DC Circuits in PE Power Exam

Welcome to the study guide on DC Circuits in the PE Power exam. In this comprehensive guide, we will explore the essential concepts and analysis techniques of DC circuits, focusing on their crucial role in power engineering.

As emphasized in NCEES® exam guidelines for the PE Power exam, understanding DC circuits in the PE Power exam is paramount for success. 

DC circuits are building blocks for power distribution systems, electronic devices, and control circuits, making them a fundamental aspect of power engineering. By mastering the principles and calculations involved in DC circuits, you will gain the necessary skills to tackle complex engineering problems and excel in your Power exam and career. 

Let’s dive into DC circuits and learn how to achieve success with your preparation for DC circuits in the PE Power exam.

Fundamentals of DC Circuits

fundamentals of dc circuits

Direct Current or DC Circuit in the PE Power exam involves studying an electric circuit where the current flows in one direction only, maintaining a constant polarity.

In a DC circuit, the voltage remains steady over time and does not change direction periodically. Familiar sources of DC include batteries and DC power supplies. The critical characteristics of DC circuits are as follows:

Let’s discuss more critical aspects of DC circuits.

DC vs. AC circuits – Waveform Difference

dc vs ac circuits

The primary difference between DC and AC circuits lies in the type of waveform each circuit carries.

  • DC circuits have a constant voltage and current, representing a straight horizontal line on a graph, indicating no change over time.
  • AC circuits, on the other hand, have alternating voltage and current, which periodically changes direction, represented as sinusoidal waves.

DC vs. AC circuits – Source Difference

difference between dc and ac lies in the source

The other crucial difference between DC and AC lies in the source, the nature of the electric current they produce, and the behavior of the voltage over time.

DC Source (Direct Current)

  • Unidirectional Current Flow – A DC source generates a constant electric current that flows in a single direction.
  • Steady Voltage Polarity – The voltage from a DC source remains constant (positive or negative) over time.
  • No Periodic Reversals – The direction of the current remains unchanged, and there are no periodic reversals or oscillations.
  • Battery and DC Power Supplies – Batteries or DC power supplies often represent DC sources.

AC Source (Alternating Current)

  • Bidirectional Current Flow – An AC source produces an electric current that alternates its direction periodically, continuously reversing its flow.
  • Varying Voltage Polarity – The voltage from an AC source fluctuates between positive and negative values over time, creating a sinusoidal waveform.
  • Periodic Reversals – The current changes direction periodically, typically at a fixed frequency (e.g., 50 Hz or 60 Hz).
  • Power Grid and Generators – AC sources are commonly used in power grids to distribute electrical energy and are generated by alternators in power plants.

The primary technical difference between DC and AC sources lies in how the electric charge moves and the behavior of the voltage waveform. In a DC source, electrons flow in one direction through the circuit, maintaining a constant voltage polarity.

On the other hand, in an AC source, electrons periodically oscillate back and forth, changing the voltage polarity and creating an alternating waveform.

The difference in the behavior of DC and AC sources has significant implications for their applications. DC sources are commonly used in electronics, battery-powered devices, and certain specialized applications requiring constant voltage.

Due to their periodic reversals, AC sources are ideal for transmitting electrical power over long distances efficiently and running motors, transformers, and most household and industrial electrical devices.

Ohm’s Law and Kirchhoff’s Laws in DC Circuits

Studying Ohm’s and Kirchhoff’s laws is vital to crack problems of DC circuits in the PE Power exam. Let’s have a look at both.

Ohm’s Law

Ohm’s Law relates voltage (V), current (I), and resistance (R) in a DC circuit and can be expressed as:

V = I R

Where:

  • V = Voltage across the resistor (in volts)
  • I = Current flowing through the resistor (in amperes)
  • R = Resistance of the resistor (in ohms)

Let’s consider a resistor of 10 ohms. If a current of 2 amperes flows through it, then the voltage across the resistor would be:

V = 2 A * 10 Ω

V = 20 V

Kirchhoff’s Laws

Kruchhoff’s laws are divided into two different approaches for current and voltage.

