13 Component Architecture: Capacitors and Selection
13.1 Start With the Decision
A sensor resets when its radio transmits even though the average current looks safe. The capacitor, supply path, and material limits must cover the short load step.
13.2 Route Overview
This is part 2 of 3. Review Electronics Materials: Conductors to Components for the preceding evidence.
13.3 Learning Objectives
- Select capacitor type and value for decoupling and energy buffering.
- Relate dielectric and semiconductor properties to circuit behaviour.
13.4 Chapter Roadmap
- Checkpoint: Architecture and Power
- Checkpoint: Materials
- Phoebe’s Field Notes: Why Materials Turn Into Circuit Numbers
- Knowledge Check: Decoupling Capacitors
- For Kids: Meet the Sensor Squad!
- Capacitor Type Selection
Checkpoint: Architecture and Power
You now know:
- A typical IoT device combines wireless, energy management, microcontroller, memory, mixed-signal, and sensor or actuator subsystems.
- Active currents can sit around 1-50mA for the microcontroller and 10-100mA for wireless transmit, while sleep currents can fall to microamp levels.
- With a 1000mAh battery and 11.4µA average sleep current, the chapter’s calculation gives 87,719 hours, about 10 years.
Now move from whole-device blocks to the materials underneath them. The same device only works because copper, FR4, plastic, and silicon each play a different electrical role.
13.4.1 Material Classification
Now that we have surveyed the electronic components used in IoT systems, let’s examine the underlying material science that makes them possible. All materials fall into three categories based on how freely their electrons can move.
Use the diagram in Figure 13.1 to ground material classification: it shows material Conductivity Classification: Conductors, Semiconductors, and Insulators rather than leaving the relationship implicit.
Follow Figure 13.1 from Material Conductivity Classification, then test the conductor branch against Free electrons move easily and its examples Copper, Gold, Aluminum. The lower labels connect that property to Copper traces, wires and Function: Carry current. The sequence makes conductivity a functional selection criterion: traces need predictable current flow, while semiconductor and insulation layers are chosen for controlled or blocked flow.
Bring the visual evidence in Figure 13.2 into material classification. Its purpose is to reveal device anatomy view: Every IoT device uses all three material types. CONDUCTORS (copper traces, wires) carry current between components. SEMICONDUCTORS (chips, ICs) actively control and process signals. INSULATORS (FR4 board, plastic case) prevent unwanted current flow and provide safety before a design claim is accepted.
Figure 13.2 places Three Material Types in Every IoT Device beside Copper traces, wires and qualifies the relationship with Function: Carry current. Read together, the labels demonstrate device anatomy view: Every IoT device uses all three material types. CONDUCTORS (copper traces, wires) carry current between components. SEMICONDUCTORS (chips, ICs) actively control and process signals. INSULATORS (FR4 board, plastic case) prevent unwanted current flow and provide safety. The chapter carries that evidence into material classification so the selection rests on circuit behaviour rather than appearance.
| Type | Electron Mobility | Resistance | Examples | IoT Use |
|---|---|---|---|---|
| Conductor | Free to move | Very low (~10^-8^ Ω·m) | Copper, gold, aluminum | Wires, PCB traces, contacts |
| Insulator | Cannot move | Very high (~10^15^ Ω·m) | Rubber, glass, plastic | Cable insulation, PCB substrate |
| Semiconductor | Conditionally mobile | Medium (variable) | Silicon, germanium | Transistors, diodes, ICs |
Bring the visual evidence in Figure 13.3 into material classification. Its purpose is to reveal classification of materials: insulators, conductors, and semiconductors before a design claim is accepted.
Figure 13.3 places Material Classification by Conductivity beside Type and qualifies the relationship with Examples. Read together, the labels demonstrate classification of materials: insulators, conductors, and semiconductors. The chapter carries that evidence into material classification so the selection rests on circuit behaviour rather than appearance.
Inspect this evidence before proceeding: a reliable material classification starts by viewing Figure 13.4; it identifies semiconductor knowledge check and key concepts for the calculation or selection that follows.
Inspect Semiconductor Knowledge Check in Figure 13.4 as the starting prompt, then contrast Insulators with Semiconductors. One blocks charge movement under normal conditions; the other permits deliberately controlled conduction after doping and device construction. That contrast closes the classification argument by asking whether each board material must carry current, modulate it, or keep neighbouring conductors isolated.
