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Return to Previous Lesson: Light Bulb vs. LED and Limiting Current

Lesson Overview:

Now we'll learn about limiting the flow of current!

Step 1: Introduction

In this lesson you’ll learn a bit more about a very common component, which is found in almost every circuit: the resistor!

Resistors don’t require user input, like a button, and it doesn’t make light and noise like an LED or buzzer. But as you saw in the previous lesson, it is a very important way to control the flow of current in a circuit.

Resistors restrict or slow down the flow of current through a circuit. Limiting current flow can be used to lower the brightness of a light, the volume of a speaker, or the speed of a motor. In this lesson we will use the light bulb as an output that we can control with resistors.

  1. Continue to the next step.

Step 2: Measuring Resistance

Resistors can be combined in two ways: SERIES and PARALLEL. In this part of the lesson you will use the multimeter to measure the resistance of resistors that are combined together.

Resistance is measured across - or in PARALLEL - with the resistor. This means that the terminals on the multimeter are each connected to one of the terminals on the resistor.

Let’s start by measuring the resistance of a single resistor that you bring into the Workplane.

  1. Open the Components + tab and bring the multimeter into the Workplane. Place it above the resistor.
  2. Connect a wire from the black (negative) terminal of the multimeter to the right terminal of the resistor.
  3. Next connect a wire from the red (positive) terminal of the multimeter to the left terminal of the resistor.
  4. Select the multimeter by clicking on it and then use the drop-down menu to select “resistance” mode.
  5. Simulate the circuit!
  6. The resistance that you measure will match the resistance setting in the resistor’s dropdown menu. Check this by highlighting the resistor and looking at the dropdown menu.
  7. Press the “next” button below to continue.

Step 3: Connecting Resistors in Series

The diagram below shows resistors in SERIES. They are linked together at one terminal.

The more resistors you link together (no matter what value) the slower current becomes. It’s like driving down a dirt road, then a road with potholes, then a construction zone.

Two resistors in series can be thought of as one larger resistor. Let’s demonstrate this using two 100 Ohm resistors linked together.

In the next step, we will take some measurements with the multimeter.

  1. Select the resistor that is already in the Workplane and rotate it so it lies horizontally. Place this resistor so it lies near the left edge of the breadboard.
  2. Select this resistor again and change its resistance value to 100 ohms
  3. Copy and paste this resistor in the Workplane. Place it on the breadboard so that the left terminal shares a connection with the right terminal of the first resistor.
  4. Highlight each resistor and name them R1 and R2.
  5. Press the “next” button below to continue.

Step 4: Connecting Resistors in Series (part 2)

For the next step, open the Components+ tab and bring a multimeter into the Workplane. We’re going to measure the resistance of 2 resistors in series.

Resistors ADD in series. The calculation is simple. The resistance of the resistors in series R1 + R2. This formula works for any number of resistors! If you had 5 resistors in series the total resistance would be R1 + R2 + R3 + R4 + R5

  1. After you bring a multimeter into the Workplane, connect the two terminals of the multimeter across two of the resistors. Remember to select “resistance” from the dropdown menu.
  2. Simulate the circuit. What is the resistance measurement?
  3. While the simulation is running, try changing the resistor values and watch the change in the total resistance!
  4. Press the “next” button below to continue.

Step 5: Connecting Resistors in Parallel (part 1)

Resistors in parallel share connections at both of their terminals. When current runs through parallel resistors, it takes the easiest path.

Parallel resistors are like multiple roads running between the same starting point and ending points. If you have the choice between a five-lane highway and a dirt path, most drivers will want to take the highway! Even if you have a small resistor and a large resistor parallel to each other, the total resistance will still be low.

Let’s take a look at some real resistors. You can keep your resistors in series and start working with the bottom bank of breadboard sockets.

  1. Open the Components + menu and bring a resistor into the Workplane.
  2. Select this resistor again and change the value of its resistance to 100 ohms
  3. Copy and paste this resistor in the Workplane one time and name the resistors R1 and R2.
  4. Now we can start placing the two resistors in parallel. Start by placing R1 on the breadboard in the bottom bank of sockets.
  5. Place R2 in the sockets immediately above R1. The two resistors will share connections at both terminals.
  6. Bring another multimeter into the Workplane from the Components+ menu.
  7. Connect the terminals of the multimeter to the two columns of sockets on either side of the resistors. Make sure you choose “resistance” from the dropdown menu.
  8. Simulate the circuit. What is the resistance measurement now?
  9. While the simulator is running, try changing the resistor values to see what happens to the total resistance.
  10. Press the “next” button below to continue.

Step 6: Connecting Resistors in Parallel (part 2)

You should see a pattern! The total resistance is always less than either of the two resistors by themselves.

Putting resistors in parallel always decreases the resistance. The more paths for current that you have -- even if one is really slow -- it still helps bring a electrons from start to finish more effectively.

There is a slightly more complicated math equation for resistors in parallel:

1/ Rtotal = 1/R1 + 1/R2 (you can keep adding 1/R3, 1/R4, 1/R5 etc if you have more than two resistors)

  1. Continue to the next step.

Step 7: Resistor Challenge

In the following resistor combinations, we challenge you to find the missing resistor values. In each setup, try to put resistors together on the breadboard and measure the total resistance to check your work.

  1. Suppose you have a 200 ohm and a 2000 ohm resistor in series. What is the total resistance? Note: the 2000 ohm resistor is the same as a 2 kohm resistor.
  2. Now put two 200 ohm resistors in parallel. What is the total resistance?
  3. You have two resistors with the same value, but you don\u2019t know what it is. You do know that if the resistors are in parallel, the total resistance is 400 ohms. What is the value of each resistor?
  4. Extra challenge: Now you have two resistors with unknown value. You do know that in parallel the resistance is 75 ohms and in series the resistance is 400 ohms. What is the value of each resistor?
  5. Stuck? HINT: 1) 2200 ohms 2) 100 ohms 3) 800 ohms 4) Solve a system of equations and the values are 100 and 300 ohms.

Step 8: How Will You Use a Resistor?

You have just learned about a very important component, the resistor.

Don’t forget that a resistor is made out of a conductive material -- just a poor one! You will continue to use resistors in future lessons, particularly in LED circuits.

In the time that you have remaining, you can try connecting a resistor in series with other output components, like the LED.

In the next lesson you will learn to about batteries and voltage!

Next Lesson:Supply a Voltage with Batteries

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