  • Kirchhoff’s Current Law (KCL) – The total current entering a junction (or a node) in a DC circuit equals the total current leaving that junction. It’s based on the principle of conservation of charge.
  • Kirchhoff’s Voltage Law (KVL) –  In a closed loop of a DC circuit, the sum of all voltage drops equals the sum of all voltage rises. It’s based on the principle of conservation of energy.

Series and Parallel Connections in DC Circuits

Series Connection

In a series connection of resistors, they are connected end-to-end, creating a single path for the current to flow through all resistors. 

  • The current remains the same across all resistors, but the total resistance is the sum of individual resistances.
  • Total resistance in series (R_series): R_series = R1 + R2 + R3
  • The current in the circuit (I_series) remains the same for all resistors.

Consider three resistors with resistances R1 = 5 ohms, R2 = 10 ohms, and R3 = 15 ohms in series. Let’s calculate the total resistance and current flowing through the circuit when connected to a 12V DC source.

R_series = R1 + R2 + R3 = 5 Ω + 10 Ω + 15 Ω = 30 Ω

Using Ohm’s Law, I_series = V / R_series = 12V / 30Ω = 0.4 A (or 400 mA)

Parallel Connection

A parallel connection of resistors is connected across the same two points, creating multiple paths for the current. 

  • The voltage across all resistors remains the same, but the total resistance is calculated differently.
  • Total resistance in parallel combination (R_parallel): 1/R_parallel = 1/R1 + 1/R2 + 1/R3
  • The voltage across the resistors is the same (V_parallel), but the current varies for each resistor.

Consider three resistors with resistances R1 = 5 ohms, R2 = 10 ohms, and R3 = 15 ohms in parallel. Let’s calculate the equivalent resistance:

To find the equivalent resistance (R_parallel) for the first three resistors in parallel:

1/R_parallel = 1/R1 + 1/R2 + 1/R3 = 1/5Ω + 1/10Ω + 1/15Ω = (6 + 3 + 2)/30 = 11/30

R_parallel = 30/11 ≈ 2.73 Ω

Consider a circuit having 5 resistors of 6Ω connected to a DC voltage source of 120V. How can we solve this circuit if three resistors are connected in parallel, and their parallel combination is attached in series with the other 2 resistors?

Solving a circuit means calculating the voltages and currents passing through each load or resistor. Let’s see how.

Step 1: Find the equivalent of three resistors connected in parallel.

To find the equivalent resistance (R’) for the three resistors in parallel:

1/R’ = 1/R1 + 1/R2 + 1/R3

Since, R1 = R2 = R3

1/R’ = 1/R + 1/R + 1/R = 3/R

R’ = R/3 = 6Ω/3 = 2Ω

Step 2: Combine the parallel combination with the other two resistors in series.

Now, we have the equivalent resistance (R’) from Step 1 and two more resistors in series. Let’s calculate the equivalent resistance for this series combination.

To find the total equivalent resistance (R) for the series combination of parallel resistors and the two additional resistors in the series:

R = R’ + R4 + R5 = 2Ω + 6Ω + 6Ω = 14Ω

Step 3: Calculate the current flowing through the circuit.

Using Ohm’s Law, we can find the current (I) flowing through the circuit:

I = V / R_total

I = 120V / R_series_total

I = 120V / 14Ω

I ≈ 8.571A (approximately 8.57 A)

Step 4: Calculate the current through each resistor in the first parallel combination.

Since the resistors in the last combination are connected in series, they all have the same current across them but different voltages.

So,

I’ = I4 = I5 = 8.57 A

Since the combination R’ is a parallel combination, all the voltages connected in parallel have the same voltage as R’.

So,

By V = IR

V’ = I’R’

V’ = (8.57)(2) = 16.54 A

Now

V1 = V2 = V3 = V’ = 16.54 A

Now by Ohm’s law, I = V/R

I1 = V1/R1

I1 = 16.54/6 = 2.76 A

Similarly,

I2 = V2/R2

I2 = 16.54/6 = 2.76 A

And,

I3 = V3/R3

I3 = 16.54/6 = 2.76 A

Now we have,

Total Equivalent Resistance, R = 14Ω

I1 = I2 = I3 = 2.76 A

I3 = I4 = 8.56 A

Power and Energy in DC Circuits (Watt’s Law)

The power and energy of DC circuits in the PE Power exam are calculated using Watt’s law. Let’s have a look at it.