Checkpoint: Materials
You now know:
- Conductors such as copper have very low resistivity, around 10^-8 Ω·m, and carry current in traces, wires, and contacts.
- Insulators such as rubber, glass, and plastic have very high resistance, around 10^15 Ω·m in the chapter table, and protect against unwanted current paths.
- FR4 appears later with about 10^12 ohm per square surface resistance, which is why contamination and moisture matter on high-impedance circuits.
13.5 Phoebe’s Field Notes: Why Materials Turn Into Circuit Numbers
The mathematical gist. One resistor value can set several circuit behaviours. With 5.00 V, a 10 kΩ lower divider leg, and a 100 nF capacitor, choosing the upper resistance sets (V_{out}=V_{in}R_2/(R_1+R_2)), divider current, (\tau=R_1C), and (f_c=1/(2\pi R_1C)). Material physics fixes the available resistance; these equations fix its circuit effect.
13.5.1 The Semiconductor Advantage
Key Property: Semiconductors can be switched between conducting and insulating states by applying external energy (voltage or current).
This controllability makes ALL modern electronics possible.
13.5.2 Wire and Cable Engineering
The sections above classify what conducts and what insulates. The wire that connects your components is itself an engineering choice: which metal, which construction, and which shielding trade cost, flexibility, and noise immunity against each other.
Before continuing wire and cable engineering, trace Figure 13.5; the drawing makes even before insulation, stranding, and shielding are added, conductor material and surface condition affect resistance, termination quality, flexibility, and long-term reliability. Photo: Chemicalinterest, public domain available for a component-level check.
The specimen in Figure 13.5 should be read from the exposed conductor outward. Even before insulation frames the bare-metal choice, and shielding are added names later cable layers, and surface condition affect resistance points to the termination interface. The photograph therefore supports the cable-engineering rule that conductor material, strand condition, and contact surface must be settled before insulation or shielding can deliver reliable field service.
| Metal | Conductivity vs Copper | Strengths | Weaknesses | IoT Use |
|---|---|---|---|---|
| Copper | Reference (best common metal) | Cheap, flexible, good for wiring | Corrodes in air and water, softer than steel | Default choice for wiring and PCB traces |
| Aluminum | Lower | Lightweight, corrosion-resistant, cheap for large runs | Creeps and loosens at connections over time, less flexible, breaks more easily | Enclosures, heatsinks, corrosion-resistant wiring and foils |
| Silver | Highest of all metals | Best thermal and electrical conduction, withstands high heat | Very expensive, poor corrosion resistance | Thin high-performance wire and plating, not bulk wiring |
| Nickel | Lower, very high melting point (1453°C) | Strong, corrosion-resistant, ductile for intricate parts | Reacts with other metals (useful for forming alloys) | Plating for corrosion protection |
| Tin | Lower, soft | Ductile, solders easily, resists water corrosion | Weak alone, poor acid/alkali resistance | Solder joints, protective plating |
| Steel | Much lower than aluminum | Strong, heavy, reinforces cable runs | Rusts in water, poor conductor on its own | Structural cable reinforcement, springs, struts |
Plating extends what a base metal can do without switching to a costlier conductor outright. Tinned copper trades a little conductivity for corrosion resistance and easier soldering — common on cabling exposed to water, such as ships and industrial Ethernet runs. Silver-plated copper keeps copper’s cost down while extending the usable temperature range to roughly -65°C to 200°C, the choice for aerospace-grade wiring. Nickel-plated copper adds corrosion resistance and can survive up to about 750°C with a thick plating, at some cost to high-frequency performance.
A conductor is either one solid piece of wire or many thin strands twisted or bunched together. Solid wire is cheaper and seats more reliably into jacks and insulation-displacement connectors, but it work-hardens and cracks under repeated flexing. Stranded wire trades a little of that seating reliability for flexibility and a much longer flex-fatigue life — the reason patch cables are stranded while permanent in-wall runs are often solid.
Stranding pattern changes the trade further:
- Bunched: cheap and simple to build, but packs loosely (the geometric “circle packing” problem wastes cross-section).
- Concentric, unilay: all strands twisted the same direction, for lighter weight, a smaller diameter, and better torsional flex.
- Concentric, contra-helical: alternating layers twisted in opposite directions, for greater mechanical strength, crush resistance, and continuous flex life.
More twists per strand generally trade added flexibility for mechanical strength. Ethernet cable is manufactured both ways: solid conductors for plenum and fixed in-wall runs, stranded for standard patch cables and patch panels.
