What is a Circuit?

Circuits are the basis of electronics, so, let’s take a few moments to break them down and discuss what they actually are.

A circuit is a loop through which current can flow. A power source, such as a battery, provides the energy for the circuit to work. Electrons flow from the negative side of the power source, through the circuit and back to the positive side of the power source. Once the electrons return to the power source, the circuit is complete.

circuit diagram

Our favorite example of a simple circuit is an LED connected to a battery. When the negative lead on the LED is connected to the negative side of the battery and the positive lead is connected to the positive side, power flows from the battery through the LED, causing the LED to light up.


simple LED and battery circuit

Components of a Circuit

At their base level, circuits can be broken down into three parts:

  • Voltage Source - this provides the electrons that flow through the circuit in order to power it. Common voltage sources are batteries and electrical connections such as outlets.
  • Load - this consumes the power created by the voltage source. Loads are what make a circuit light up, make noise, run a program and more. In simple circuits, the load may be a single light bulb, but in more complex circuits, the load may be made up of a combination of resistors, capacitors, light bulbs, buzzers and more.
  • Conductive Path - this is the route the current follows through the circuit. It must be made of conductive materials in order to allow electricity to flow. The path starts at the voltage source, travels through the load and returns to the voltage source. In order to create a closed circuit, this path must form a loop.

Open versus Closed Circuits

In order to provide power, circuits must be closed. This happens when a complete loop is formed from one side of the voltage source to the other side. If there are any interruptions in this loop, the electrons cannot complete their loop, creating an open circuit.

Technically speaking, the term “open circuit” is an oxymoron since the very definition of a circuit requires a closed loop. However, the term “open circuit” is often used to refer to a circuit that has been broken on purpose (through something like a switch or button) or through an error (such as a broken or loose component).

Short Circuits

When the conductive path of a circuit connects directly from one end of the voltage source to the other without first powering a load, the result is a short circuit.

short circuit example

Photo Credit: Dummies

Current flows everywhere it can, and if it can find a shorter path, it will take it. This is why conductive wires are coated in an insulator - to prevent accidental short-circuiting through wires touching.

Short circuits can be very dangerous and cause wires to burn up, damage the power supply, drain the battery, start a fire and more. Most of the time your power supply will have some sort of safety mechanism built into it to limit the maximum current in the event of a short circuit, but not always. This is the reason all homes and buildings have circuit breakers, to prevent fires from starting in the event of a short circuit somewhere in the wiring. If you notice a part of your circuit suddenly becoming hot or a part suddenly burns out, immediately turn off the power and look for possible short circuits.

It is important to note that current does not limit itself to choosing just one path, it will take every available path it can find. Which means that even if a short circuit is present, a small amount of power may still be supplied to your load.

Going Further

SparkFun Inventor's Kit LED blink circuit

Now that you know what a circuit is, it’s time to try building one. As we mentioned above, we love starting with just a battery and an LED. From there, try adding more components through a paper circuit, which uses copper tape to build the conductive path or by building a circuit on a breadboard using jumper wires, resistors and LEDs. Take a look at the SparkFun Inventor’s Kit Guidebook to learn more about building your own circuits.

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