Direct Current: What is it? (AC vs DC & DC Current Symbol)

What is DC Current

What is DC Current?

DC stands for Direct Current, although it is often referred to as “DC Current”. DC current is defined as a unidirectional flow of electric charge. In DC current, the electrons move from an area of negative charge to an area of positive charge without changing direction. This is unlike alternating current (AC) circuits where current can flow in both directions.

DC current can flow through conducting material like wire and also flow through the semiconductors.

The battery is the best example of a DC source. In a battery, the electrical energy produced from the chemical energy stored in the battery. When a battery is connected in a circuit, it provides a constant flow of charge from negative to the positive terminal of the battery.

A rectifier is used to convert alternating current to direct current. And the inverter is used to convert direct current to alternating current.

DC Current Symbol

The DC current is a constant current. Therefore, the symbol of DC current is a straight line. The symbol of DC and AC current is as shown in the figure below.

Symbol of DC and AC Current
Symbol of DC and AC Current

Difference between AC and DC Current

The electrical energy is available in the form of Alternating current (AC) or Direct current (DC). In alternating current, the current reverses direction 50-60 times in a second depending on the frequency.

The main differences between AC and DC have been summarized in the below table;

Alternating Current (AC) Direct Current (DC)
The direction of flow of currentWhen an alternating current flowing through a circuit, it reverses its direction.When a direct current flowing through the circuit, it will not change the direction.
FrequencyThe frequency of alternating current decides how many times it reverses its direction. If the frequency is 50 Hz, it means the current changes direction 50 times.The frequency of the direct current is always zero. Because it never changes its direction.
Movement of ElectronElectrons keep changes its direction from forward to backwardElectrons move only in a forward direction.
Current magnitudeThe magnitude of the instantaneous current is varying with time.The magnitude is constant at each instant of time for pure DC. But it is variable for pulsating DC.
Power Factor It ranges between 0 and 1.It is always 1.
Passive ParameterImpedance (Combination of Reactance and Resistance).Resistance only.
TypesSinusoidal, Trapezoidal, Square, TriangularPure DC and Pulsating DC
Transmission of electrical energyIn a power system, the conventional method to transmit power is the HVAC transmission system. The losses are less but more than the HVDC transmission system.In a power system, most emerging technology for transmission systems is the HVDC Transmission system. The losses are very less in the HVDC transmission system.
ConvertIt can convert from DC supply with the help of Inverter.It can convert from AC supply with the help of a rectifier.
Type of loadIt can connect with the resistive, inductive and capacitive type of load.It can connect only with the resistive type of load.
SourceAC GeneratorDC Generator and battery
DangerousIt is dangerous.But it is more dangerous than AC for the same power rating.
Waveform Waveform of AC Current Waveform of DC Current
ApplicationMost of the household, industrial and commercial equipment operate on DC.Cell phones, Electric Vehicle, Electroplating, Flashlight, etc.

What uses DC Current

The DC current can easily get from the battery and solar cells. Most of the power electronics circuits require DC supply. The application of DC current in various fields are listed as below;

  • DC supply used in many low voltage applications like charging mobile batteries. In a domestic and commercial building, DC used for emergency lighting, security camera, and TV, etc.
  • In a vehicle, the battery is used to start the engine, lights and ignition system. The electric vehicle runs on the battery (DC current).
  • In communication, a 48V DC supply is used. Generally, it uses a single wire for communication and uses a ground for the return path. Most of the communication networking devices operate on the DC current.
  • High voltage Power Transmission is possible with the HVDC Transmission line. There are many advantages of HVDC Transmission system over conventional HVAC Transmission system. An HVDC system is more efficient than an HVAC system, as it does not experience power losses due to the corona effect or skin effect.
  • In a solar power plant, energy generated in the form of DC current.
  • The AC power cannot be stored like DC. So, to store electrical energy, always DC is used.
  • In a traction system, the locomotive engines are run on DC current. In diesel locomotive also, the fan, lights, AC, and sockets are operating on DC current.

How to measure DC Current

The DC current can be measured by a multimeter. The multimeter is connected in series with the load.

The Black (COM) probe of a multimeter is connected with the negative terminal of the battery. The positive probe (red probe) is connected with the load. The positive terminal of the battery is connected with the load. The connection diagram is as shown in the below figure.

Measure DC Current
Measure DC Current

Set type of current DC in a multimeter. The reading shows the value of DC current flowing the load. The clamp-on meter is also used to measure DC current flowing through a conductor.

Which way does DC Current Flow

The current is known as the flow of charge or electrons. The direction of the current depends on the direction of the flow of charge.

The electrons flow from the negative end of the battery to the positive end of the battery. Then also, the current indicates directions in a direction from positive to the negative end.

Benjamin Franklin observed that something moving through the conductor. But at that time, protons and electrons are not discovered. So, he does not know what is moving through the conductor.

He assumed that current flow from higher concentration region to lower concentration region. And he called a higher concentration region as positive and lower concentration region as negative. Therefore, the current flows from positive to negative. And this direction is known as the conventional direction of the flow of current.

After the invention of electron and proton, it is confirmed that current moves from negative to the positive end of the battery. But still, we are assuming the direction of current as of the conventional method.

Who invented DC Current

The DC was first introduced by Italian physicist Alessandro Volta’s battery. At that time direction of current was not introduced. French physicist Ampere gives an opinion that current travel in one direction from positive to negative.

In the late 19th century, three inventors, Nikola Tesla, George Westinghouse, and Thomas Edison combat on choosing the electricity system.

The company of Edison promoted the DC system as the dominant electric system and it is better than the AC system. He built a first power plant and started to transmit DC power to a home in New York.

The competition began between Edison and Tesla. Because tesla supports the AC system and it can transmit the AC power over a long distance. After this war, Westinghouse made the first hydro-electric generator placed on Niagara fall. And the winner of the current war. From now, the AC system is dominant over the DC system.

But nowadays, due to more power electronics equipment, DC current is used to powered low-voltage power electronics devices.

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