Electricity Fundamentals: Chapter 2
Types of Electric Circuits
This module focuses on identifying the different types of circuits and on applying the formulas covered in the previous unit. We will also introduce new electrical laws to allow for calculation in more complex circuits. Skip to quiz!
Series Circuits
Series circuits are any circuit where current has only one path to follow. Series circuits can have multiple power sources, loads, and other components, but only one electrical path.
For series circuits, current must pass through every part of the circuit before returning to its source. If a series circuit is open all components of the circuit stop working. This could be because a switch is open, as shown in this image, or because a component failed.
One common example of a series circuit is the light switch on your wall, which is in-series with the light bulb.
If the switch is used to open the circuit, all electricity stops flowing to the light bulb.
Fuses are another example of a series circuit. Fuses are designed to protect equipment sensitive to overload by stopping the flow of current if it exceeds the rated amount. If more current flows through a fuse than it is rated for, it will break to open the circuit.
The circuits in many of our devices require very precise voltages and currents to work properly. Knowing some of these values means we can calculate the others. For this, we can use Ohm’s Law as discussed earlier, which states that V=I*R, as well as new laws for circuits.
In order to better understand how to solve for variables in circuits, we must first understand Kirchhoff’s Circuit Laws. Kirchhoff established the following 2 fundamental laws for understanding circuits.
Kirchhoff’s Current Law
Kirchhoff’s Voltage Law
Kirchhoff’s Current Law states: “The algebraic sum of currents in a network of conductors meeting at a point is zero.” In other words, the current entering equals the current exiting for any junction or node.
Kirchhoff’s Voltage Law states: The directed sum of the potential differences (voltages) around any closed loop is zero. In other words, if we trace the path of electricity from its start back to its origin, the total voltage in a closed circuit is equal to zero.
We know that the current entering any junction, or node, is equal to the current leaving that junction. Series circuits only have one path for current to flow, meaning that for any given point in a circuit, the current in equals the current out. In a series circuit, the current is the same at all parts of the circuit.
In the example to the right, the current flowing through each voltage source and each resistor is the same and is also equal to the total current for the circuit.
ITotal = IVS1 = IVS2 = IR1 = IR2 = IR3
In any circuit, if we trace the path of electricity back to its origin, the total voltage is equal to zero. For a series circuit, there is only one path for electricity to travel, so the sum of all voltage changes in the loop adds to zero.
Voltage sources facing the same direction add together to increase voltage. Voltage sources facing opposite directions subtract. Resistances cannot increase voltage in any direction, they only subtract. So for our example:
VTotal = VVS1 + VVS2 - VR1 - VR2 - VR3 = 0
It is also useful to know that all resistances in a series circuit, can be simplified to represent a single resistance for that circuit. In other words, the total resistance of a circuit is the sum of all resistances in the circuit.
In the example to the right, the total resistance for the circuit is equal to the sum of all 3 resistances.
RTotal= R1+ R2+ R3
If each resistor has a resistance of 2Ω, the total resistance of the circuit is 6Ω.
RTotal= 2Ω + 2Ω + 2Ω = 6Ω
Parallel Circuits
Parallel circuits are any circuit where current has more than one path to follow. Even if one part of the circuit is open, electric current may still be able to flow through other branches of the circuit.
One common example of a parallel circuit is the wiring in our homes. If a breaker trips in your kitchen, it doesn’t shut off all appliances in the house. Generally it only affects a few of the appliances in your kitchen.
In a parallel circuit, the total current for the circuit equals the sum of current in each parallel branch. In the example to the right, the total current from the batteries equals the sum of current flowing through each resistor.
ITotal = IR1 + IR2 + IR3
When multiple resistors are added in parallel, they decrease the overall circuit resistance. The total resistance in a parallel circuit will always be less than the resistance in any one branch.
1/RTotal = 1/R1 + 1/R2 + 1/R3
In a parallel circuit, voltage is equal in each branch. In the example to the right, if the two batteries produce a total voltage of 12V, then that is how much flows through each parallel branch and also through each resistor.
VTotal = VR1 + VR2 + VR3
Combined Circuits
In a combined or series-parallel circuit, there are elements of both series and parallel circuits. The rules that apply for either series or parallel circuits cannot be applied directly for combined circuits. Instead, the circuits need to be simplified into series and parallel parts. Then the rules can be applied.
Most circuits are combined circuits. Turning off the light in your bathroom may turn off the outlet for a curling iron, but it won’t turn off your refrigerator. Also, your home’s electrical panel can be used to shut off power to parts of your home, without affecting the whole house.
