What is the difference between AC and DC

What is Alternating current (AC)?

AC, or alternating current, refers to a voltage or current scale whose size and direction change regularly and periodically over time.

The waveform diagram of alternating current is shown in the figure below:

What is Direct current (DC)?

Direct Current, referred to as the DC,also known as “constant current”. The magnitude and direction of the direct current remain unchanged. Common DC power sources include batteries, lead-acid batteries, and dry batteries.

The waveform of DC power is shown in the figure below:

Several basic concepts about current:

Take sinusoidal alternating current as an example:

  • Peak: The maximum value of sinusoidal alternating current in a cycle, denoted as Vpk.
  • average value: The waveform of sinusoidal alternating current is symmetrical, so the average value of sinusoidal alternating current in a cycle is 0. Such an average value cannot describe the characteristics of alternating current. Therefore, we often calculate the absolute average value of alternating current, the formula is as follows:
  • Instantaneous value:It can also be expressed as:
    ω is the angular frequency of alternating current, ϕ is the initial phase angle of alternating current.
  • Valid value: The effective value of alternating current is usually defined by the thermal effect of current, and the formula is as follows:

Please note that the following signals also belong to alternating current, and they can all be converted into sine waves by Fourier transformation.

Since the magnitude and direction of the direct current are constant, the peak value, instantaneous value, effective value, and average value of the direct current are all equal to a constant.

What is the difference between AC and DC?

Now, we use 12V DC power and 12V AC power to analyze the difference between DC and AC power from loss, use, measurement, and safety.

Loss
DC: Direct current is more suitable for long-distance and large-capacity transmission. Therefore, HVDC transmission has become a hot topic.
AC: The AC circuit has inductance parameters, so the loss is large in long-distance transmission.

Use
DC voltage stability, no big noise, it is suitable for the use of electronic products. (such as televisions, radio computers, etc.)
AC power to go through the rectifier/switching power supply into DC power can be used for electronic products.

Measure the difference between 12V AC and DC:
A) with a digital universal measurement, respectively, with 20V AC voltage and 20V DC voltage file measurement, the results will be different.
B) simple measurement method: with a stylus pen (non-ordinary pen) on the wire foreskin, 12V AC will still show, 12V DC no display.

Safety
12V DC is safer than 12V AC. Body resistance decreased when the 12V AC still may cause the dead, 12V DC will not be in 100%. However, the hazard degree of electric shock to the human body mainly depends on the size of the current passing through the human body and the length of the energization time.

Peak
According to the voltage pattern, 12V AC and DC instantaneous peak voltage is not the same, the instantaneous peak voltage (12V straight) ≡ 12V, instantaneous peak voltage :

About Rectifier and Inverter Circuit

Rectifier: The conversion of alternating current to direct current is called Rectifier. The single-phase bridge rectifier circuit diagram is shown below. VT1 and VT4 are a set of switches. VT2 and VT3 are another set of switches. Two sets of switches are turned on alternately to obtain direct current.

Inverter Circuit: The conversion of direct current to alternating current is called Inverter Circuit. The single-phase bridge inverter circuit diagram is shown below. S1 and S2 are one set of circuits; S3 and S4 are another set of circuits. In simple cases, for a resistive load, two sets of switches are turned on alternately to get alternating current at both ends of the load.

Remarks: In order to get good waveforms for the rectifier and inverter circuits, filters should be used in actual situations.

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