--- title: "Design rules and ordering" subtitle: "Your own board · From breadboard to board · Lesson 3" --- # Design rules and ordering **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** The motor board works on the bench, then a trace turns brown and lifts off. Why that trace, and not the others? :::teacher-note **In one sentence:** Students set trace widths against current from a table, find a trace too wide to squeeze between header pins and route it around, then work out the cost of a board order (setup plus per board). **Students should be able to say:** I can choose a trace width from the current it carries. I can explain why two design rules can force a longer route. I can work out what an order of boards costs, per board. **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 **Every trace is 0.25 mm wide. The motor draws 1.5 A. Which trace is most likely to overheat?** ☐ SIG, the control signal ☐ None: copper never overheats ☐ VM, the motor's power ☐ They all heat the same My prediction, and why: {{lines-3}} ::: :::teacher-note **What happens:** VM (and GND, MA, MB): they carry 1.5 A. SIG carries a few thousandths of an amp, so a thin trace is fine for it. ::: ## Run Try it on the board in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Worked example: width from current :::keep-together ### Choose the width 1. $\text{VM carries } 1.5 \text{ A}$ — Find the current each net carries (the motor's datasheet gives it). 2. $0.5 \text{ mm} \to 1.4 \text{ A} < 1.5 \;\times$ — Too thin: it would run hot. 3. $1 \text{ mm} \to 2.4 \text{ A} \ge 1.5 \;\checkmark$ — The narrowest width that passes, with a margin. 4. $\text{between header pins: } \le 0.4 \text{ mm}$ — A second rule: the gap between pins leaves room for 0.4 mm only. 5. $1 \text{ mm} > 0.4 \text{ mm} \Rightarrow \text{route around}$ — A trace that needs 1 mm can't squeeze between pins, so it takes the long way. **A heater trace carries 2.8 A. Which is the narrowest width that passes?** ☐ 1.5 mm ☐ 1 mm ☐ 2 mm ☐ 0.5 mm ::: :::teacher-note **Answer:** 1.5 mm 1.5 mm is safe to 3.2 A, which is at least 2.8 A. ::: :::keep-together ### Two rules meet **MA must carry 1.5 A, and its shortest path squeezes between two header pins (0.4 mm of room). What should the designer do?** ☐ Make MA 1 mm and squeeze it through anyway ☐ Lower the motor's current ☐ Make MA 0.4 mm and squeeze it through ☐ Route MA around the header, at 1 mm wide ::: :::teacher-note **Answer:** Route MA around the header, at 1 mm wide Two rules meet: width for current, and room between pins. The longer route around satisfies both. ::: :::typical ## Vary: a squeeze Work in the tutorial. Plan first: what will you change, and how will you test it? {{box-4}} ::: ## Rules and orders :::keep-together ### Width **A trace carries 0.9 A. Which is the narrowest width that passes? (0.25 mm 0.8 A · 0.5 mm 1.4 A · 1 mm 2.4 A)** ☐ 0.5 mm ☐ 0.25 mm ☐ 1 mm ::: :::teacher-note **Answer:** 0.5 mm 0.25 mm is safe only to 0.8 A; 0.5 mm to 1.4 A. ::: :::keep-together ### An order **A board maker charges $20 to set up an order and $1.50 a board. What do 10 boards cost, in dollars?** Answer: {{field-15}} ::: :::teacher-note **Answer:** 35 20 + 10 × 1.50 = $35. ::: :::keep-together ### Per board **Same prices: what does each board cost when you order 40, in dollars?** Answer: {{field-15}} ::: :::teacher-note **Answer:** 2 20 + 40 × 1.50 = $80, and 80 ÷ 40 = $2 a board. The setup cost is shared by more boards. ::: ## Make: ready to order **Pass the rules.** **Order.** **Evaluate.** Work in the tutorial. Plan first: what will you change, and how will you test it? {{box-4}} ## Exit ticket :::keep-together **A trace carries 2 A. Which width (0.5 mm 1.4 A, 1 mm 2.4 A, 1.5 mm 3.2 A) do you choose, and why not the others?** {{lines-3}} ::: :::teacher-note **Answer:** 1 mm: the narrowest safe for 2 A; 0.5 mm would overheat, and 1.5 mm takes more room than needed. :::