A switch can be used to open the circuit and prevent current flowing through R3. However, current will keep flowing through both R1 and R2 after the switch is opened.
We can visualize combined circuits as a combination of series and parallel circuit rules. In the example to the right, R1 and R2 are in series, as are R1 and R3. These resistances can be added because of series circuit rules. R2 and R3 are parallel instead, so they must use parallel circuit rules.
There are three main types of circuits: series, parallel, and combined circuits. Using values that can be obtained through measurement together with Kirchhoff’s Circuit Laws and Ohm’s Law, we can solve for almost any missing values.
Switches and Loads
This module introduces the basics of electric switches, as well as some of the most common switches. It also defines loads and introduces some of the more common electric loads. Skip to quiz!
Switches
Switches are devices that can open or close an electric circuit, and can be activated at will. Recall that a closed circuit is one where electric current can flow, forming a closed-loop. An open circuit is one where no current can flow.
Given the wide variety of uses that switches can have, there are many ways of classifying them. Two of the most common ways to classify switches are by:
Contacts, or by
Actuators
Describing a switch by its contacts means describing it by how it can be used in a circuit.
Describing a switch by its actuator means describing how the switch is mechanically activated. For example, the switch shown has a toggle actuator and only two contacts.
Switches have conductive pieces called contacts, which connect to the external electrical circuit. Normally open switches have contacts that ‘close’ when activated. Normally closed switches have contacts that ‘open’ when activated.
The variation on number of contacts and possible positions a switch can have are used to describe switch variations. The term ‘pole’ is used to describe the number of contacts a single switch has. ‘Throw’ is the term given to the amount of operating positions of a switch
An SPST switch has only one set of terminals that can be open or closed. An example of this type of switch is a regular light switch, with off/on positions.
A DPST switch has two sets of terminals that are opened or closed by the same mechanism. This is analogous to having two SPST switches controlled by one single mechanism.
An SPDT switch has three contacts, a single input and two outputs. This means that the input can be connected to either of the outputs, but not to both at the same time. One of the outputs will be normally closed, and the other will be normally open.
A DPDT switch has six contacts, two inputs and four outputs. This is analogous to having two SPDT switches controlled by a single mechanism.
Another way to classify switches is by means of their actuator. The actuator is the moving part that applies the force that makes the contacts change position. They can be easily distinguished by how a person activates the switch.
A simple switch that is normally open, and closes the circuit after being pressed. These switches are generally used for ON/OFF functions, or to send signals.
A thermostat is a device that combines a switch and a temperature sensor. The switch opens and closes the heating or cooling circuit, and the sensor tells it when to do so. In this case, a person is not required to provide physical force to activate the switch.
A pressure switch activates an electrical switch when a pre-established fluid pressure is achieved. They are often found in HVAC systems, as part of the operation or safety controls.
Another switch that includes a sensor, flow switches are used to control the flow of liquid and gas through a channel. They activate if the flow of liquid or gas gets either too high or too low.
Loads
Recall that the loads in a circuit are the devices that consume power. They are the reason a circuit is created, since providing them electric power is the objective of a circuit. They take electric power and transform it to another form of useful energy, such as heat, movement, or light.
Loads can be resistive, capacitive, or inductive in nature. They can also be a combination of those three. It is important to understand the size and type of load that will be connected to an electric circuit.
Electric heaters are devices that take advantage of the ability of resistors of heating up. By running a high current through a heating element (resistor) they transform electricity into heat. The most common metal used as the heating element is called nichrome.
Inductive loads are devices made up of inductors. These devices take advantage of the magnetic field created by the inductors to perform a task. Electric motors and transformers are examples of inductive loads.
The term solenoid is used for a coil of wire (inductor) when it is used as an electromagnet. This means converting electrical energy, to magnetic, to mechanical. They are commonly used in mechanical switches, valves, and as the starters on automobiles.
These are lights used to convey messages in displays, and consume very little power. HVAC systems sometimes have them in the control boards to indicate status or troubleshooting codes. These lights come in different colors to transmit a wide variety of messages.
Knowing and understanding how different switches and loads operate is essential to creating and using circuits properly. Switches come in many forms and are very versatile. Loads have three basic components, but can be combined to create a wide array of circuits.
Circuit Components
This module will introduce common circuit components used in electrical installations. These include relays, contactors, transformers, motors, and circuit protection devices. Skip to quiz!
Controllers
Operations such as the closing or opening of switches or contacts are carried out by magnetic controllers. A magnetic controller will automatically perform all intended operations in the proper sequence after the closure of a switch.
