Parallel vs. Series Circuit: How They’re Different and Full Comparison

Key Points

• Components in a series circuit are connected in a single branch, with the same current at each component but a different voltage.
• If one part fails in a series circuit, the other elements also fail.
• Components in a parallel circuit are connected in multiple branches, with each branch having full voltage and equal voltage among all branches.
• If one component fails in a parallel circuit, the other components will still function correctly.
• Parallel circuits are commonly used in home wiring, while series circuits are used in appliances like refrigerators and headlamps.

Understanding the fundamentals of parallel vs. series circuits sounds confusing, but is easy to understand. Components in a series circuit are connected in a single branch. The branch has the same current at each component (like a light bulb) but a different voltage.

If one part fails, the other elements also fail. Have you ever tested every single light in a strand of Christmas tree lights to find the “bad” bulb? This is a perfect example of a series circuit. One bulb went bad, so the entire string stopped working.

Components in a parallel circuit are connected in multiple branches. Each branch has full voltage, and the voltage is equal among all branches. If one component fails, the other components in the circuit will still function correctly.

Thankfully, most Christmas tree light strands are now parallel circuits, so we don’t need to spend hours testing each individual bulb in a strand.

Parallel vs. Series Circuit: Side-by-Side Comparison

Parallel vs. Series Circuit: What’s the Difference?

Let’s dig into each type of circuit: a parallel and a series circuit. To keep things simple, we’ll ignore a series-parallel circuit.

Alessandro Volta

In 1800, Alessandro Volta found that he could generate a steady flow of electricity. Volta stacked disks of silver and zinc. In between the disks, he inserted a brine-soaked cloth.

When Volta attached a wire from one of the stack disks to the other end of the stack of disks (positive and negative!), an electrical current began to flow. As Volta increased the pile of silver and zinc (alternating) disks, the current increased.

Electricity wasn’t new, as it had been around for approximately fifty years. The challenge until Volta’s discovery was what to “do” with electricity since there wasn’t a method to generate current on an ongoing basis.

Alessandro Volta’s demonstration of current flow heralded the introduction of a battery and the beginning stages of understanding and developing a series circuit.

Series Circuit

A series circuit is fairly straightforward to understand. Let’s look at the diagram below and briefly chat about it.

In the diagram above, the electrical current flows from the positive battery terminal through light bulb one (on the left), light bulb two (on the right), onto the switch, and back to the negative battery terminal. Here’s what we know about how this circuit operates as a series circuit.

1. The current is equal through all the light bulbs in this circuit. The current of the light bulb on the left is the same as the current of the right bulb on the right.
2. The total voltage of the circuit is equal to the sum of the voltage drops across both light bulbs. Let’s call the light bulb on the left V, and the right bulb on the right V₁. V₁+V₂=V_total.
3. In a simple example, both light bulbs would be identical, but that’s not necessarily how electronics work! If the light bulb on the left is drawing more voltage (let’s say it’s much bigger), then the same formula is true.
4. The resistance is based on the voltage drop from each component in the circuit. The individual resistances add up to the total resistance.
5. What would happen if we had a series circuit with two light bulbs but added two additional light bulbs? The same current needs to power all light bulbs, and with four light bulbs, the available current to each one is decreased. The series circuit with four light bulbs would be “dimmer” than the series circuit with two.

A Short Word About Christmas Tree Lights

Suppose we get a wild idea and decide to unscrew a light bulb. Guess what!?! The entire strand of light bulbs will turn off! Have you ever had the pleasure of checking every light bulb in a string of Christmas lights??

That’s a series circuit! When the current is interrupted (the light bulb is removed), the current can’t flow. That’s a big difference from a parallel circuit, which we should discuss now.

Parallel Circuit

A parallel circuit is quite different from a series circuit. Let’s take a look at the diagram below. While the diagram is fairly simple, we can use it to understand the fundamentals of a parallel circuit.

In the diagram above, the electrical current flows from the positive battery terminal through both light bulbs (top and bottom). Each light bulb has its own “branch.”

The current travels to the switch and back to the negative battery terminal. Here’s what we know about how this circuit operates as a parallel circuit.

1. While this series circuit has one “branch,” a parallel circuit has multiple branches. A branch may include various electric devices like capacitors, resistors, and inductors.
2. The current is divided among the branches. The current that enters each branch equals the current leaving the same branch. So, if 5A enters the branch, 5A leaves the branch.
3. The voltage at each branch is the same (equal) to the voltage at the source (battery.)
4. The sum of the resistors is not equal to the circuit’s total resistance. (Unlike a series circuit.)
5. Adding more branches to the circuit causes the total resistance to decrease. As more branches are added to the circuit, the total resistance decreases, and the current increases. (Ohm’s Law: E = IR)
6. In the diagram above, both lights are illuminated at the same brightness. If we added two more lights, all four lights would be illuminated at the same intensity. In a parallel circuit, the light bulbs have the same voltage passing across them as the battery. Each light bulb has its separate branch. And each branch is independent of the other and receives full voltage from the battery. This is different from a series circuit.

Back to the Christmas Tree Lights

1. If you remove a light bulb, the circuit will still function correctly, unlike a series circuit. In a series circuit, if you remove a light bulb, the circuit stops working.
2. Parallel circuits are commonly used in (newer) Christmas tree lights, outdoor lighting, indoor lighting, etc. The ability to have a single branch (light bulb) stop working and the other branches (the remaining light bulbs) still function properly is a key function of parallel circuits.
3. While we’re focused on “light bulbs” in our example, you can replace them with anything else that suits your fancy — an Xbox, microwave, electric toothbrush, bedside clock, cell phone charger, etc.

Parallel vs. Series Circuit: 6 Must-Know Facts

1. The current is the same through each component in a series circuit: I₁ = I₂ =I₃ = I₄.
2. In a series circuit, the total voltage is the sum of the voltages across each component: V₁ + V₂ = V_total.
3. The voltage is equal through each component in a parallel circuit: V₁ = V₂.
4. The total current of the parallel circuit is the sum of the current across each component: I₁ + I₂ = I_total.
5. A common use of the series circuit is a fuse or circuit breaker. If the fuse or circuit breaker is tripped, no energy passes through that circuit, protecting other connected devices.
6. A common use of the parallel circuit is the wiring in your home. One device can stop functioning (like an overhead light), and the remainder of the devices on the circuit will continue to function properly.

Parallel vs. Series Circuit: Which One Is Better? Which One Should You Use?

Overall, there’s no choice when selecting a parallel vs. series circuit. The choice is already made for us when we plug our appliances into a power outlet on the wall (parallel).

Our devices themselves have hundreds, if not thousands, of parallel circuits within them. We’re not electrical engineers, but we have a few general rules of thumb to help you out.

A parallel circuit delivers the same voltage to each component. Voltage is equally divided between all the components in a series circuit. Parallel circuits can deliver different levels/ amounts of current to each device in the circuit.

A series circuit will provide the same current to all devices in the circuit. This sounds problematic if you want to connect appliances to the circuit requiring different levels of current, like an LED lamp (very low current requirement) and a microwave (very high current requirement).

The independent current flowing through each branch means that if one branch is not working (broken light bulb), the entire circuit won’t work in a series circuit. Refrigerators and freezers use series circuits on the temperature control switches and compressors. You’ll also find series circuits in headlamps, toasters, and brake lights.

The image featured at the top of this post is ©haryigit/Shutterstock.com.