Understanding the difference between series and parallel circuits is fundamental to working with electricity, whether you are a student, a hobbyist building your first project, or an engineer designing a complex system. These two circuit configurations behave very differently in terms of voltage, current, and how they respond to component failures.
This guide covers the theory, the math, real-world applications, and hands-on examples to help you truly understand series and parallel circuits.
What Is a Series Circuit?
In a series circuit, all components are connected end-to-end in a single path. Current flows from the power source through each component in sequence and back to the source. There is only one path for current to flow.
Think of a series circuit like a single-lane road -- all traffic (current) must pass through every stop (component) along the way. If one stop is blocked, all traffic stops.
Key Properties of Series Circuits
1. Current is the same through all components.
Since there is only one path, every component carries the same current. If 0.5 amps flows through the first resistor, 0.5 amps flows through every other component in the circuit.
2. Voltage divides across components.
The total voltage from the power source is split among the components based on their resistance. Each component "drops" a portion of the total voltage proportional to its resistance relative to the total resistance.
For a series circuit with resistors R1, R2, and R3 powered by voltage V:
- Voltage across R1 = V x (R1 / R_total)
- Voltage across R2 = V x (R2 / R_total)
- Voltage across R3 = V x (R3 / R_total)
The sum of all voltage drops equals the source voltage: V_R1 + V_R2 + V_R3 = V
3. Total resistance is the sum of individual resistances.
R_total = R1 + R2 + R3 + ...
Adding more components in series increases the total resistance, which decreases the current.
4. If one component fails open, the entire circuit stops.
A burned-out bulb in a series string breaks the single current path. This is why old Christmas lights would all go out when one bulb failed.
What Is a Parallel Circuit?
In a parallel circuit, components are connected across each other, providing multiple paths for current to flow. Each component has the same voltage across it, but the current divides among the branches.
Think of a parallel circuit like a multi-lane highway -- traffic (current) splits across the available lanes (branches). If one lane is blocked, traffic continues on the other lanes.
Key Properties of Parallel Circuits
1. Voltage is the same across all branches.
Every component in a parallel circuit sees the full source voltage. If you connect three resistors in parallel across a 12V battery, each resistor has 12V across it.
2. Current divides among branches.
The total current from the source splits among the parallel branches. Each branch carries a current determined by its resistance: I_branch = V / R_branch.
The total current is the sum of all branch currents: I_total = I1 + I2 + I3
3. Total resistance is less than the smallest individual resistance.
For parallel resistors, the total resistance is calculated as:
1/R_total = 1/R1 + 1/R2 + 1/R3 + ...
For two resistors: R_total = (R1 x R2) / (R1 + R2)
Adding more resistors in parallel decreases the total resistance, which increases the total current drawn from the source.
4. If one component fails open, the others continue to operate.
Each branch is independent. A burned-out bulb in a parallel circuit only affects that one branch. This is why modern Christmas lights and all home wiring use parallel circuits.
Side-by-Side Comparison
| Property | Series Circuit | Parallel Circuit |
|---|---|---|
| Current | Same through all components | Divides among branches |
| Voltage | Divides across components | Same across all branches |
| Total Resistance | R_total = R1 + R2 + R3 | 1/R_total = 1/R1 + 1/R2 + 1/R3 |
| Component Failure | Entire circuit stops | Only failed branch stops |
| Adding Components | Increases resistance, decreases current | Decreases resistance, increases current |
Worked Example: Series Circuit
Let us calculate the values for a series circuit with a 12V battery and three resistors: R1 = 100 ohms, R2 = 200 ohms, R3 = 300 ohms.
Total resistance:
R_total = 100 + 200 + 300 = 600 ohms
Current (same through all):
I = V / R_total = 12 / 600 = 0.02A (20 mA)
Voltage drops:
- V_R1 = 0.02 x 100 = 2V
- V_R2 = 0.02 x 200 = 4V
- V_R3 = 0.02 x 300 = 6V
- Total: 2 + 4 + 6 = 12V (matches the source)
Power dissipated:
- P_R1 = 0.02 x 2 = 0.04W
- P_R2 = 0.02 x 4 = 0.08W
- P_R3 = 0.02 x 6 = 0.12W
- Total: 0.24W
Worked Example: Parallel Circuit
Same components, but connected in parallel across a 12V battery.
