Series and Parallel Connections

One-sentence summary

Placing parts one after another along a single path is called a series connection, while giving the current more than one separate path is called a parallel connection.

Why does it matter?

In the previous lesson we saw that a circuit needs a closed path for current to flow. But when a circuit has more than one part, how do we arrange them?

There is no single way to connect two lamps. You can line them up one after another on a single path, or you can give each of them its own path. These two choices directly change how bright the lamps glow, whether one keeps working if the other fails and how long the battery lasts.

That is why the difference between series and parallel is one of the most basic ideas in electronics. You see both kinds of connection everywhere, from the lights in your home to the battery holder in a toy. In this lesson we will study them step by step using simple text schematics with a battery, a resistor and an LED.

Short definition: A series connection gives the current one single path; a parallel connection gives the current more than one separate path.

Series connection: One path, same current

In a series connection all the parts are lined up one after another on a single path. There is only one route the current can follow. Because of this, the same amount of current flows through every part.

You can compare this to a garden hose. Water flows through a single pipe, and the same amount passes every point along it. If you block one spot, the flow stops completely.

A series circuit with two AA batteries and one LED

The schematic below shows two AA batteries (about 3 V), a resistor and an LED on a single path. The resistor protects the LED by limiting the current that reaches it.

[+] Battery (2×AA, ~3 V)
 |
 +---> Resistor (limits the current)
        |
        +---> LED  (long leg = +, short leg = -)
               |
[-] Back to the battery

Because the parts are lined up one after another, the current has only one route: it leaves the battery, passes through the resistor, lights the LED and returns to the battery.

If we connect two LEDs in series

Now let us add a second LED to the same path:

[+] Battery (2×AA, ~3 V)
 |
 +---> Resistor
        |
        +---> LED 1
               |
        +---> LED 2
               |
[-] Back to the battery

There are two important things to notice here:

• The voltage is shared. Each LED uses part of the voltage to light up. A red LED needs about 1.8–2 V. Connected in series, two LEDs together need about 3.6–4 V. A 3 V battery cannot fully supply this, so the two LEDs glow dimly together, or do not light at all.
• If the chain breaks, everything stops. In a series circuit, if one part fails or one leg comes loose, the single path is cut and every LED in the circuit goes dark.

This second point is the most memorable feature of a series connection: one path, one chain.

Parallel connection: Many paths

In a parallel connection the parts are not lined up one after another. Instead they sit side by side on separate branches. This opens more than one path for the current. Each branch connects on its own to the plus and minus ends of the battery.

Let us return to the hose comparison: if you split the water from one main pipe into two separate hoses, each hose flows on its own. If you block one, the other keeps flowing.

If we connect two LEDs in parallel

In the schematic below the two LEDs are placed on two independent branches. Each LED has its own resistor.

[+] Battery (2×AA, ~3 V)
 |
 +---> Resistor A ---> LED 1 ---+
 |                              |
 +---> Resistor B ---> LED 2 ---+
                                |
[-] Back to the battery  <------+

Two important differences appear in this parallel circuit:

• Each branch gets the full voltage. Both LED 1 and LED 2 see the whole battery, about 3 V. Because of this, both LEDs glow brightly, as if each were lit on its own.
• If one branch breaks, the other keeps working. Even if LED 1's leg comes loose, LED 2's path is still closed and it keeps glowing.

On the other hand, in a parallel connection the battery has to supply more current, because each branch draws current separately. This means the battery drains faster.

Why does each LED have its own resistor?

We put a separate resistor on each branch so that the current to each LED is limited on its own. If two LEDs share a single resistor, the current may not divide evenly, and one LED can end up brighter than the other, or even hot enough to be damaged over time. A separate resistor keeps the sharing safe and balanced.

Two everyday examples

Example 1: Home lighting

Think about the lamps in your home. The lamp in the living room can be on while the kitchen lamp is off; turning one off does not affect the other. This is because home lighting is wired in a parallel arrangement. Each lamp uses its own path, so they work independently and each one glows at full strength.

If all the lamps in the house were connected in series, one burnt-out bulb would leave the whole house in darkness, and the lamps would all be dim because they would split each other's share of the voltage. That is why parallel wiring is preferred for lighting.

Note: Home lighting runs on mains electricity and is dangerous. Here we only use the connection idea as an example; never experiment with wall sockets or house wiring.

Example 2: Old string lights

In some old strings of holiday lights, all the tiny bulbs were connected in series. When a single bulb burnt out, the one path of the chain was cut, so the whole string went dark, and finding the broken bulb took a lot of patience. Newer strings usually use parallel-like arrangements, so even if one bulb fails, the others keep glowing. This is a nice example of the difference between series and parallel being felt directly in daily life.

