Electric Circuits

• Simple Circuits

• Resistance, Voltage & Current

• Series & Parallel Circuits (Defined)

• The Resistor Colour Code

• Circuit Diagram Symbols

• Voltmeters and ammeters

• Batteries (Cells); series and parallel

A. Circuit fundamentals

A circuit is a closed path so that electrons (current) can easily move from the voltage source to the load and back to the source.

The path of a circuit is generally made up of conductors (wires, metal strips--on a printed circuit board for example, etc..)

Conventional current flow is defined from the positive terminal to the negative terminal when a circuit is connected to a voltage source.  Within the voltage source,  in a battery for example, electron current flow takes place from the negative terminal to the positive terminal.
 

A circuit must have four key elements:

1. A voltage source

2. A load

3. A complete conductive path

4. A switch

Current is (how fast) the measure of the flow of electricity in a circuit and it is measured in Amps.  “Amps” is a short form for Amperes.  The symbol for Current is “I”.   One Amp is one Coulomb per Second.

Voltage is the force in a voltage source and is measured in VOLTS.  The symbol for voltage is “V”.  An example of a voltage source is a battery. Another name for voltage is Electric Potential (also known as EMF Electromotive Force).  A Voltage source will cause a voltage rise in a circuit.  Whereas a load will cause a voltage drop.

Loads can be generally defined as the resistance to the flow of current in a circuit.

Resistance is the opposition to current flow.  The symbol for resistance is “R”.    A resistor is a small electronic component used to add resistance (or load) in a circuit.   Resistor values are given in ohms (W - this is the symbol for Greek letter "omega").

B. The Resistor colour code

• The code is a convenient way to tell the resistance of a resistor.

• (It is also used for other components such as capacitors)

•  Coloured bands are placed on a resistor.  Each band has a numerical value.

•  Bands are read from left to right. 

The last band (usually separated from the others) = the tolerance (how accurate the R value is)

 Resistors colour codes - The following table should be memorized

The Resistor Colour Code

EXAMPLE:  What is the value of resistance of a resistor with the following coloured bands:

Yellow,  green,  red,  gold

Solution
Yellow = 4   Therefore, the first number = 4
Green  = 5   Therefore, the second number = 5
Red (in the third band is  a multiplier) Therefore Red = X100

Therefore,  THE VALUE OF THE RESISTOR IS 45x100 = 4,500  ohms   OR   4.5k W (4.5 KILO-OHMS).

Since the last band is gold, the accuracy of this resistor is ± 5%. 

This means that it can have a value that can range from a high value of :
 [4500 + 5%*(4500)] = [4500 + 225] = 4725 W  to a low value of       [4500 – 5%*(4500) ] = [4500  - 225] = 4275 W

Types of Circuits

There are basically two types of circuits:

• Series Circuits
• Parallel Circuits

 Series Circuits
In series circuits there is only one path for current flow. This is an example of series circuit

	Equivalent circuit diagram
One single path of current.  Both lights are ON.
.	When one light bulb is removed, the other light goes off too.  Why? By taking out the other light bulb, the path of current was broken.  The circuit is no longer complete.

   Parallel circuits
In parallel circuits there may be two or more paths for current flow. This is an example of a parallel circuit

	Two separate paths of current.  Both lights are ON.	Equivalent circuit diagram
	One light bulb was taken out, but the other stays on.  Why? The two paths of current are independent from each other.  Even though one path is interrupted the other is still closed.

Standard Electric Circuit Diagram Symbols
(Some of the most common components)

We use standard symbols to draw electronic circuits or schematics. Here are some of the standard symbols used.

Component	Symbol
Light Bulb
Resistor
Battery	Note: The longer wire indicates the positive terminal
Diode	allows current to flow in one direction only
LED	Light Emitting Diode
Switches
Wire
Voltmeter(short form of Voltage-meter)	measures voltage in a circuit
Ammeter (short form of Amp-meter)	measures current in a circuit
Ohmmeter	measures resistance in a circuit

Essential Rules to Connect Meters in a Circuit

When connecting a meter in a circuit we must observe some very important rules for safety reasons and to prevent damage to the meter or the circuit.

1. Always observe the polarity of a meter - positive to positive, negative to negative

2. Voltmeters are connected across a load to measure its voltage drop

3. Voltmeters are connected across a voltage source to measure its voltage rise

4. Ammeters are connected in series in a circuit to measure current flow

5. Ohmmeters are connected in a circuit  when the power supply of the circuit is OFF

Example of a circuit with properly connected meters

In this circuit you can see the voltmeter measuring the voltage rise across the battery and the ammeter measuring the current coming out of the battery. Note the polarity of the meters. Also note that the voltmeter is across the source (in parallel with the source or battery); the ammeter is in series with the battery and the load.

Batteries & Cells

REVIEW EXERCISES

A. Find the resistance for the following resistors and state the high/low range for each.
1. brown, black, red, gold 
2. orange, yellow, red, silver 
3. blue, black, green, silver

B.  Draw a circuit diagram, using proper symbols to show the following

1. A series circuit with a battery, an ammeter, and a light bulb

2. A parallel circuit with a battery, a voltmeter, an a light bulb

  Back Home