AC (Alternating Current) voltage is clearly the opposite of DC voltage. AC voltage is a variable signal, which continuously oscillates between positive and negative values. Here it is shown how a 120V voltage source varies through the time.
This means that the tension "v" have an initial value of zero, then it gradually rises until reaching maximum positive peak, then slopes down and comes into negative values, until the negative peak, to finally return to the null value, completing the cycle and starting again. But, how fast? Very, sixty cycles per second! Please try to imagine that.
The signal shape is called sinusoidal because corresponds to the mathematical function sine. In a circuit schematic, an AC voltage source is represented with this symbol.
It have been mentioned some parameters which need to be defined accurately.
If we need to express the magnitude of a voltage source, we cannot use the instantaneous "v" value, because it is not constant but it is always changing. When we say "at home I have 120 Volt" we are referring to the Vrms value.
The rms value is the root mean square of the sinusoidal signal, and its used as a nominal value of the tension, and applies to all calculations, beginning by the ohm law. The rms value in United States and other countries is 120VAC.
Another important parameter is the peak value (Vpeak), which is the highest value that the signal reaches both at positive and negative polarity. It is related to Vrms by the following equation.
So the peak value is a 41% higher than rms value. In the case of USA, where Vrms=120V, Vpeak results:
We mentioned before that the voltage signal completes a cycle sixty times per second, this alludes to the frequency and its unit is the Hertz, for example in USA the frequency is 60 Hertz. A parameter related to the frequency is the period, which is the duration of one cycle, and is the reciprocal of the frequency:
By the other hand, there are some places like Europe where the standard household voltage and frequency are 230V and 50Hz.
Here the peak voltage value is:
And the period corresponding to the frequency of 50 Hertz:
Each electronic application needs a suitable voltage source to work, eg. 12V, 5V or +-15V, so the household 120 or 230V usually must be reduced.
This job is carried out by power transformers which are electromagnetic devices that work only with AC voltage, and converts an input voltage to a different output voltage, also providing isolation between input high-voltage source and output low-voltage circuit.
An electric transformer consists basically of one high-voltage primary winding and one low-voltage secondary winding. Windings are not connected to each other, they are wound around a ferromagnetic core, thus the energy passes from the primary side to the secondary magnetically. We can see on the right the schematic symbol for a power transformer.
Nominal Primary Voltage (V1N): it is the recommended voltage to be applied at the primary winding (for example 120V). If a higher voltage is applied, the transformer could get damaged. Of course, this parameter must be selected equal to the line voltage of the place where is going to be used.
Nominal Secondary Voltage (V2N): it is the voltage at secondary terminals (eg 12V) that the transformer is designed to deliver when the input is connected to the Nominal Primary Voltage. Depending on load conditions, the real output voltage may vary a bit, but it should keep a value close to its nominal value.
No-Load Secondary Voltage (V20): it is the output voltage that the transformer delivers when there in no load connected to the secondary terminals (the output current is null). It is often slightly higher than the nominal value.
Nominal Secondary Current (I2N): it is the maximum recommended current to be taken by the secondary load for continuous work.
Full-Load Secondary Voltage (V2FULL): it is the voltage at secondary terminals when current at secondary is I2N. We could expect a slight voltage drop compared to nominal value for a commercial transformer.
Nominal Power (PN): is the product of V2N by I2N.
When giving examples, henceforth we will specify the primary voltage as "line voltage". If your country's household voltage is 120V, please assume that "line voltage" means 120V, and so for other different line voltages.
Transformer with a resistive load:
Let's suppose we have a resistor connected to a transformer, where the secondary voltage is 9 volts rms.
Multiplying the rms value by square root of two (1.41), we get the peak value, see the next graphic:
So we have an alternating current voltage applied over the resistor, and this will generate an alternate current on it, with the same sine shape, but... what value?
Fortunately the ohm law also applies to ac voltages and currents, the only thing to remember is to use the rms values. For example, if the resistor value is 1KΩ, then:
By using transformers we are starting to convert the line voltage to a more suitable level for electronic applications, but the outcome is still an alternate current. On next chapters we will learn how to process this signal in order to implement a regulated power supply. Particularly on the next chapter we will meet the diode, which will help us to convert this AC signal to a direct current signal.