February 23, 2025

Circuit Design in Practice

Designing a circuit well involves more than getting the schematic right. Simulation, PCB layout, power management, and knowing what to verify — these are the parts that determine whether a design that works in theory actually works on the bench. Here's the workflow that makes real projects succeed.

The Design Process

  1. Define requirements first. Input voltage range, output parameters, power budget, operating temperature, cost constraints. Write these down before touching a schematic. Changes to requirements after layout are expensive.
  2. Select components with margin. A resistor's rated power, a capacitor's voltage rating, a transistor's current capacity — always leave headroom. 2× margin is conservative; 1.5× is acceptable; at spec is asking for failure.
  3. Schematic before layout. The schematic captures intent. Get it right and readable before thinking about board layout. A confusing schematic produces a confusing layout produces a board that's hard to debug.
  4. Simulate the tricky parts. Not everything needs simulation, but analog signal paths, power supply stability (especially switching supplies), and filter response are worth checking before building.
  5. Prototype on breadboard or protoboard. Verify functionality, find the problems, fix them cheaply. Breadboards have parasitics that matter at RF; at audio and below frequencies they're fine.
  6. PCB layout. Apply the rules below. Order and test.

Simulation Tools

  • LTspice — free, from Analog Devices (formerly Linear Technology). The best free SPICE tool. Extensive model library, good documentation, active user community. The default choice for analog simulation.
  • KiCad simulator — SPICE integration in KiCad's schematic editor. Convenient for simulating what you're already drawing.
  • Falstad Circuit Simulator — browser-based, interactive, no installation. Good for quick sanity checks and teaching. Limited to simpler circuits.
  • Qucs-S — open-source, good for RF and S-parameter simulation.

Schematic Good Practices

  • Signal flows left to right. Power flows top to bottom. VCC at the top, GND at the bottom — consistently.
  • Label nets with meaningful names. "ADC_IN" and "MOTOR_PWM" are better than "net001".
  • Include reference designators (R1, C1, U1) and values on the schematic — not just in the BOM.
  • Add decoupling capacitors on every IC power pin in the schematic. They'll remind you to place them properly in layout.
  • Add test points explicitly. A TP1 marker in the schematic means a test pad in the layout means a place to hook a probe.

PCB Layout Rules That Matter

Layout can make or break a design that works perfectly in simulation.

Ground Plane

Use a solid copper pour on one layer as a ground plane. This provides a low-impedance return path for every signal, reduces radiated emissions, and makes decoupling caps actually work. Split ground planes (analog ground / digital ground) are sometimes used but often cause more problems than they solve — a single ground plane with careful placement is usually better.

Decoupling Capacitors

Place 100nF ceramic caps as close to IC VCC pins as physically possible — measure in millimetres. A decoupling cap on the other side of the board is nearly useless. The via inductance between the cap and the pin is also significant; where possible, use via-in-pad or place the via directly at the cap pad.

High-Current Paths

Trace resistance matters at high current. Estimate the voltage drop:

R_trace = ρ × L / A
For 1oz copper: R ≈ 0.5mΩ per square (for a trace width equal to its length)

Rule of thumb: 1A per mm of trace width for 1oz copper (with 20°C rise)
For 2A: use 2mm minimum trace width

Differential Pairs

For USB, Ethernet, LVDS, and other differential signals: route the two traces together, equal length, same width, with consistent separation. Most PCB tools have a differential pair router. The goal is matched impedance and matched delay between the two conductors.

Thermal Management

Calculate power dissipation for every component that gets warm. Linear regulators, power MOSFETs, bridge rectifiers, and high-power resistors all need to dissipate heat somehow.

Thermal resistance: T_junction = T_ambient + P × θ_JA

Where:
θ_JA = junction-to-ambient thermal resistance (°C/W, from datasheet)
P = power dissipated in watts

Example: 7805 in TO-220 package, θ_JA = 65°C/W
P = (12V - 5V) × 0.5A = 3.5W
T_junction = 25 + 3.5 × 65 = 252°C  ← junction would fail (max 125°C)
→ Need a heatsink: target θ_JA ≤ (125 - 25) / 3.5 = 28.6°C/W
→ Use heatsink with θ_SA ≤ 28.6 - 5 (device θ_JC) ≈ 23°C/W