Power (P) in DC Circuits

Power in a DC circuit is the rate at which energy is consumed or supplied by the circuit components. It can be calculated using Watt’s Law:

P = V I

Where:

  • P = Power (in watts)
  • V = Voltage (in volts)
  • I = Current (in amperes)

By substituting values of V and I from Ohm’s law, we get two more formulas for power, including:

P = I^2 * R and P =V^2 / R

Energy (E) in DC Circuits

Energy consumed or supplied by a circuit can be calculated by multiplying the power (P) by the time (t) for which the circuit is active:

E = P * t

Where:

  • E = Energy (in watt-hours)
  • P = Power (in watts)
  • t = Time (in hours)

Power Dissipation in DC Circuits

In DC circuits, power dissipation refers to the energy lost as heat due to the resistance of components like resistors. When current flows through a resistor, it encounters resistance, resulting in power dissipation in the form of heat.

The power dissipated by a resistor can be calculated using the abovementioned formulas as P = V I or  P = I^2 * R or P = V^2 / R.

Interconversion of Energy

The interconversion of energy in DC circuits is based on the relationship between power (P) and energy (E) mentioned above. Energy can be obtained by multiplying the power by time (E = P * t), and power can be determined by dividing energy by time (P = E / t).

For example, if a device consumes 100 watts of power and is used for 2 hours, the energy consumed by the device is:

E = 100 watts * 2 hours = 200 watt-hours.

Similarly, if a battery provides 500 watt-hours of energy to power a device, and the device is used for 5 hours, then the average power consumption would be:

P = 500 watt-hours / 5 hours = 100 watts.

This interconversion between power and energy is essential for understanding electrical devices’ and circuits’ efficiency and usage.

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DC Circuit Analysis Techniques

This critical topic of DC circuits in the PE power exam is critical due to its depth of detail and significance. Let’s look at some valuable techniques used for DC circuit analysis in the PE Power exam and field.

Mesh Analysis in DC Circuits

Mesh analysis is a circuit analysis technique based on Kirchhoff’s Voltage Law (KVL) used to determine currents in different loops (meshes) of the circuit.

The steps involved in mesh analysis are:

  • Assign a current variable to each loop (mesh) in the circuit.
  • Apply KVL to each mesh, considering the voltage drops and rises around the loop.
  • Form a system of linear equations using KVL equations.
  • Solve the equations to find the values of the mesh currents.

Nodal Analysis in DC Circuits

Nodal analysis is a circuit analysis technique based on Kirchhoff’s Current Law (KCL) used to determine node voltages.

The steps involved in nodal analysis are:

  • Identify and label the nodes in the circuit.
  • Assign a voltage variable to each node.
  • Apply KCL at each node, summing up the currents entering and leaving the node.
  • Form a system of linear equations using KCL equations.
  • Solve the equations to find the values of the node voltages.

Learn more about Mesh and Nodal Analysis.

Thevenin and Norton Equivalent Circuits in DC Systems

Thevenin and Norton’s equivalent circuits are simplification techniques used to replace a portion of a complex circuit with an equivalent single voltage source (Thevenin) or a current source (Norton) with an associated equivalent resistance.

Thevenin Equivalent Circuit

  • Find the open-circuit voltage (V_th) across the load terminals when the load is removed from the original circuit.
  • Find the Thevenin resistance (R_th) as seen from the load terminals when all voltage sources are replaced with short circuits and all current sources are replaced with open circuits.
  • The Thevenin equivalent circuit has a series voltage source (V_th) with a resistor (R_th).

Norton Equivalent Circuit

  • Find the short-circuit current (I_N) flowing through the load terminals when the load is replaced with a short circuit.
  • Find the Norton resistance (R_N) as seen from the load terminals when all voltage sources are replaced with short circuits and all current sources are replaced with open circuits.
  • The Norton equivalent circuit has a current source (I_N) parallel to a resistor (R_N).

