Series vs. Parallel Circuits

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Introduction: Why Circuits Matter

Electricity is the invisible train delivering energy where it needs to go but the “tracks” (circuits) come in different layouts.
Two of the most common are series and parallel. Understanding them is essential because:
  • They determine how much voltage and current each component gets.
  • They affect what happens if one component fails.
  • They’re used everywhere, from Christmas lights to your phone’s battery management.

Series Circuit

Components connected end-to-end so there’s only one path for current. The same current flows through each component, but the supply voltage is divided among them. It's like a conga line — if one person stops, everyone stops. One failure stops the whole circuit.

Simple circuit diagram:

Below is a simple series circuit diagram of a circuit you can make yourself
Series Example Calculation:

Given:
  • Supply voltage: 9V
  • Red LED forward voltage: ~2V each
  • Desired LED current: 20mA (0.02A)

Step 1: 

To reach the desired current flow in the circuit, we must calculate the total LED voltage drop by adding the voltage drop of each LED together.
Total LED voltage drop = 2V+2V+2V 
Total LED voltage drop = 6V

Step 2: 

Next, calculate the remaining voltage after the LEDs.
Voltage left for resistor = Supply Voltage - Total LED voltage drop 
Voltage left for resistor = 9V−6V = 3V

Step 3: Using Ohm’s Law:

To get the desired current to 20mA we must use Ohm's law to calculate the resistor required to match our voltage demands.
Where V = Voltage, I = Current and R = Resistance
R= V/I
R= 3/0.02 = 150Ω
So, one 150Ω resistor will set the current close to 20mA.

Step 4: Breadboard Example

Using a breadboard we can create the series circuit and measure the voltages across each led and resistor to confirm our calculated values. The voltage drop can be measured using a multimeter set to measure voltage in the appropriate range.
The pictures below show the circuit we have created and where to measure.


Step 5: Total resistance

In series the total resistance of the circuit can be calculated by adding the sum of all the resistances together.
Rt = R1+R2+R3+R4 e.t.c

In our example the total resistance should be the sum of all resistances of the LEDs and the 150Ω resistor together.
We can calculate the approximate resistance of one LED by R = 2/0.02 = 100Ω
Therefore the total resistance in the circuit can be calculated by: -
Rt = 100+100+100+150 =
Rt = 450Ω
Because LEDs are diodes and do not have a fixed resistance, we cannot measure their resistance directly with a multimeter. For demonstration purposes, replace each LED with a 100 Ω resistor to show how to measure total resistance in the circuit.
You can measure this in the circuit using the multimeter set to measure resistance in the appropriate range. The power should be turned off and disconnected before measuring. The picture below shows how to measure and the measurement value obtained: -

Parallel Circuit

Components connected across the same two points, so each has its own path to the power supply. Each component gets the full supply voltage, but the total current is split between each branch. Like multiple water slides from the same pool one blocked slide doesn’t stop the others.

Simple circuit diagram:


Parallel Example Calculation
Given:
  • Supply voltage: 9V
  • LED forward voltage: ~2V
  • Desired LED current: 20mA (0.02A) per LED
Step 1:
First, find the voltage across each resistor by subtracting the LED’s forward voltage from the battery voltage
Voltage across resistor for each LED = 9V−2V=7V
Step 2:
To get the desired current to 20mA we must use Ohm's law to calculate the resistor required to match our voltage demands.
Using Ohm’s Law:
Closest standard values: 330Ω (slightly brighter) or 360Ω (slightly dimmer).
Each LED has its own resistor of this value.
Step 3: Breadboard Example
Using a breadboard we can create the parallel circuit and measure the voltages across each led and resistor to confirm our calculated values. We can measure this voltage drop by using a multimeter set to read the appropriate voltage range.
The picture below shows the circuit we have created and where to measure the voltages across each LED.


Step 4: Total Resistance
In parallel circuits the total resistance is calculated by the inverse sum of all the resistances in each branch added together: -
1/Rt = 1/Ra + 1/Rb + 1/Rc
In our example each branch has the same resistance Ra = Rb = Rc
Ra can be found by using the previously calculated approximate resistance value for the LED and adding the resistor in the branch to it. Each branch is effectively a small series circuit made of an LED and its resistor: -
Ra = 100+ 350
Ra = 450Ω

So the total resistance can be found by: -
1/Rt = 1/450 + 1/450 + 1/450

1/Rt = 3/450
3/450 is the inverse of the total resistance so we can re-arrange the formula to: -
Rt = 450/3
Rt = 150Ω
Because LEDs are diodes and do not have a fixed resistance, we cannot measure their resistance directly with a multimeter. For demonstration purposes, replace each LED with a 100 Ω resistor to show how to measure total resistance in the circuit.
You can measure this in the circuit using the multimeter set to read resistance in the appropriate range. The power should be turned off and disconnected before measuring. The picture below shows how to measure and the measurement value obtained: -

Key takeaway:

In series, components share voltage; in parallel, they share current but each gets full voltage.

Conclusion

Understanding series vs. parallel is the foundation for all circuit design whether you’re wiring LEDs for a project or designing the next electric car battery pack.

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