Resistor Types & Ratings: A Practical Guide for PCB Designers

Resistors are simple passive components, but choosing the right type and rating is critical for reliable designs. They control current, shape voltages, and protect circuits, making them essential in nearly every electronic design.

What Resistors Actually Do

Resistors oppose current flow and stabilize voltages. Common uses include:

• Biasing transistors
• Setting amplifier gain
• Pull-up and pull-down networks
• Current sensing
• Protecting components from overcurrent

Resistors fall into two main categories:

• Linear resistors: Obey Ohm’s law with near-linear voltage-current relationship.
• Non-linear resistors: Resistance changes with voltage, temperature, or light; used as sensors or protectors.

Core Resistor Types for Modern Designs

Fixed Resistors (Through-Hole & SMD)

Fixed resistors have constant nominal resistance and are standard for most circuits.

• Carbon composition: High noise, poor tolerance; mostly for legacy or surge applications.
• Carbon film: Moderate tolerance, low cost; general-purpose electronics.
• Metal film: Low noise, tight tolerance, good stability; precision analog, instrumentation.
• Wirewound: High power, low temp coefficient; used in power and current-sense applications.

SMD Resistors

Compact chips in sizes like 1206, 0805, 0603, 0402, and 0201. Key points:

• Size correlates with power and voltage rating.
• Thick-film SMD: cost-effective for general logic.
• Thin-film SMD: precise, stable for analog and RF.
• Standardize on 1–2 sizes per design for assembly and procurement simplicity.

Variable & Adjustable Resistors

• Potentiometers (pots): Three-terminal adjustable voltage dividers.
• Rheostats: Two-terminal high-power current control.
• Trimmers: PCB-mounted, factory calibration.

Non-Linear Specialty Resistors

• Thermistors (NTC/PTC): Temperature-dependent, used in sensing and inrush limiting.
• Varistors (MOVs/VDRs): Voltage-dependent, surge protection.
• LDRs (photoresistors): Light-dependent, simple sensing circuits.

Key Resistor Ratings

Resistance Value & Tolerance

Resistors follow E-series (E6, E12, E24, E96). Tolerance indicates allowable deviation:

• ±5%: Adequate for pull-ups, pull-downs, LED current limiting.
• ±1% or better: Needed for precision analog, references, and sensing.

Power Rating

Defines maximum continuous dissipation. Use at least 2× expected power for safety. Common ratings:

• Leaded: 0.25 W, 0.5 W, 1 W+
• SMD: 1/16 W, 1/10 W, 1/8 W, 1/4 W+

Voltage & Temperature Considerations

• Voltage rating: max continuous working voltage.
• Temperature coefficient (TCR): ppm/°C; lower is better for precision.
• Environment: humidity, vibration, and contamination can affect performance; derate appropriately.

Practical Selection Guidelines

• Digital logic & general biasing: Thick-film SMD or carbon/metal film; ±5% tolerance; 1/10–1/8 W SMD or 0.25 W through-hole.
• Precision analog & instrumentation: Thin-film SMD or metal film; ±1% or better, low TCR; verify power to avoid drift.
• Power supplies & current sense: Wirewound or dedicated current-sense resistors; at least 2× safety margin; NTC thermistors for inrush, MOVs for surge.
• Sensing & protection: Thermistors, LDRs, varistors selected for range and protection needs rather than strict tolerance.

Visual & Resource Suggestions

• Table comparing carbon film, metal film, wirewound, thick-film SMD, thin-film SMD: cost, tolerance, noise, power rating.
• Diagram showing SMD sizes on a PCB.
• Chart of power rating vs package size.
• Internal links: "How to Read Resistor Color Codes," "PCB Layout Tips for High-Power Components."
• External references: distributor design guides (DigiKey, RS Components).

Conclusion

Choosing the right resistor type and rating early prevents failures, noise, and redesigns. Default to thin- or thick-film SMD for most designs, and layer in wirewound, thermistors, and varistors only where needed. Standardize a selection checklist for repeatable, reliable designs and simplify procurement and stocking.