Total resistance:
1/R_total = 1/100 + 1/200 + 1/300 = 6/600 + 3/600 + 2/600 = 11/600
R_total = 600/11 = 54.5 ohms
Branch currents:
- I_R1 = 12/100 = 0.12A (120 mA)
- I_R2 = 12/200 = 0.06A (60 mA)
- I_R3 = 12/300 = 0.04A (40 mA)
- I_total = 0.12 + 0.06 + 0.04 = 0.22A (220 mA)
Verification:
I_total = V / R_total = 12 / 54.5 = 0.22A (matches)
Voltage across each (same for all):
12V
Power dissipated:
- P_R1 = 12 x 0.12 = 1.44W
- P_R2 = 12 x 0.06 = 0.72W
- P_R3 = 12 x 0.04 = 0.48W
- Total: 2.64W
Notice the parallel circuit draws significantly more power from the same battery because the total resistance is much lower.
Series-Parallel (Combination) Circuits
Real circuits often combine series and parallel elements. For example, two resistors in parallel might be connected in series with a third resistor. To analyze these circuits:
- Identify which groups of components are in series and which are in parallel.
- Calculate the equivalent resistance for each parallel group.
- Add the series resistances together.
- Solve for total current, then work backward to find individual voltages and currents.
Building these circuits in a tool like CircuitDiagramMaker and running the built-in simulator can verify your hand calculations and help you develop intuition for how current and voltage distribute across complex networks.
Real-World Applications
Series Circuit Applications
- Voltage dividers: Use series resistors to create a lower voltage from a higher source (e.g., powering a 3.3V sensor from a 5V Arduino pin).
- Current limiting: A resistor in series with an LED limits the current to a safe level.
- String lights: Some decorative lighting still uses series connections.
- Fuses and circuit breakers: These are always in series with the circuit they protect -- when they "blow," they break the single current path.
Parallel Circuit Applications
- Home electrical wiring: Every outlet and light fixture in your home is wired in parallel. Each device gets the full 120V (or 240V) regardless of how many other devices are on the same circuit.
- USB ports: Multiple USB ports on a hub are wired in parallel, each providing 5V.
- Battery banks: Batteries in parallel increase capacity (amp-hours) while maintaining the same voltage.
- Redundant systems: Critical systems use parallel paths so that a single failure does not take down the entire system.
Batteries in Series vs Parallel
- Series: Voltages add up. Three 1.5V AA batteries in series = 4.5V. Capacity stays the same.
- Parallel: Voltage stays the same. Three 1.5V batteries in parallel = 1.5V. Capacity triples.
- Series-Parallel: Combine both to increase voltage and capacity. Four batteries in a 2S2P configuration (two series pairs in parallel) give 3V at double capacity.
Common Mistakes
1. Shorting a parallel branch. If you accidentally create a zero-resistance path in parallel with a component, all the current flows through the short. This can blow a fuse, damage the power source, or start a fire.
2. Forgetting that parallel resistance is always less than the smallest branch. Students often calculate parallel resistance incorrectly by adding resistances instead of using the reciprocal formula.
3. Overloading a parallel circuit. Adding more devices in parallel draws more total current. If the total current exceeds the wire or fuse rating, you have a problem. This is why home circuits have breakers -- too many appliances on one circuit trips the breaker.
4. Ignoring internal resistance of the power source. Batteries have internal resistance. As you draw more current (by adding parallel loads), the voltage drops due to the internal resistance. This is why a weak battery can light one LED but dims when you add more.
Build and Simulate Your Own Circuit
The best way to internalize series and parallel circuit behavior is to build and test circuits yourself. With CircuitDiagramMaker, you can:
- Drag and drop resistors, batteries, LEDs, and switches onto the canvas
- Connect them in series, parallel, or combination configurations
- Run the built-in SPICE simulator to see real voltage and current values
- Change component values and instantly see the effect
- Compare series vs parallel behavior side by side
Build and simulate your own circuit -- free
Key Takeaways
- In series circuits, current is the same everywhere and voltage divides across components.
- In parallel circuits, voltage is the same across all branches and current divides.
- Series resistance adds up; parallel resistance is always less than the smallest branch.
- A failed component breaks a series circuit entirely but only affects one branch in a parallel circuit.
- Home wiring, USB ports, and battery banks are parallel. Voltage dividers, fuses, and current limiters are series.
- Use a simulator to verify your calculations and build intuition for circuit behavior.
Originally published at https://circuitdiagrammaker.app/blog/series-vs-parallel-circuits-explained.
