--- title: "Why the resistor" subtitle: "Circuits with the micro:bit · Lighting an LED · Lesson 2" --- # Why the resistor **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** Every LED circuit in every kit has a small striped part next to the LED. Leave it out and the LED works brilliantly, for about a second. What is it doing? :::teacher-note **In one sentence:** Students take the resistor out of an LED circuit, compare resistor sizes, use Ohm's law with the LED's 2 V taken off to find the current and the smallest safe resistor, test that the resistor works on either side of the LED, and choose one for a target current. **Students should be able to say:** I can explain what a resistor does in an LED circuit. I can use Ohm's law to work out the current. I can choose a resistor for a safe current. **Width:** the unmarked parts are the floor every student does (predict, run, investigate and make). Typical adds the modify practice; wider adds the real-world contexts. They add contexts, not difficulty. Students work in the online tutorial alongside this sheet. ::: ## Predict :::keep-together **You take the resistor out and join the LED straight to pin 0. What happens?** ☐ The LED is brighter, and that's fine ☐ Far too much current flows, and the LED is damaged ☐ Nothing changes: the resistor does nothing ☐ The LED goes out: the loop is broken My prediction, and why: {{lines-3}} ::: :::teacher-note **What happens:** Far too much current: about 100 mA, five times the LED's limit. The resistor is what keeps the current safe. ::: ## Run Try it on the board in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Worked example: how much current? :::keep-together ### Ohm's law with an LED 1. $3.3 - 2 = 1.3\text{ V}$ — Pin 0 gives 3.3 V. The LED uses about 2 V, so 1.3 V is left across the resistor. 2. $I = \frac{V}{R} = \frac{1.3}{220}$ — Ohm's law: current = voltage ÷ resistance. 3. $I = 0.0059\text{ A}$ — In amps. 4. $0.0059 \times 1000 = 5.9\text{ mA}$ — Times 1000 for milliamps. 5. $5.9 < 20 \;\checkmark$ — Under the LED's 20 mA: safe. Now do the same for 470 Ω. **Now 470 Ω. How many milliamps? (1 decimal place)** Answer: {{field-15}} ::: :::teacher-note **Answer:** 2.8 1.3 ÷ 470 = 0.00277 A ≈ 2.8 mA. ::: :::keep-together ### Work backwards **Work backwards: what resistance gives exactly 20 mA (0.02 A)? (ohms)** Answer: {{field-15}} ::: :::teacher-note **Answer:** 65 R = 1.3 ÷ 0.02 = 65 Ω. Anything bigger is safe; kits use 220 Ω to leave a margin. ::: :::keep-together ### Find the mistake **Sam is sure a resistor after the LED won't protect it. Click the line where Sam's reasoning first goes wrong.** ``` 1 The current comes out of pin 0. 2 It reaches the LED first, so the LED gets the full current. 3 So the resistor has to go before the LED to protect it. 4 After the LED it would be too late. ``` Line: {{field-15}} Why is it wrong? {{lines-2}} ::: :::teacher-note **Answer:** line 2 Line 2: in a single loop the current is the same everywhere. It doesn't arrive at the LED first and then slow down: the resistor limits the current in the whole loop, wherever it sits. ::: :::typical ## Vary: change one thing **Move it.** **Very big.** Work in the tutorial. Plan first: what will you change, and how will you test it? {{box-4}} ::: :::typical ## Build it in Tinkercad Tinkercad's LED and pin are not exactly the board's model, so expect close, not identical, numbers. Work in the tutorial. Plan first: what will you change, and how will you test it? {{box-4}} ::: ## Ohm's law :::keep-together ### 1 kΩ **1 kΩ (1000 Ω): how many milliamps? (1 decimal place)** Answer: {{field-15}} ::: :::teacher-note **Answer:** 1.3 1.3 ÷ 1000 = 0.0013 A = 1.3 mA. ::: :::keep-together ### Choose for 10 mA **You want 10 mA (0.01 A). What resistance? (ohms)** Answer: {{field-15}} ::: :::teacher-note **Answer:** 130 1.3 ÷ 0.01 = 130 Ω. ::: :::keep-together ### Review: dark LED **Review: the LED is dark with a 220 Ω resistor in the loop. Which is NOT a likely cause?** ☐ Pin 0 is off ☐ The LED is the wrong way round ☐ The resistor is too big ☐ The GND wire is off ::: :::teacher-note **Answer:** The resistor is too big 220 Ω still gives about 6 mA: plenty to see. ::: ## Make: choose and place Work in the tutorial. Plan first: what will you change, and how will you test it? {{box-4}} ## Exit ticket :::keep-together **With a 330 Ω resistor and a 2 V LED on a 3.3 V pin, how much current flows? Is it safe?** {{lines-3}} ::: :::teacher-note **Answer:** 1.3 ÷ 330 ≈ 0.0039 A ≈ 3.9 mA: safe, under 20 mA. :::