Both Thevenin and Norton equivalent circuits provide a simplified representation of a portion of a complex circuit, making analysis easier when dealing with more extensive networks or when connecting various loads to the circuit. Learn in more detail about Thevenin and Norton Equivalent Circuits.

DC Circuit Components

Resistors (R), capacitors (C), inductors (L), and voltage source (battery or power supply) constitute a DC Circuit. The DC circuits carrying all these components are called RLC circuits.

Resistors

Resistors are passive electrical components that resist the flow of electric current. They are commonly used to control current flow, divide voltage, and limit current in electronic circuits. The resistance of a resistor is measured in ohms (Ω) and determines how much it resists the current flow.

Impact of Adding Resistors to DC Circuit

  • Adding resistors in series increases the total resistance in the circuit. The total resistance in series is the sum of individual resistances (R_total = R1 + R2 + …).
  • Adding resistors in parallel decreases the total resistance in the circuit. The total resistance in parallel can be calculated using the formula: 1/R_total = 1/R1 + 1/R2 + …

Capacitors

Capacitors are passive components that store electrical charge and are commonly used in filtering, smoothing, and energy storage applications. Capacitance measures a capacitor’s ability to store electric charge. It is measured in farads (F).

Impact of Adding Capacitors to DC Circuit

  • Adding capacitors in parallel increases the total capacitance in the circuit. Total capacitance in parallel is the sum of individual capacitances (C_total = C1 + C2 + …).
  • Adding capacitors in series decreases the total capacitance in the circuit. Total capacitance in series can be calculated using the formula: 1/C_total = 1/C1 + 1/C2 + …

Inductors

Inductors are passive components that store energy as a magnetic field when current flows through them. They are commonly used in energy storage, filtering, and circuit protection. Inductance is the measure of an inductor’s ability to store energy, measured in henrys (H).

Impact of Adding Inductors to DC Circuit

  • Adding inductors in series increases the total inductance in the circuit. Total inductance in series is the sum of individual inductances (L_total = L1 + L2 + …).
  • Adding inductors in parallel decreases the total inductance in the circuit. Total inductance in parallel can be calculated using the formula: 1/L_total = 1/L1 + 1/L2 + …

DC Electricity Source

Batteries and power supplies are sources of electrical energy in DC circuits. They provide a potential difference (voltage) that drives the current flow through the circuit.

Voltage or EMF of Voltage Source

Voltage (V) is the potential difference between two points in a circuit. It represents the energy per unit charge available to move charges from one point to another. The voltage or potential difference is specifically used to denote the power supply’s potential that tends to pass the current across different loads.

Electromotive Force (EMF) is the total energy per unit charge delivered by a source, specifically a battery. EMF is the maximum voltage that a source can provide when no current is flowing.

When a current flows through a source, there might be a voltage drop due to internal resistance or other factors, which reduces the effective voltage.

Mathematical Notation for DC Circuit Components

  • Resistors: Resistance (R) measured in ohms (Ω).
  • Capacitors: Capacitance (C) measured in farads (F).
  • Inductors: Inductance (L) measured in henrys (H).
  • Batteries and Power Supplies: Voltage (V) or Electromotive Force (EMF) measured in volts (V).

Circuit Theorems and Laws in DC Circuits

Circuit theorems and laws help solve and analyze DC circuits in the PE Power exam. Let’s look at some valuable theorems and laws formulated to ease the complex process of solving and analyzing DC circuits.

Superposition Theorem in DC Circuits

The Superposition Theorem states that in a linear circuit with multiple independent sources, the response (voltage or current) at any point in the course can be calculated by considering the individual effects of each source, one at a time, with all other sources, turned off (replaced with their internal resistances).

For a circuit with multiple independent sources, the total response (voltage or current) is the algebraic sum of the responses due to each individual source acting alone. When analyzing the effect of one source, all other sources are treated as short circuits for voltage sources or open circuits for current sources.

*You can use the Superposition Theorem when analyzing circuits with multiple independent sources. This theorem simplifies circuit analysis by breaking down the effects of each source individually and then summing the responses.