At high frequencies, current stops flowing uniformly through a conductor’s cross-section and crowds toward the surface — the skin effect, driven by eddy currents from the conductor’s own changing magnetic field. Stranding helps less against this than intuition suggests, because touching strands still act electrically like one larger conductor; only strands that are individually insulated from each other (Litz wire) actually force current to use more of the total surface area. This is also why a low-resistance plating such as silver-tinned copper pays off disproportionately at RF: the current that matters is concentrated in exactly the thin outer layer the coating occupies.
Moving a conductor through a changing magnetic field induces a current in it — electromagnetic induction. In a cable bundle, one wire’s signal can induce unwanted current fluctuations in its neighbors (crosstalk) or pick up noise radiated from other equipment. Two countermeasures do most of the work in IoT wiring:
- Twisted pairs run a signal and its return on two wires twisted around each other and read the difference between them at the receiver (differential signaling). Because both wires sit in essentially the same field, induced noise affects them equally and cancels out of the difference measurement. The trade-off is mechanical: EMI rejection depends on every twist staying intact, so twisted-pair cable specifies a maximum pulling tension and a minimum bend radius, and cheap cable with an uneven twist rate per pair can show up as visible signal defects on video or data links.
- Shielding wraps the insulated conductor in an additional conductive layer that reflects, absorbs (acting as a Faraday cage), or conducts interference to ground instead of into the signal wire. Foil shielding is cheap and thin but has a shorter flex lifetime and suits high-frequency and RFI noise; braided shielding costs more and is bulkier but suits low-frequency and EMI noise and flexes longer. Combining foil and braid gives the best rejection across the frequency range — in one shield-effectiveness comparison across 30-210 MHz, plain aluminum/polyester foil manages roughly 10-30 dB, an 85%-coverage tinned-copper braid roughly 30-50 dB, and a combined foil-plus-braid shield roughly 40-60 dB.
Insulation matters here too: avoid air gaps in a cable’s construction, since ionization of trapped air can degrade cable quality over the cable’s life.
Any IoT sensor cable that runs through walls, ceilings, or air-handling spaces has to meet a fire-safety rating, not just an electrical one:
| Rating | Meaning | Notes |
|---|---|---|
| Plenum (PL) | Toughest, most expensive tier; meets NFPA 90A | Fire-retardant low-smoke jacket (PVC or FEP), a reinforcing filament to support the cable’s own weight, solid rather than stranded conductors — the same restrictions that make it stiffer and costlier |
| Riser (RI) | Rises between floors outside plenum spaces | Lighter-duty than plenum; not for open plenum airflow spaces |
| General purpose (GP) / Residential (RE) | Standard indoor runs | Least restrictive, lowest cost |
| Low-smoke zero-halogen (LSZH) | Eliminates toxic halogen gases when burning | Matters most in enclosed, poorly ventilated spaces or near sensitive equipment |
A sensor deployment that looks purely electrical on the schematic still needs an answer for “which of these does the run pass through?” before the wire gauge, stranding, and shielding choices above are actually complete.
The Sensor Squad discovers why materials matter!
Temperature Terry was curious about something. “Max, why are some things made of copper and other things made of plastic?” the microcontroller loved this question!
“Imagine a hallway full of kids,” Max explained. “In a CONDUCTOR like copper, all the kids can move freely — they’re like electrons zooming through the hallway. That’s why copper wires carry electricity so well!” the LED nodded. “That’s how electricity gets to me!”
“Now imagine a hallway where all the kids are stuck to their desks and can’t move at all,” Max continued. “That’s an INSULATOR like rubber or plastic. No electrons can flow, so no electricity gets through. That’s why your charging cable has a plastic coating — it keeps the electricity inside the wire and protects your fingers!”
“But here’s the really cool part,” said Max, getting excited. “A SEMICONDUCTOR like silicon is a hallway where the kids are sitting at their desks, but if someone gives them a push — like adding a little bit of special material called a dopant — some of them can start moving! It’s like a teacher saying ‘OK, you can get up now!’”
the battery was amazed. “So semiconductors can be switched between ‘conducting’ and ‘not conducting’?” “Exactly!” said Max. “That’s what makes transistors work — tiny switches that can turn on and off billions of times per second. That’s how I think and make decisions!”
Sammy looked at the circuit board. “So the copper traces are conductors, the green board is an insulator, and all those tiny chips are semiconductors?” “You got it!” cheered the whole Sensor Squad. “Every IoT device uses all three materials working together!”