Mini practice

You can try this with an adult using a 2×AA battery holder, two LEDs and two 220 Ω resistors. If you do not have the parts, you can do the same steps by drawing them on paper.

Steps

1. First build the single-LED series circuit: battery → resistor → LED → battery. Make sure the long leg of the LED faces the plus end.
2. Look at the brightness of the LED and note it down.
3. Add a second LED on the same path to connect two LEDs in series. How did the brightness change?
4. Now take the circuit apart and connect the two LEDs in parallel: give each LED its own resistor, and connect both to the battery ends on separate branches.
5. Compare the brightness in the two connections.

Observation questions

• In the series connection, did lighting two LEDs together make them brighter or dimmer?
• In the parallel connection, if you pull out one leg of one LED, what does the other LED do?

Draw the schematic of both circuits in your notebook and write down what you observed. There is no single correct answer; what matters is seeing the difference with your own eyes.

Common mistakes

Connecting two LEDs in parallel with one resistor

If you do not put a separate resistor on each parallel branch, the current may not divide evenly. One LED can glow very brightly and heat up while the other stays dim. Give each branch its own resistor.

Not noticing the low voltage in a series circuit

When you connect two red LEDs in series, together they need about 3.6–4 V. A 3 V battery cannot supply this, so the LEDs glow dimly. Before deciding an LED is "broken" and replacing it, think about whether the voltage is enough.

Connecting the LED backwards

An LED only lets current flow in one direction. If the long leg does not go to plus and the short leg to minus, the LED will not light at all. If an LED does not light, check its direction first.

Using no resistor at all

Without a resistor, the current to the LED rises too high, and the LED can burn out in a few seconds. Every LED branch must have a current-limiting resistor.

Safety note

• Use only low-voltage, educational power sources: 2×AA batteries (3 V), USB, a micro:bit or an Arduino. Never experiment with mains electricity, wall sockets or exposed house wiring.
• Have an adult with you whenever you work with batteries, motors, cutting tools or hot surfaces.
• Connect the polarity correctly, use a current-limiting resistor on every LED branch, and never short-circuit the battery by touching its plus and minus ends directly together.
• Lithium batteries can heat up and become dangerous when short-circuited. In this lesson use simple AA cells; if a battery gets warm, open the circuit right away and tell an adult.

Review questions

1. What stays the same through components connected in series?
2. What stays the same across branches connected in parallel?
3. Why does one broken component often stop an entire simple series circuit?
4. How can parallel branches affect total current drawn from a battery?
5. What observation can distinguish a series LED arrangement from a parallel one?
6. Why should a circuit plan include current limits before it is powered?

Answers

1. The same current passes through each component in one unbranched series path.
2. Each branch is connected across the same two nodes, so the voltage across the branches is the same.
3. A break opens the only available path, so current cannot continue through the circuit.
4. Adding branches can lower total resistance and increase the current demanded from the source.
5. Removing one LED in series may stop all of them; in parallel, other correctly wired branches can continue working.
6. Current limits protect LEDs, wires, controllers and the power source from excessive current and heat.

Lesson summary

• In a series connection the parts sit on a single path; the current has one route and flows the same through every part.
• In a series circuit the voltage is shared between the parts, and if one point in the chain breaks, everything stops.
• In a parallel connection the parts sit on separate branches, opening more than one path for the current.
• In a parallel circuit each branch gets the full voltage, and if one branch breaks, the others keep working.
• Home lighting is wired in parallel so that each lamp works independently and at full brightness.

Check your understanding

1. In a series connection, how many separate paths does the current have?
2. In a parallel connection, if one of the two LEDs loses a leg, what happens to the other LED?
3. If we connect two red LEDs in series to a 3 V battery, why do they glow dimly?
4. Why is home lighting wired in parallel?
5. Why do we put a separate resistor on each parallel LED branch?

Answers

1. There is one single path. Because of this, the same amount of current flows through every part.
2. The other LED keeps glowing, because its branch is a separate path and is still closed.
3. Together the two LEDs need about 3.6–4 V; a 3 V battery cannot fully supply this voltage, so both glow dimly.
4. So that each lamp uses its own path and works independently at full brightness. That way, if one goes off, the others are not affected.
5. To limit the current to each LED separately and keep the sharing balanced and safe.

Source and verification note

For “Series and Parallel Connections”, verification focuses on whether the relationship between Series connection: One path, same current and If we connect two LEDs in series remains consistent across examples. Activities are designed only for low-voltage direct-current circuits; mains electricity is not used. Component values must be rechecked for the actual circuit, and connections should be reviewed with an adult before power is applied.

Next lesson

Ohm's Law: You will learn the relationship between voltage, current and resistance, and calculate how to choose the right resistor for an LED.