Relays
Recall that switches are devices that can connect and disconnect circuits, activated by physical force. If the switch is operated electrically, and not manually, the device is called a relay.
When the first circuit is closed, an electric current flows through the coil in the relay. This coil
creates a magnetic field that attracts or repels the moving portion of the relay. This closes the secondary circuit, which powers the load to which the relay is connected.
As with switches, there are many types of relays that accommodate different purposes. Control relays operate control circuits and light duty loads, or even control the coil of another relay in the case of pilot duty control relays. Fan duty or service duty relays are mostly used to control fan motors.
Relays have throws and poles, just like switches. Recall that ‘pole’ refers to the number of contacts, and ‘throw’ to the number of operating positions. This means relays are also classified as SPST, SPDT, DPST, DPDT, and so on.
The control side of a relay has a voltage rating required to trigger the switch. The contacts side of a relay has a rating for both voltage and current, and should be operated within this rating.
Contactors
Like relays, contactors are electrically control switches to open and close circuits. Unlike relays, they are designed for high current applications, such as above 15 amps.
Recall that normally open switches are those whose contacts are open until they are activated. The contacts on a normally closed switch are closed until activated. Contactors are almost exclusively built to be normally open.
Just as switches and relays, contactors can have different amounts of poles, and one or two throws. Throws are limited to two since there are only two positions available for the mobile part.
Just as with relays, contactors have a voltage rating for their control circuit and a current and voltage rating for the contacts. There are three current ratings:
Resistive rating,
Full load amps (FLA) rating, and
Locked rotor amps (LRA).
Transformers
Transformers are used to transfer electrical power from one circuit to another. The two circuits are not physically connected with wires. They are commonly seen mounted on poles along distribution lines.
Transformers work by using the properties of inductors. Recall inductors create a magnetic
field around them, and that field can induce a voltage in conductors inside them. Transformers have, at minimum, one pair of physically isolated inductors.
The main purpose of a transformer is to transfer electrical energy between two circuits while changing the voltage value. Transformers are highly efficient, so the power transferred between circuits remains essentially the same. This means the amount of current in the secondary circuit changes depending on the voltage change.
Transformers can only work if using alternating current (AC), and not with direct current (DC). Recall that in AC current flows in both directions, while in DC current flows in only one direction.
The two circuits connected to the coils in transformers are named primary and secondary circuits. If the voltage in the secondary circuit is bigger than the voltage in the primary, the transformer is a step-up transformer.
If the voltage in the secondary circuit is smaller than the voltage in the primary, the transformer is a step-down transformer. The number of turns each of the coils have determines the change in voltage between primary and secondary windings.
Multi-tap transformers have several taps on either primary or secondary windings. By choosing different taps, the transformer can be step-up or step-down, providing options according to changing needs.
The high side of a transformer is the one that has the high-voltage and low-current. The low side of a transformer is the one that has the low-voltage and high-current.
Motors
Electrical motors use electrical energy to create mechanical rotational. The attracting and repelling forces of magnets and electromagnets inside the motor cause it to spin.
There is a very wide variety of electric motors, that cover many different needs. One of the
most popular types of motors is the induction motor. Induction motors use the property of induction of current to create a rotational movement.
There are different approaches to get an induction motor to start turning.
A split-phase motor utilizes an extra small winding to provide the initial rotation direction to the motor. A permanent split capacitor (PSC) motor uses a permanently connected capacitor to correctly start.
Electronically commutated (EC) motors are a type of permanent magnet motor that is highly efficient and controllable. The control of the motor is provided by electronic components, making it very accurate and versatile.
CIrcuit Protection Devices
Sometimes electrical circuits can experience excessive amounts of current caused by overloads or short circuits. These currents can damage the electrical equipment, which is why
there are devices to protect them in these cases.
One of the most basic devices for protection, disconnectors are used to ensure that a circuit is completely de-energized. Usually manually controlled by a lever or switch, they physically open the circuit to prevent current flow.
Fuses are devices that burn up if too much current flows through them, opening the circuit and avoiding damage. Circuit breakers can also open a circuit if too much current flows through them. Fuses are a one-time use, while circuit breakers can be reset.
Circuit components come in many different varieties, accomplishing many varied objectives. Most of them are loads, but there are also controllers and protective equipment. They have to be well understood before any attempt to use them.
Question #1: Which type of switch is activated by one mechanism, but opens or closes two sets of contacts?
