# What is a Series Circuit?

## Introduction

A series circuit is a type of electrical circuit in which the components are connected end-to-end in a single loop. The same current flows through each element in a series circuit, but the voltage drops across each component can be different. Understanding series circuits is fundamental for analyzing DC and AC networks. This article provides an in-depth overview of series circuit fundamentals, analysis methods, characteristics, applications, and related concepts.

## Series Circuit Basics

A basic series circuit consists of a voltage source, like a battery, connected to two or more electrical elements like resistors, inductors, capacitors, etc. The elements are chained together with wires in a single path for current flow.

Simple series circuit with battery and three resistors

Some key properties of ideal series circuits:

• Single loop – Only one path for current to flow around the circuit.
• Same current – The current is the same at every point due to single path configuration.
• Voltage divider – Total voltage equals the sum of the individual voltage drops.
• Series resistance – Total resistance is the sum of individual resistances.
• Power distribution – Total power from source is distributed across each element.

These concepts allow systematic analysis of series circuits using basic circuit theory principles.

## Series Circuit Analysis

Several important methods are used to analyze series circuits:

### Kirchhoff’s Voltage Law (KVL)

This fundamental law states that the algebraic sum of all voltages in a loop must equal zero. This is applied to find unknown voltage drops:

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`Vs = V1 + V2 + V3 + ... + Vn`

Where Vs is the total source voltage and V1 to Vn are the individual voltage drops across each element.

For example, in a circuit with a 45V battery and three resistors with voltages V1, V2, and V3, KVL gives:

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`45V = V1 + V2 + V3`

If two of the voltages are known, the third can be found by subtracting the known values from the total.

### Voltage Divider Rule

The voltage divider rule is a short-cut method to determine the voltage across an individual element using its resistance relative to the total series resistance:

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`Vx = (Vx/RT) * Vs`

Where Vx is the voltage across element X, RT is the total series resistance, and Vs is the source voltage.

For example, if R2 is 220Ω, and the total series resistance is 1500Ω, with a 120V source, the voltage across R2 is:

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`V2 = (220/1500) * 120V = 18V`

This avoids having to find every intermediate voltage drop.

### Current Calculation

For an ideal series circuit, the current is the same through every element, and can be found from Ohm’s Law:

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`I = Vs / Rt`

Where I is the constant current in Amps, Vs is the source voltage, and Rt is the total resistance from adding all individual resistances.

### Power Ratings

The power dissipated in each element is calculated as:

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`P = I2 * R`

Where P is power in Watts, I is the series current, and R is the element’s resistance.

The total power dissipated is the sum of the individual powers.

## Characteristics of Series Circuits

Beyond the basic principles, series circuits exhibit some key characteristics:

### Current is Constant Everywhere

Due to the single path configuration, current cannot vary within a series circuit. Each passive component must have the same current flowing through it. This makes analysis using a single loop current straightforward.

### Voltage Divider Effect

The total source voltage is divided up across each element proportional to its resistance, according to the voltage divider rule. Elements with lower resistance have larger voltage drops than high resistance components.

For the overall circuit, series resistances simply add together. This holds true even for nonlinear devices like diodes or lamps, when their incremental resistances are added.

### Impedances Add in General Series Connections

When reactive elements like inductors or capacitors are connected in series, their impedances add together rather than just resistances. For example, three impedances Z1, Z2 and Z3 in series have a total impedance of:

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`Ztotal = Z1 + Z2 + Z3`

### Current Leads Voltage in Inductive Series Circuits

In a series L-R circuit, the current leads the voltage across the inductor due to its reactance. The opposite happens with a series C-R circuit, where current lags voltage.

### Parallel Resistances Concept

Any branch resistance in parallel can be reduced to an equivalent series resistance using:

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`Rseries = (R1*R2)/(R1+R2)`

This allows simplification of branches to a single equivalent resistor.

## Simple Series Circuit Examples

### Example 1

Simple series circuit with two resistors

• Vs = 10V
• R1 = 5Ω
• R2 = 15Ω

We can directly analyze this circuit as:

• Total resistance:
• Rt = R1 + R2 = 5Ω + 15Ω = 20Ω
• Circuit current:
• I = Vs/Rt = 10V/20Ω = 0.5A
• Voltage drops:
• V1 = I*R1 = (0.5A)(5Ω) = 2.5V
• V2 = I*R2 = (0.5A)(15Ω) = 7.5V
• Total voltages:
• V1 + V2 = 2.5V + 7.5V = 10V = Vs

Using KVL and Ohm’s law gives the same results, verifying the analysis.