Millman’s Theorem in DC Circuits

Millman’s Theorem finds the voltage at a particular point in a circuit with multiple parallel voltage sources. It states that the voltage at a node where several voltage sources are connected in parallel can be calculated by dividing the sum of the products of individual source voltages and their respective conductances (reciprocal of resistance) by the sum of the conductances.

For a circuit with ‘n’ parallel voltage sources (V1, V2, …, Vn) and resistances (R1, R2, …, Rn) connected to a common node, the voltage (V) at that node is given by:

V = (V1/R1 + V2/R2 + … + Vn/Rn) / (1/R1 + 1/R2 + … + 1/Rn)

*You can apply Millman’s Theorem when analyzing circuits with multiple parallel voltage sources connected to a common node. This theorem helps to find the voltage at the common node efficiently.

Maximum Power Transfer Theorem in DC Circuits

The Maximum Power Transfer Theorem states that the maximum power is transferred from a source to a load when the load resistance is equal to the source’s internal resistance.

For maximum power transfer, the load resistance should be matched to the source’s internal resistance.

For a DC circuit with a source having an internal resistance (R_source) and a load resistance (R_load), the maximum power (P_max) transferred to the load occurs when the load resistance is equal to the internal resistance of the source (R_load = R_source). The maximum power is given by:

P_max = (V_source^2 / 4 * R_source)

Where:

V_source = Voltage of the source.

*You can use the Maximum Power Transfer Theorem when designing or optimizing a DC circuit to ensure maximum power is delivered to the load. This theorem is commonly used in power systems and audio amplifier design.

Remember to identify which theorem or law to use by considering the specific type of circuit and the nature of the sources and loads involved.

Choose the theorem or law that simplifies the analysis, optimizes power delivery, or provides an efficient solution to the problem at hand.

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Safety Considerations in DC Circuits

Adhering to proper safety considerations in DC circuits is crucial to ensure the safety of personnel, prevent equipment damage, and maintain the electrical system’s reliability.

Neglecting these considerations can lead to severe consequences, including hazards, equipment failure, and financial losses.

1. Grounding and Bonding

Grounding and bonding are critical safety measures in DC systems to protect equipment and personnel from electrical hazards. Proper grounding provides a low-impedance path for fault currents to flow safely to the ground, preventing electric shocks and minimizing potential damage. 

Bonding, conversely, ensures all metallic components of a system are interconnected to prevent voltage differences that could lead to electrical shocks or fires.

Use-case

Consider a metal chassis accidentally charges in a DC power distribution system due to an insulation failure. The chassis may pose an electric shock hazard without proper grounding and bonding.

However, if the system is correctly grounded, any fault current will be directed safely to the ground, preventing harm to users and equipment.

Consequences of Neglecting Grounding and Bonding

  • Electric Shock Hazard – Electrical faults can energize conductive surfaces without proper grounding and pose electric shock hazards to personnel.
  • Equipment Damage – Lack of grounding can cause excessive voltage potential between components, leading to equipment damage due to electrical arcing or insulation breakdown.
  • Fire Hazard – Without bonding, voltage differences between metallic components may cause arcing or spark discharges, leading to potential fires.

Learn more about Grounding Resistance Testing Methods and discover how to implement the Equally-spaced 4-pin Method, Unequally spaced 4-pin Method, and Driven Rod Method for ground testing.

2. Ensure Overcurrent Protection

Overcurrent protection is crucial in DC circuits to prevent excessive currents that may damage components or cause hazardous conditions. Overcurrent protection devices, such as fuses and circuit breakers, interrupt the current flow when it exceeds safe levels, protecting the circuit and connected devices.

Use-case

In a DC circuit with sensitive electronic components, a sudden surge in current may lead to component burnout. The excessive current is interrupted by incorporating an appropriate overcurrent protection device, like a fuse, protecting the components from damage.

Consequences of Neglecting Overcurrent Protection

  • Component Damage – Excessive current can cause components to overheat and fail, leading to costly repairs or replacements.
  • Fire Hazard – Uncontrolled overcurrent conditions may generate excessive heat, increasing fire risk.
  • Circuit Failure – Unprotected circuits may experience permanent damage or complete failure due to excessive currents.

