Parts of a circuit

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An electric circuit is a closed loop made of conductors and other electrical elements through which electric current can flow


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A schematic is a diagram that shows the physical connections of components in a circuit. Here is a schematic for a simple motor control circuit.


As we start to investigate circuits, we need to define and classify each individual part of the circuit. It’s the combination of these parts that make a circuit possible. On this page we go through the individual parts in detail.


  1. Power supply / Power source

Every circuit needs a power supply to supply the energy. We will further divide this category in to 2 types of power sources, AC and DC below. You need to understand both types of power supplies.


DC or Direct Current Power supply

Current that moves in only one direction

In DC circuits, like ones using batteries as the power supply, the negative and positive ends of the circuit are in fixed positions. This means they never change position. Because of this electrons always move in the same direction on their way to the positive.

Electricity flows from negative to positive.

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In the schematic a battery symbol is shown on the left side along with + and - labels for its terminals.

Since electrons are negative and attracted to the positive. The flow of current is shown starting at the negative side of the battery and flowing around to the positive side of the battery.


Ac or alternating Current Power supply

Current that alternates or changes its direction periodically.

In an AC circuit the positive side and negative side of the circuit switch positions repeatedly and causes the the electrons to continually change direction always towards the positive.

electrons move forwards when the signal is positive and backwards when the signal is negative. they change direction 60 times a second

electrons move forwards when the signal is positive and backwards when the signal is negative. they change direction 60 times a second

When you look at the AC diagram above you will see what is known as a sine wave. This is a graph that shows how AC works.

  1. When the red sine wave is positive. The yellow electrons move in one direction.

  2. When the the sine wave is negative, the electrons move the opposite or alternate direction.

  3. Hertz is the unit of measurement for how fast the signal changes direction, in the US alternating current runs at 60Hz (changing direction 60 times every second)

Videos on AC and DC

https://youtu.be/vN9aR2wKv0U?t=5

https://www.youtube.com/watch?v=q-DKc4KHPqU

 
symbol for an AC Power Supply

symbol for an AC Power Supply

Symbol for a DC Power Supply

Symbol for a DC Power Supply


 

2. Load

An electrical load is the part of the circuit that consumes power in contrast the power supply, which supplies power. The load converts electrical energy into some other form as we have already discussed. We used the example that a light bulb converts electrical energy into light and heat energy. The light bulb is the load.

Load is the generic term for the component that you are trying to turn on. The basic loads we will be discussing are lights and motors.

All circuit Resistance is considered to be in the load, measured in ohms.

symbol for a motor

symbol for a motor

symbol for a light bulb

symbol for a light bulb


 

3. Conductors

Conductors are the part of the circuit that provide a path for current. Electrons need paths of low resistance so they can flow from the negative side of the circuit, through the load, and finally to the positive side of the circuit.

Any conductive material will allow electrons to flow and become a conductor given the chance. We use the third rail on city train system as a conductor to feed power to the trains motors.

Wires are beneficial because they come wrapped in insulation. This allows us to handle the wires safely and not become part of the circuit. We use various sizes of electrical wires as conductors in circuits. Bigger wires are needed for circuits that have more Current. Small wires can’t handle all those electrons!!

The lines drawn to connect the 3 components are wires.

The lines drawn to connect the 3 components are wires.


 

4. Control Device

Control Devices allow you to control the current flow in a circuit. This allows you to turn the circuit on an off. The most common control device is a switch. Other control devices you will use in future classes will include Relays, Variable Frequency Drives, PLCs,

Switches come in many different styles, designs and packages to meet very basic and also very complex needs. We will cover switches in greater detail in another lecture very soon. The symbol below represents a Single Pole-Single Throw switch. This is the most basic switch.

a SPST (single pole-single throw) switch acts like a drawbridge for electrons. The current cant go through until the switch closes.

a SPST (single pole-single throw) switch acts like a drawbridge for electrons. The current cant go through until the switch closes.

 
Industrial control panel with switches

Industrial control panel with switches


 

5. Protection Device

Fuses and Circuit Breakers are used to protect electrical circuits from dangerous amount of energy. If a component receives more energy than it is rated for, it will likely be damaged. However these protection devices are not meant to protect the technician or user. Fuses and breakers are in place to protect the wiring and the loads in the circuit.

The electrical wiring in your house is protected at the breaker box or service panel. If you’ve ever had to go flip a breaker, that’s because part of your house had too much current flowing. In winter this can be common when running space heaters.

A—General type fuse. B—Slow blow fuse. C—A fuse symbol representing a 1/2 amp fuse.

A—General type fuse. B—Slow blow fuse. C—A fuse symbol representing a 1/2 amp fuse.