Direct Current (DC) Electrical Circuits
by Ron Kurtus (revised 14 August 2005)
A direct current (DC) electrical circuit consists of a source of DC electricity with a conducting wire going from one of the source terminals to a set of electrical devices and then back to the other terminal, in a complete circuit. A DC circuit is necessary for DC electricity to exist. DC circuits may be in series, parallel or a combination. Understanding DC circuits is important for learning about the more complex AC circuits, like those used in the home.
Questions you may have include:
- What does an electric circuit consist of?
- What is a series circuit?
- What is a parallel circuit?
This lesson will answer those questions. Useful tool: Units Conversion
If you take a continuous source of DC electricity, such as a battery, and connect conducting wires from the positive and negative poles of the battery to an electrical device such as a light bulb, you have formed an electric circuit.
The battery, bulb and switch inside a flashlight form a DC circuit
In other words, the electricity flows in a loop from one end of the battery (or source of electricity) to the other end in a circuit. The concept of electric circuits is the basis for our use of electricity.
One nice feature of an electrical circuit is that you can install a switch in the circuit to turn the power on or off when you want.
Note: Although electrons move from a negative (−) area toward the positive (+), the convention for the motion of electricity was established before electrons were discovered. Thus, the convention is that that electricity is designated as moving from (+) to (−).
A DC circuit requires a source of power. Typically, a battery is used to provide continuous DC electricity. A DC generator is another source of energy. Alternating current (AC) electricity can be modified through a rectifier or adapter to create DC electricity. The common adapter used for some of your small DC-powered devices will transform 110V AC house current into 12V DC current for your device.
Voltage, current and resistance
The electricity moving through a wire or other conductor consists of its voltage (V), current (I) and resistance (R). The voltage or potential energy of a source of electricity is measured in Volts. The current of amount of electrons flowing through the wire is measured in Amperes or Amps. The resistance or electrical friction is measured in Ohms.
The wire and electrical devices must be able to conduct electricity. Metal such as copper is a good conductor of electricity and has a low resistance. The tungsten filament in a light bulb conducts electricity, but it has high resistance that causes it to heat up and glow.
Series DC circuit
In an electrical circuit, several electrical devices such as light bulbs can be placed in a line or in series in the circuit between the positive and negative poles of the battery. This is called a series circuit.
Two light bulbs in a series circuit with a battery
One problem with such an arrangement is if one light bulb burns out, then it acts like a switch and turns off the whole circuit.
Every device in a DC circuit--whether a light bulb or electrical motor--can be represented by an electrical resistance or resistor Usually, when drawing a circuit diagram or schematic, you use certain symbols for the battery and resistors.
Schematic of a DC circuit with three resistor in series
Parallel DC circuit
Devices can also arranged in a parallel configuration, such that if any bulbs go out, the circuit is still intact. Not only is a parallel circuit useful for holiday lighting, the electrical wiring in homes is also in parallel. In this way lights and appliances can be turned on and off at will. Otherwise if you turned one light off--or one burned out--all the other lights in the house would go off too.
Two light bulbs in a parallel circuit
If either light bulb would go out, the other would still shine. You could add other bulbs or even appliances such as electric motors in parallel to this circuit, and they would remain independent of each other.
Schematic of parallel DC circuit
You could also replace a bulb with a series circuit of bulbs or add bulbs or devices in series between parallel items. There are many combinations possible.
DC electrical circuits consist of a source of DC electricity with a conducting wire going from one of the terminals to a set of electrical devices and then back to the other terminal, in a complete circuit. DC circuits may be in series, parallel or some complex combination.
Always strive for success
Resources and references
Basic Electricity by Bureau of Naval Personnel; Dover Pubns; (1970) $14.95 - Provides thorough coverage of the basic theory of electricity and its applications
Basic Electricity and DC Circuits by Charles Dale, Prompt (1995) $54.95 - Large book with basic concepts to harness and control electricity
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