Learn more about fault current calculations to get an in-depth insight into this critical topic.

3. Employee Best Practices as per Industry Regulations and Guidelines

Adhering to electrical safety practices is essential to minimize risks and ensure the well-being of personnel working with DC circuits. Safety practices include using proper personal protective equipment (PPE), de-energizing circuits before maintenance, and following lockout/tagout procedures.

Moreover, adhering to industry standards, codes, and compliances is paramount in DC circuits and technical systems to ensure safety, reliability, and interoperability. Experts and organizations set and follow industry standards and codes to define best practices, design guidelines, and safety requirements for specific technologies or applications.

Compliance with these standards ensures that the DC circuits meet the established quality and safety benchmarks, making them more reliable and reducing the risk of accidents or malfunctions.

Additionally, adherence to electrical codes and regulations set by governing bodies ensures that the circuits meet legal requirements and maintain the safety of personnel, equipment, and property. 

Meeting industry standards and compliances also foster trust among customers, partners, and stakeholders, demonstrating the commitment to quality and safety in the design, installation, and operation of DC circuits.

Use-case

A technician working on a live DC circuit without wearing appropriate PPE may be at risk of electric shock if a fault occurs. The technician would wear insulated gloves and other PPE by following safety practices, significantly reducing the risk of injury.

Consequences of Neglecting Electrical Safety Practices

  • Electric Shock Hazard – Neglecting safety practices may result in electric shock injuries or fatalities.
  • Equipment Damage – Mishandling live circuits without proper precautions can damage sensitive electronic components.
  • Legal Consequences – Failure to follow safety regulations and practices may result in legal liabilities and penalties for the individuals and organizations involved.

Read our detailed guide on codes and standards in the PE Power exam.

4. Conduct Regular Audits and Maintenance

Proper troubleshooting and maintenance practices are essential to identify and rectify issues in DC circuits. Routine inspections, preventive maintenance, and thorough diagnostics are critical to ensure the reliability and safety of DC systems.

Moreover, regular audits are essential in technical systems, including DC circuits, to ensure that all aspects of the system are functioning as intended and to identify any potential issues or deviations from the desired performance. 

Audits involve thorough examinations and evaluations of the system’s components, safety measures, maintenance records, and compliance with industry standards. 

By conducting regular audits, organizations can proactively detect and address issues before they escalate into critical failures, reducing downtime, minimizing repair costs, and enhancing overall system reliability. 

Additionally, audits provide insights into the effectiveness of maintenance practices, allowing organizations to optimize their maintenance strategies and improve the operational efficiency of DC circuits.

Use-case

Consider a DC circuit with intermittent and loose connections that may result in equipment malfunctions. Regular maintenance, including tightening connections and identifying faulty components, would prevent such issues.

Consequences of Neglecting Troubleshooting and Maintenance

  • System Failures – Neglecting maintenance can lead to unexpected system failures, downtime, and potential financial losses.
  • Safety Risks – Undetected faults in DC circuits may pose safety hazards for personnel working with or around the equipment.
  • Reduced Efficiency Energy efficiency has both short and long-term impacts on energy producers, distributors, and consumers. Neglected circuits may operate inefficiently, leading to increased energy consumption and decreased performance.

Check our detailed guide on reliability in the PE Power exam for in-depth reading.

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Conclusion

Understanding and cracking DC circuits in the PE Power exam is crucial. As outlined in the NCEES®exam roadmap and guidelines, DC circuits play a crucial role in various power engineering applications, DC circuit is the must-to-cover topic that creates a foundation of power distribution to electronic control systems.

Explore Study for FE – a recognized and reputable PE preparation platform for comprehensive PE Power exam preparation. With its expert resources and practice materials, Study for FE will guide you through every aspect of the exam, providing the confidence and knowledge needed to succeed as a professional engineer.

wasim-smal

Licensed Professional Engineer in Texas (PE), Florida (PE) and Ontario (P. Eng) with consulting experience in design, commissioning and plant engineering for clients in Energy, Mining and Infrastructure.