### Example 2

• Vs = 120V
• R1 = 10Ω
• R2 = 30Ω
• R3 = 15Ω
• R4 = 20Ω
• Total resistance:
• Rt = R1 + R2 + R3 + R4 = 10Ω + 30Ω + 15Ω + 20Ω = 75Ω
• Circuit current:
• I = Vs/Rt = 120V/75Ω = 1.6A
• Individual voltage drops:
• V1 = I*R1 = (1.6A)(10Ω) = 16V
• V2 = I*R2 = (1.6A)(30Ω) = 48V
• V3 = I*R3 = (1.6A)(15Ω) = 24V
• V4 = I*R4 = (1.6A)(20Ω) = 32V
• Total voltages:
• V1 + V2 + V3 + V4 = 16V + 48V + 24V + 32V = 120V = Vs

Again, methodical analysis using series concepts matches the expected results.

More complex scenarios involve combined resistance and reactance, or nonlinear elements like diodes:

### Series RLC Circuit

• Analysis involves vector addition of complex impedances
• Impedances must consider both resistive and reactive parts
• Allows determining overall circuit resonance and current

### Series Diode Circuit

• Must consider diode IV curve and nonlinear resistance
• Resistance changes with current flow due to diode conduction voltage
• Allows analyzing diode biasing and turn-on based on series resistance

These advanced tools enable thoroughly analyzing complex series circuits with diverse elements.

## Common Applications of Series Circuits

Some typical applications that leverage series circuits:

### Voltage Dividers

One of the most common uses of series connections is for creating voltage dividers. For example, measuring a high voltage using two resistors in series, where the lower resistor converts the high voltage into a lower measurable value.

### Current Limiting

A resistor or inductor in series can purposely limit the current in part of a circuit. This protects components from excessive currents.

### Impedance Matching

Inserting series inductors or capacitors allows matching the impedance looking into a circuit to the desired source/load impedance for maximum power transfer and efficiency.

### Voltage Regulation

A series linear regulator uses a voltage sensing resistive divider combined with a series pass transistor to maintain a steady DC output voltage even with variations in supply voltage or load current.

### EMI Filtering

Series inductors and capacitors can filter out electromagnetic interference by blocking high frequency noise while allowing lower frequency signals to pass through.

### Signal Coupling

A series capacitor can couple AC signals from one stage to another while blocking DC voltages, allowing simple isolation of AC amplified signals.

## Related Concepts

### Open Circuit

If a break occurs in a series circuit, it becomes an open with no current flow. This is equivalent to a series element with infinite resistance blocking current.

### Short Circuit

When two nodes in a series circuit contact each other, a “short circuit” occurs that bypasses part of the series loop. This often leads to excessive currents and is to be avoided in most cases.

### Parallel Circuits

In contrast to series, parallel circuits provide multiple paths for current flow. Complex networks combine series and parallel connections, which are analyzed with techniques like nodal analysis or mesh current methods.

### Series-Parallel Circuits

Some circuits contain both series and parallel combinations within an overall network. These compound connections can be reduced to simplified series-parallel equivalents for analysis.

### Series Resonance

In series RLC circuits, resonance occurs when the total series impedance is minimized at the resonant frequency. This creates a bandpass filter effect around this frequency.

### Series DC Motors

DC motors have the field and armature windings connected in series. This results in high starting torque since the initial current is limited only by the total resistance.

## Conclusion

In summary, series circuits provide a fundamental topology for analyzing electric networks and understanding key concepts including current, voltage division, resistance addition, and load power distribution. While ideal series connections represent the basics, practical circuits require considering complex impedances, nonlinearities, and combined series-parallel networks. Facility with series circuit techniques forms the foundation for more advanced circuit analysis and design for electronics and power systems.

### What are the main characteristics of an ideal series circuit?

The key characteristics are: single loop topology, same current throughout, total voltage divides across elements, total resistance is the sum of individual resistances, and total power from source distributes among the components.

### Why is current constant at every point in a series circuit?

Due to the single path configuration, charge carriers have no alternative route to flow so the current cannot change within a series circuit. Each element must pass the same current in steady state.

### What happens if one resistor opens in a series circuit?

If one resistor opens, creating infinite resistance, current flow would stop. An open resistor is equivalent to disconnecting that part of the loop, so the circuit becomes open and voltage drops across the remaining components go to zero.

### How do you calculate total resistance in a series circuit?

The total resistance is simply the arithmetic sum of the individual resistances. This holds true for any passive linear resistive elements. For nonlinear components, incremental resistances must be summed at the operating point.

### Why is voltage division important in series circuits?

The voltage divider effect allows finding voltages across individual components from their resistances and the total voltage. This avoids tediously finding voltage drops across each preceding resistor to determine the voltage across a specific element.

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