What is a Direct Current Circuit – Analysis and Example

Direct current circuit or dc circuit is a circuit which has one direction of voltage and current. To be more specific, this circuit only has one direction or unidirectional flow of electrical charge. One of the simplest examples for DC voltage is a battery.

If DC is the abbreviation of “Direct Current” then AC is the abbreviation of “Alternating Current”. The idea behind the “direct” or “alternating” is a “direction” of their flow of charges.

Basic DC Theory

If someone asks you to define a dc circuit, can you answer it with your best and simplest explanation? From some literature, books, or journals, they may give you the most complete explanation. But for some reason, we only need to learn the simplest definition about them and go straight to circuit analysis.

Just as the name implies, Direct Current means it only uses one direction. Here we know another important word and that is “polarity”. Polarity indicates “a direction” just as we need to know before analyzing an electrical circuit.

A DC circuit is a circuit that only has one polarity, one direction of voltage or current. This circuit has constant value, zero frequency, or slow to almost none varying mean value for voltage or current.

Another explanation is

  • A DC voltage source will always have a constant voltage across it.
  • A DC current source will always have a constant current through it.

In conclusion, a DC circuit is where the voltage and current in every part in the circuit has constant value.

Maybe it is confusing to think that the abbreviation of DC current is Direct Current current. This is why DC also refers to “constant polarity”. A DC circuit may have a varying voltage but it will not cross the negative polarity or vice versa just as shown in the voltage waveform of a rectifier below.

direct current circuit 1

Above is the common DC circuit graph where it never crosses another polarity.

What is DC Circuit

A DC circuit is an electrical circuit that only consists of

  • Constant voltage and/or current sources,
  • Resistive loads (resistors).

This circuit is independent from time. If a circuit doesn’t need any past value of its voltage and current, we can assume that circuit is a DC circuit.

It doesn’t need its past value hence it doesn’t use any complex calculation consisting of time-varying, transient period, or something like that.

We only need to analyze its steady-state calculation, of course with a DC solution.

Keep in mind that not every electrical circuit can be solved with a DC solution. Get ready to use another solution if the circuit has inductive (inductor) and capacitive (capacitor) loads.

Furthermore, it is safe to refer to a circuit as a DC circuit if it is energized by DC sources (battery or DC power supply).

Application of DC Circuit

DC is commonly found in extra low and low voltage applications, especially for batteries and solar cells. Most of our electronic devices also require DC voltage and DC power supply.

Keep in mind that when we are using DC electrical devices, we need to pay attention to the polarity or else you will break it. Even if your devices have a diode bridge element, it is still a wise thing to connect the proper polarity.

There are also many uses of a DC voltage in the automotive sector.

Take example of the developing electric vehicles, they are using batteries meaning a DC voltage. Diesel engines in heavy vehicles, trucks, farm equipment, etc also use DC voltage systems.

DC power supplies are also found in telecommunication technology.

This power supply will maintain the connection lines for subscribers if there is an interruption during operating. DC-DC converter is also popular as a power source for communication devices.

For efficiency, high voltage power transmission uses DC electricity. This is because in long distance transmission, the losses by High Voltage DC Voltage (HVDC) are lower and cheaper.

Simple DC Electric Circuit

A DC electric circuit, the electric charge is flowing from higher potential to lower potential. This is why the symbol of a DC voltage source or battery has an indication of high potential and low potential.

direct current circuit 2

Symbol above is a common DC voltage source where:

  • The higher potential is represented by positive symbol
  • Lower potential is represented by negative symbol

direct current circuit 3

Symbol above is a battery where:

  • The longer line with the positive symbol represents the higher potential.
  • The shorter line represents the lower potential.

Moving on, let us proceed to a simple direct current circuit diagram below. We will use a flashlight to make a simple direct current circuit diagram.

direct current circuit 4

From the diagram above we can conclude that our parts of a simple dc electric circuit this time are:

  • A battery,
  • A switch, and
  • A light bulb.

Our next concern is “how the bulb will light up in a simple direct current circuit”. It is very easy because if it doesn’t have a switch then the bulb will always ON until the battery dries out.

This simple bulb circuit can show us how a DC circuit behaves when there is a DC current flow or DC voltage applied to the circuit.

There are two conditions:

  • When the switch is closed, the current flows to the bulb and turns it ON.
  • When the switch is open, the current doesn’t flow to the bulb and turns it OFF.

Of course this circuit alone can be improved for better safety because we only connect a battery to a bulb with a conductor wire. Keep in mind that every electrical device, elements, and components have thresholds for both voltage and current.

Overcurrent in the circuit will blow the bulb and you need to buy another one.

Thinking back to how we learn Ohm’s Law, we can absolutely apply it here.

Using

    \begin{align*}&V=I\times R\\&\mbox{or}\qquad\\&I=\frac{V}{R}\end{align*}

We can reduce the current by increasing the resistance or decreasing the voltage.

You can forget about decreasing the voltage because we use a battery which has a fixed voltage value. Then we can only add a resistor to reduce or limit the flowing current to the bulb.

direct current circuit 5

This is how to make your own simple direct current circuit.

Direct Current Circuit Analysis Example

What is a dc circuit and in what direction does it flow?

After reading until this point, you should have answered that question to yourself. Just observing a DC circuit for a second and you will know the flow direction of the current. Our next focus is:

How much current flows in that direction?

To answer that we will use a direct current series circuit as shown below:

direct current circuit 6

The DC circuit current flow is indicated by the letter I along with its arrow direction. The voltage in the dc circuit is represented by V.

We can assume that V is a battery then R1 and R2 can be any resistive loads such as light bulbs.

Since there is only one current path then the current will be the same at every point in the circuit. Or we can write it in mathematical expression:

    \begin{align*}I=I_1=I_2\end{align*}

The voltage in a DC circuit is the sum of the voltage drops across the circuit elements. In this case, voltage source V is equal to the sum of the voltage drop across R1 and R2.

Hence we can turn it into mathematical equation:

    \begin{align*}V=V_1+V_2\end{align*}

After we have these two equations, we can freely use Ohm’s law to find the desired variable.

Using Ohm’s law:

    \begin{align*}V&=V_1+V_2\\V&=IR_1+IR_2\\V&=I(R_1+R_2)\end{align*}

Thus,

    \begin{align*}R_{eq}=R_1+R_2\end{align*}

We will limit ourselves with the series DC circuit and not continue into the parallel DC circuit.

DC Circuit Theory and Laws

In order to understand an electric circuit fully, make sure to read:

Don’t forget to learn:

When dealing with a simple DC circuit, we can always use:

Furthermore when the circuit becomes more and more complicated and complex, you will start to learn and use the analysis methods below:

Sometimes the analysis methods above make things more complicated with numerous equations, hence we will use the analysis theorem below:

Frequently Asked Questions

What is a direct current circuit?

A DC circuit is a circuit that only has one polarity, one direction of voltage or current. This circuit has constant value, zero frequency, or slow to almost none varying mean value for voltage or current.

What is an example of a DC circuit?

The most basic example of a DC circuit is a flashlight where we only connect a battery, a switch, and a light bulb or LED.

Which country uses DC current?

We can say every country uses DC current if they are still using DC voltage. Of course most of the electronic devices we use also operate in DC electricity.

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