The Source

The source is a device, which supplies electrical energy to a circuit.

• Sources are categorized as being one of two types:
  • voltage sources or current sources.
    
    
• The voltage of a voltage source is specified, but the current is determined by the rest of the circuit.
  
  
• The source may consist of:
  • a simple dry cell (as in a flashlight)
  • a storage battery (as in an automobile)
  • or alternating current (AC) power supply (such as a socket outlet at homes and offices).
    

Reference Point

A reference point is an arbitrarily chosen point to which all other points in the circuit are compared (see Figure 7.1).

• Any point in a circuit may be considered as a reference and the electrical potential at all other points may be determined in reference to that point.

• The reference point of a circuit is always considered to be at zero potential.

• Since the earth (ground) is said to be at a zero potential, the term ground is used to denote a common electrical point of zero potential.

Open Circuit

A circuit element whose resistance approaches infinity (R = ?) is called an open circuit and accordingly no current can flow in the circuit (see Figure 7.2). This may happen when a break exists in a closed loop. Although an open circuit occurs when a switch is used to de-energize a circuit, an open circuit may also develop accidentally. To restore a circuit to proper operation, the open circuit must be located, its cause determined, and repairs made.

Occasionally an open circuit may be located visually by a close inspection of the circuit components. Defective components, such as burned out resistors, may usually be discovered by this method. Others, such as a break in wire covered by insulation or the melted element of an enclosed fuse, are not visible to the eye.

Short Circuit

Another type of circuit is the short circuit. This is a circuit that has very little or no resistance to limit the flow of current. According the voltage across the circuit or part of the circuit will approach zero, regardless of the current flowing through it. A short circuit generally occurs when the conductors leading from and back to the power source become connected (see Figure 7.2).

Grounded Circuit

Another type of circuit is the grounded circuit which may cause an excessive amount of current flow just as a short circuit does. Grounded circuits occur when a path other than the intended, is established to ground. Many circuits contain an extra conductor called the grounding conductor, which provides the return path and completes the circuit back to the power source. The grounding conductor is generally connected to the case of the appliance to provide a low resistance path to ground.