Ohm's Law

In this tutorial, we’ll learn about a fundamental concept in circuit design known as Ohm’s Law and apply it to ensure that we can safely light up an LED.

In all circuits, electricity flows from the positive side (+) of your power source (like a battery) to the negative side (-). The name for the flow of electricity is current.

When current flows through a components, like an LED or a resistor, there is an associated change in potential energy across the component that is known as voltage. The amount of voltage “drop” across the a component depends on its resistance, or how much it restricts the flow of current. This can be really important for making sure your components don’t fry because they aren’t able to support the current coming from your battery, as we will soon see when it comes to parts like LEDs.

These three concepts (current, voltage, and resistance) are all related to one another through a law called Ohm’s Law, where voltage is equal to the product of current and resistance:

Voltage (V) = Current (I) * Resistance (R)

or

V = IR

Let’s test this out!

In the editor, you’ll find a simple circuit connecting an LED directly to a 9V battery. If we power the circuit, we expect that current will flow from the positive side of the battery, through the LED, and then into the negative side of the battery.

1. Turn on the simulator to see what will happen with our LED.
2. Hover over the LED to reveal a message about the amount of current flowing through the LED.
3. It looks like there is too much current flowing through the LED with the current design - this could really damage your LED! Let’s see how we can prevent this from happening in the next step.

In the last step, you saw how there was too much current flowing through the LED. Let’s recall Ohm’s Law, where Voltage is equal to Current multiplied by Resistance.

V = IR

LEDs have a maximum rated current level, or a maximum amount of current it can safely handle. In this case, let’s account for a maximum current of 20mA, a fairly standard rating for LEDs – that's 0.02 A if we convert millamps to amps.

If we want to ensure that there is only 20mA of current flowing through the LED, we will need to add some resistance to our Circuit. We know the voltage because we are using a 9V battery. Applying Ohm’s law, we have 9V/0.02A = 450 ohms (ohms is our unit for Resistance). This means that if we want to ensure there is only 20mA of current flowing through the LED, we want to add a resistor with a value of 450 ohm to our circuit.

1. Click + Components and add a resistor.
2. Click on the resistor and change its value to 450 ohms.
3. Add the resistor to the circuit by removing the red wire and adding the resistor between the positive side of the battery and the Anode leg of the LED.
4. Continue to the next step.

Congratulations! You learned how to apply ohm’s law to limit the amount of current through the LED.

Tips:

• In a circuit in which there is a single path for current to flow like this one, the current through each element of the circuit will be the same. For this reason, it actually doesn’t matter whether the resistor comes before or after the LED - either way, it will limit current flow. You can test this out for yourself if you’d like by placing the resistor between the cathode leg of the LED and the negative side of the battery.
• Did you notice how the colored bands on the resistor changed when you edited its resistance value to 470 ohms? Colored bands on the resistor are used to visually determine its value. You can read more about that here.