13.5.3 Key Words for Kids
| Word | What It Means |
|---|---|
| Conductor | A material that lets electricity flow easily (like copper wire) |
| Insulator | A material that blocks electricity (like rubber or plastic) |
| Semiconductor | A material that can switch between conducting and blocking electricity |
| Silicon | The most common semiconductor material, found in every computer chip |
| Doping | Adding special ingredients to silicon to control how it conducts electricity |
| Circuit Board | The green board inside electronics where copper conductors connect all the chips |
Context: You need to add capacitors to your IoT circuit. Choosing the wrong type can cause circuit failure, noise issues, or premature component failure. Use this framework to select the optimal capacitor for each application.
Start with the job, because “add a capacitor” is not yet a specification. A decoupling part must supply fast local current at an IC pin; bulk storage must support a slower rail transient; an ADC reference filter must preserve low noise and settling behaviour; and a timing capacitor must keep its value across voltage and temperature. Those roles lead to different acceptable dielectric, capacitance, equivalent series resistance, leakage, tolerance, and package choices.
Next establish the electrical envelope. Record the normal and transient voltage, ambient and self-heating temperature, ripple current or pulse demand, useful frequency range, and required lifetime. Nominal capacitance alone is insufficient: a high-value ceramic can lose substantial effective capacitance under DC bias, an electrolytic is polarised and ages with temperature and ripple, and a film or C0G/NP0 part may be physically larger or unavailable at the required value. Derating is therefore a reasoned margin based on the technology and vendor data, not a universal substitute for checking the datasheet.
Finally connect the part to placement. High-frequency decoupling works only when the current loop is short and low-inductance; bulk capacitance can sit farther from the IC but must still be close enough to the load it supports. Regulator input and output capacitors must satisfy the regulator manufacturer’s stability conditions. The tables and checklist below summarise these decisions after the explanation; use them to build a component record, then verify the selected manufacturer’s voltage-bias, ESR, ripple, tolerance, and lifetime curves.
Before comparing dielectric families, inspect Figure 13.6 to connect a physical part with the first row of the selection table. The photograph helps with recognition, but it does not reveal the capacitance, tolerance, dielectric class, or voltage rating.
In Figure 13.6, begin with the two radial leads, then the ceramic disc body, and finally the absence of a polarity stripe. That distinguishes this non-polarised physical form from common aluminium electrolytics, but the printed marking and datasheet are still needed to identify value and dielectric behaviour. This carries visual recognition into the electrical-envelope and placement checks below.
| Capacitor Type | Capacitance Range | Key Properties | Typical Applications | Cost | Pitfalls |
|---|---|---|---|---|---|
| Ceramic (MLCC) | 1pF - 100µF | Low ESR, high frequency response, temperature-stable (X7R), small size | Decoupling, bypassing, high-frequency filtering | Low ($0.05-$0.50) | Voltage coefficient (capacitance drops at rated voltage), microphonic (vibration sensitivity) |
| Electrolytic (Aluminum) | 1µF - 10,000µF | High capacitance, polarized (must observe polarity), high ESR | Bulk power supply filtering, energy storage | Low ($0.10-$1) | Limited lifespan (2000-10,000 hrs), fails open/short, temperature-sensitive |
| Tantalum | 0.1µF - 1000µF | Stable, low leakage, polarized, lower ESR than aluminum | Power supply filtering where size matters, audio circuits | Medium ($0.50-$3) | Catastrophic failure mode (burns/explodes if reversed), surge-sensitive |
| Film (Polyester/Polypropylene) | 1nF - 10µF | Low loss, non-polarized, stable over temperature | Timing circuits, signal coupling, precision filters | Medium ($0.20-$2) | Large physical size limits use in compact IoT devices |
Decision Guide:
| Application | Recommended Capacitor | What to Check |
|---|---|---|
| Decoupling MCU/IC power pins | 100nF X7R or X5R ceramic MLCC | Place within 5mm of the power pin; add 10µF ceramic for local bulk storage |
