit M OHM is Mega Ohms and the uF is micro Farads.

mega = 1,000,000

micro = 1/1,000,000

Total current = V = RxI; I = V/R = 10/2,000,000 = 5 micro amps.

The time constant of this circuit is R * C = rc = 2*1 = 2 seconds. Therefore after two seconds the current will have fallen to:

I = 5(e^-2/2x1) = 5e^-1 = 1.84 micro amps.

Self-test: How much current will flow through after 5 seconds? Anwser is .41 microamps (prove this to yourself).

For all practical purposes, the time it takes to fully charge a capacitor is equal to 5 time constants.

We will now exit the world of simple circuits and move on to the basic physics and properties of semiconductors.