SPST
DPST
SPDT
DPDT
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Answer: DPST
Double pole, single throw (DPST) switches have two sets of contacts that open or close simultaneously, by the same mechanism.
Question #2: Which of these devices has a switch that is activated depending on temperature?
Push-button
Flow switch
Pressure switch
Thermostat
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Answer: Thermostat
Thermostats react to temperature by opening or closing an electric circuit.
Question #3: Which of these would qualify as an electric load?
A resistor
An inductor and a resistor
A capacitor and an inductor
All of the above
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Answer: All of the above
Any combination of resistors, inductors, and capacitors are electric loads.
Question #4: What do solenoids use to create a mechanical force?
Magnetic field
Electric field
Heat
Resistance
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Answer: Magnetic field
Solenoids create mechanical force from the magnetic field created by the inductor.
Question #5: If a series circuit is open because of a gap in the electrical path, current will stop flowing.
True
False
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Answer: True
True. If the circuit is open in a series-type circuit, current will not flow.
Question #6: Which of Kirchhoff’s Laws tells us that the sum of all voltages in a closed loop is equal to zero?
Kirchhoff’s Current Law
Kirchhoff’s Voltage Law
Kirchhoff’s Law of Electricity
Kirchhoff’s Capacitance Law
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Answer: Kirchhoff’s Voltage Law
Kirchhoff’s Voltage Law is that the sum of all voltages in a closed loop is equal to zero.
Question #7: If two batteries in series produce 3A of current, how much current passes through the 3 resistors in the circuit shown?
0A
0.3A
1A
3A
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Answer: 3A
In a series circuit, if ITotal = 3A, then IVS1, IVS2, IR1, IR2, and IR3 also equal 3A each.
Question #8: If two batteries in series produce 12V, and resistors R1 and R2 each cause a voltage drop of 5V, what is the voltage drop across resistor R3?
0V
2V
5V
12V
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Answer: 2V
0 = 12V-5V-5V-R3
R3=2V
Remember that the voltage in a series circuit must sum to zero.
Question #9: If only one branch of a parallel circuit is open, but another branch is closed, current will stop flowing for the entire circuit.
True
False
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Answer: False
False. As long as there is one closed path for current to flow from the battery, through the circuit, and back to the battery, some electrical current will flow.
Question #10: If a circuit has 3 resistors in parallel with resistances of 6Ω each, and a total voltage of 3V produced by the two batteries, what is the total current for the circuit?
0.5A
1A
1.5A
6A
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Answer: 1.5A
1/RTotal = 1/6 + 1/6 +1/6 =0.5Ω +1/Ω
RTotal = 2Ω
ITotal = VTotal/RTotal = 3/2 = 1.5A
Question #11: If we have found a voltage drop across R1 as 7V and across R2 as 1V, what is the voltage drop across R3?
0V
1V
6V
7V
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Answer: 0V
VR3=0V
As the switch is open, current travels through R2 instead of R3.
No current = no voltage drop.
Question #12: In the diagram, if the switch is closed and we have a voltage drop across R1 as 7V and across R3 as 1V, what is the voltage across R2?
0V
1V
6V
7V
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Answer: 1V
For parallel circuits, we know that the voltage through each branch must be the same, so VR2=VR3=1V
Question #13: How are relays different from a regular switch?
They can stand higher voltage
They are electrically activated
They look better in an installation
They have a better rating
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Answer: They are electrically activated
Relays are electrically activated, instead of manually activated.
Question #13: Which of the following is a type of current rating for a contactor?
Locked Rotor Amps (LRA)
Full Load Amps (FLA)
Resistive Rating
All of the above
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Answer: All of the above
Fahrenheit is the unit of measurement for temperature in the imperial system.
Question #14: What property of a conductor carrying a current allows transformers to work?
Resistance
Hardness
Inductance
Capacitance
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Answer: Inductance
Transformers work because of the principle of induction.
Question #15: If the voltage in the secondary winding of a transformer is bigger than the voltage in the primary, the transformer is a ______ transformer.
Broken
Step-up
Step-down
Moving
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Answer: Step-up
A step-up transformer increases the voltage present in the primary winding.
Question #16: Which of these motors is the most controllable?
Induction motor
EC motor
PSC motor
Split-phase motor
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Answer: EC motor
EC motors are controlled by electronics, making them more controllable than the others.
Question #17: What activates fuses and circuit breakers?
Excessive voltage
Not enough voltage
Excessive current
Not enough current
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Answer: Excessive current
An excess of current running through a circuit activates the protection offered by fuses and circuit breakers.