| Power supply bulk storage | 100µF-1000µF electrolytic | Place in parallel with 100nF ceramic; check ESR, ripple current, and lifetime |
| ADC reference filtering | 1-10µF polypropylene film, or high-quality X7R ceramic | Check voltage coefficient and noise requirements |
| Precision timing circuit | Film capacitor, or C0G/NP0 ceramic for best stability | Check tolerance and temperature coefficient |
| Energy storage | 1000µF+ electrolytic, or supercapacitor for very high values | Consider series resistance and leakage current |
Critical Selection Criteria:
1. Voltage Rating:
- Rule: Always use ≥2× operating voltage
- Example: 5V circuit → use 10V-rated minimum (16V for margin)
- Why: Capacitance drops near rated voltage (ceramic), lifetime extends with derating
2. Temperature Coefficient:
- C0G/NP0: ±30ppm/°C (best stability, small values)
- X7R: ±15% over -55°C to +125°C (good stability, larger values)
- Y5V: +22% to -82% over -30°C to +85°C (worst, avoid for critical circuits)
- Choose: X7R minimum for IoT (C0G for precision timing)
3. ESR (Equivalent Series Resistance):
- Low ESR needed: Decoupling (ceramic), switching regulators
- High ESR OK: Bulk filtering with aluminum electrolytics
- Check datasheet: For >100mA ripple current, verify ESR won’t cause excessive heating
4. Physical Size:
- 0805 ceramic: 2mm × 1.25mm (hand-solderable, common values)
- 1206 ceramic: 3.2mm × 1.6mm (easier for beginners)
- Electrolytic radial: 5-10mm diameter (through-hole, large values)
- Design tip: Reserve space on PCB BEFORE finalizing component selection
Worked Example: ESP32 Power Supply Design
Requirements:
- Input: 5V USB
- Output: 3.3V @ 500mA max (ESP32 + peripherals)
- LDO regulator: AMS1117-3.3
Capacitor selection:
1. Input decoupling (5V rail):
- C1: 100nF X7R ceramic (high frequency noise)
- Voltage: 10V rating (2× safety margin)
- Package: 0805
- C2: 10µF X7R ceramic (bulk storage near regulator)
- Voltage: 10V rating
- Package: 1206 (larger for higher capacitance)
2. Output decoupling (3.3V rail):
- C3: 100nF X7R ceramic (high frequency)
- Voltage: 6.3V rating
- Package: 0805
- C4: 10µF X7R ceramic (bulk storage)
- Voltage: 6.3V rating
- Package: 1206
- C5: 100µF aluminum electrolytic (large transient currents)
- Voltage: 6.3V rating minimum
- Low ESR type (<100mΩ) for 500mA load switching
3. ESP32 VCC pins (near chip):
- C6-C9: 100nF X7R ceramic (one per VCC pin)
- Voltage: 6.3V rating
- Package: 0805
- Critical: Place within 5mm of each VCC pin
Why this combination?
- Ceramic caps handle MHz-GHz switching noise from ESP32 digital logic
- Electrolytic cap provides energy reservoir for Wi-Fi TX bursts (200mA for 1-2s)
- Multiple 100nF caps (one per power pin) minimize inductance
- Cost: ~$1 total for all capacitors
Common Mistakes to Avoid:
- Wrong: using Y5V ceramic for power supply filtering. Capacitance can drop 80% with temperature and voltage.
- Wrong: forgetting voltage derating. Electrolytic capacitors fail early when run at their rated voltage.
- Wrong: reversing electrolytic or tantalum polarity. This creates an explosion and fire hazard.
- Wrong: using one large capacitor instead of multiple small capacitors. High inductance hurts high-frequency response.
- Wrong: ignoring ESR for high ripple current. The capacitor overheats and fails.
- Wrong: omitting the bulk capacitor on a switching regulator output. The output voltage can become unstable.
Quick Reference Card:
| Need | Use This | Value | Voltage | Notes |
|---|---|---|---|---|
| MCU decoupling | Ceramic X7R | 100nF | 2× Vcc | One per VCC pin |
| Bulk power | Electrolytic | 100-1000µF | 2× Vcc | Low ESR type |
| ADC reference | Ceramic C0G or Film | 1-10µF | 2× Vref | Ultra-low noise |
| Timing (RC) | Ceramic C0G or Film | Per calculation | 2× Vmax | Stable over temp |
| Energy storage | Electrolytic or Supercap | >1000µF | 2× Vcc | Check leakage current |
Default Recommendation: For general IoT power supply decoupling, use 100nF ceramic (X7R, 0805) + 10µF ceramic (X7R, 1206) + 100µF electrolytic (low ESR) in parallel. This combination handles high-frequency noise, moderate transients, and large current bursts — covering 90% of IoT applications.
13.6 Continue to the Next Part
Carry this evidence into Components in Systems: Passive and Active Contracts, which begins with Passive vs Active Components.