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AC Voltage Sources

AC stands for: Alternating Current. AC voltage sources don't have a positive and a negative terminal: the polarity reverses in time. Take a look at the picture below.

In this picture SW1 is a switch. In the drawn position, A is connected to C and therefore to the positive side of the DC voltage source. B is connected to E and therefore to the negative terminal of the 9V source. When you toggle switch SW1, the polarity will be reversed: A will be connected - via D - to the negative end of the voltage source, and B will be connected to the positive end. Now imagine that someone toggles switch SW1 frequently. The signal at terminals A and B will then be an AC voltage.

The top value of an AC voltage is called the amplitude. In this case, the amplitude is 9Vt. The voltage between the two tops is called the top-top value; in this case 18Vtt.

If we toggle SW1 forth and back in exactly 1 second, we create a 1Hertz signal. Hertz is the unit of frequency: the number of times a signal repeats itself in one second. Hertz is usually abbreviated to Hz. The time it takes for a signal to repeat itself is called the period time, symbol T; in this case T = 1 s. A 10Hz signal means that the signal repeats itself 10 times per second; in that case T=0.1s. So:

T = 1/f and f = 1/T

It is common practice to use symbols in capitals for DC signals and lower case symbols for AC signals. For example VA would mean the DC voltage at point A, and iR4 would mean the AC current flow in resistor R4.

Examples of AC voltage sources are: a microphone, a house outlet, and the speaker terminals of an amplifier.

An AC voltage source doesn't really have a symbol of it's own. They are usually drawn as one or two terminals with a ~ sign. If only one terminal is drawn, the other one is connected to ground.

Some AC voltage sources have their own symbols, e.g. a microphone: