Introduction

When designing a printed circuit board (PCB), selecting the right resistor and capacitor components is critical for proper functioning of the circuit. Resistors limit current flow and capacitors store electrical charge. Choosing the wrong resistor or capacitor values can lead to timing issues, noise, circuit malfunction and damage to components. This article provides a guide on key considerations when selecting resistors and capacitors for your PCB design.

Resistor Selection

Resistors are passive components that restrict current flow in a circuit. Some key factors to consider when selecting resistor values include:

Desired Resistance Value

• Calculate the required resistance value based on Ohm’s law (R = V/I) for the specific application. For example, limiting current through an LED or setting gain in an amplifier stage.

Power Rating

• Choose a resistor wattage rating sufficient to handle the maximum expected power dissipation (P = I2R). Use higher wattage for high power applications to avoid overheating.

Tolerance

• Tolerance is the allowable deviation from the stated resistance. Tighter tolerances like 1% are used when accuracy is critical. Looser tolerances like 5% are acceptable for less critical circuits to allow lower cost components.

Temperature Coefficient

• The temperature coefficient indicates the resistance change with temperature. Use low drift or temperature compensated resistors for precision circuits. Standard metal film resistors offer adequate stability for most applications.

Size

• Consider physical size limitations of the PCB. Larger wattage resistors require more board space. Smaller chip resistors save space but have lower power handling.

Termination

• Through-hole or surface mount resistors have tradeoffs. Through-hole allow easier replacement while SMD save space. Some circuits may require one style over the other.

Ease of Sourcing

• Using common resistor values that are easily obtained from suppliers simplifies procurement and reduces costs. Checking inventory and supplier availability should factor into resistor selection.

Capacitor Selection

Capacitors store and release electrical charge in circuits. Key considerations for selecting capacitors include:

Capacitance Value

• Determine the capacitance needed for timing circuits, filtering signals or supplying power. Analyze the circuit to calculate the ideal capacitance value(s).

Voltage Rating

• The voltage rating must exceed the maximum voltage across the capacitor terminals. This provides a safety margin and prevents dielectric breakdown.

Dielectric Type

• Common dielectrics include ceramic, tantalum, aluminum, polymer and mica. Different dielectric properties suit different applications based on size, ESR, ESL, frequency response, etc.

Temperature Characteristics

• The operating temperature range determines capacitor dielectric and construction style. High performance circuits require tight capacitance tolerance over temperature.

Frequency Response

• Capacitor impedance varies with frequency. Different dielectrics have different frequency characteristics based on equivalent series resistance (ESR) and inductance (ESL).

Physical Size

• Larger capacitance values require bigger case sizes. Chip capacitors occupy less space but lower capacitance values. Consider height restrictions and spacing requirements.

Mounting Style

• Through-hole or surface mount capacitors have tradeoffs. SMD capacitors reduce assembly costs. Through-hole allow easier replacement or prototyping.

Reliability Requirements

• Electrolytic and tantalum capacitors age and can fail over time.Consult manufacturer datasheets for expected lifespan under operating conditions.

Cost Considerations

• Electrolytic and ceramic capacitors offer low cost general purpose options. High stability or tight tolerance capacitors can be considerably more expensive.

Key Resistor and Capacitor Specifications

Here are some key specifications to evaluate when selecting resistors and capacitors:

Resistors:

• Resistance Value (Ohms)
• Tolerance (%)
• Power Rating (Watts)
• Temperature Coefficient (ppm/°C)
• Voltage Rating (Volts)
• Operating Temperature Range (°C)
• Size / Package Style
• Termination (through-hole or SMD)

Capacitors:

• Capacitance Value (Farads)
• Tolerance (%)
• Voltage Rating (Volts)
• Dielectric Material
• Temperature Coefficient (ppm/°C)
• Frequency Response
• ESR (Equivalent Series Resistance)
• ESL (Equivalent Series Inductance)
• Leakage Current
• Operating Temperature Range (°C)
• Lifetime Expectancy
• Size / Package Style
• Termination (through-hole or SMD)

Carefully checking these parameters against application requirements helps ensure you pick the right resistor and capacitor for your design.

Resistor and Capacitor Ratings

Understanding resistor and capacitor ratings is crucial for designing robust, reliable circuits. Key ratings include:

Power Ratings

Resistors must be rated to handle maximum expected power dissipation without overheating:

• 1/4 Watt – Low power signal circuits
• 1/2 Watt – General purpose circuits
• 1 Watt – Higher power, often wirewound
• 2+ Watts – Power resistors, wirewound

Capacitors do not dissipate power continuously like resistors, so power ratings relate more to momentary surge handling.

Voltage Ratings

Voltage ratings indicate the maximum working voltage that can be applied without risk of shorting or arcing through the dielectric. This includes considerations for transients and AC peak voltages, not just DC supply level. Minimum derating rules are recommended by manufacturers.

Temperature Ratings

Check the operating and storage temperature ranges against application environment and soldering profiles. Extreme temperature cycling can degrade resistors and capacitors over time.

Failure Rate

MIL-spec rating indicates quality level and acceptable failure rates for military systems. Consumer-grade capacitors may only guarantee 1000 hours lifetime, while MIL-spec rates components for at least 10,000 hours utilization.

Matching component ratings to circuit requirements prevents failures and ensures reliable operation over the product lifetime. Check manufacturer datasheets for detailed rating specifications.

Terminology

Here are some common terminology used when selecting resistors and capacitors:

Resistors:

• Tolerance – Allowed deviation from nominal resistance value, e.g. ±1%, ±5%.
• TCR – Temperature coefficient of resistance, measured in ppm/°C.
• Noise – Unwanted fluctuations in resistor voltage, higher in large values.
• TSP – Temperature sensitive parameter that modifies resistance with temperature changes.

Capacitors:

• Dielectric – Insulating material separating capacitor plates, affects capacitance and frequency response.
• Leakage – Small current that flows when capacitor is not actively powered.
• ESR – Equivalent series resistance, contributes to power losses.
• ESL – Equivalent series inductance, causes impedance at higher frequencies.
• Ripple Current – Maximum allowable AC current before damage occurs.

General Terms:

• Derating – Operating below full component ratings for reliability.
• SMD – Surface mount device, for automated PCB assembly.
• Polarization – Applying correct polarity voltage across polarized capacitors.
• Self-Resonance – Capacitors behave inductively above self-resonant frequency point.

Understanding this terminology helps properly interpret component datasheets and make informed selections.

Optimizing PCB Layout

Careful arrangement of resistors and capacitors during layout can improve circuit performance. Some tips include:

• Place decoupling capacitors close to integrated circuits to reduce power supply noise.
• Keep high frequency signal paths short to minimize stray inductance and resistance.
• Route traces between resistors and capacitors directly without vias where possible.
• Group associated resistors and capacitors together as functionally cohesive units.
• Orient components consistently to aid visual inspection and troubleshooting.
• Consider options like capacitor arrays to reduce part count and assembly costs.
• Provide sufficient clearance around high power resistors and heat generating components.
• Use larger pad sizes or thermal relief connections for high current traces.
• Keep high voltage nodes safely separated from potentiometers and user controls.

Following good layout practices makes assembly more straightforward and results in circuits that work as intended.

Component Packages

Resistors and capacitors come in a variety of package styles suitable for through-hole or surface mount assembly. Common types include:

Resistors:

• Axial Lead – Cylindrical leaded components for through-hole boards. Easy to inspect and replace.
• Chip Array – Multiple flat chip resistors in a single package. Saves space.
• 1206 Size – Standard metric SMD size (1206 = 0.12″ x 0.06″). Very small but easy to assemble.
• SOT-23 – Common 3 pin SMD transistor package used for some resistors.

Capacitors:

• Radial Lead – Cylindrical leaded capacitors for through-hole mounting.
• SMD Chip – Small rectangular ceramic SMD capacitors. Available in many sizes.
• SMD Tantalum – Compact SMD electrolytic capacitors with high capacitance values.
• Dipped – Axial lead capacitors with epoxy coating for protection and stability.
• Supercapacitors – High capacity capacitors for energy storage, large case sizes.

Component Sourcing

Choosing the right suppliers and stocking components helps avoid issues procuring parts for production. Some best practices include:

• Select reputable authorized distributors over gray market or counterfeit part concerns.
• Check manufacturer availability and lead times before specifying long lead-time or obsolete components.
• Qualify multiple suppliers to reduce sourcing risks.
• Review supplier inventory regularly and adjust stocking for demand changes.
• Plan for component lifecycle and develop alternatives for approaching end-of-life parts.
• Obtain reference design samples for pre-qualification prior to specifying components.
• Request manufacturer packaging and moisture sensitivity handling guidance.

Careful component sourcing practices prevent delays, minimize costs, and improve quality over the full product lifecycle.

Summary

Here are some key points when selecting resistors and capacitors for your PCB design:

• Choose resistor wattage, tolerance and TCR characteristics suitable for application requirements.
• Select capacitor dielectric, voltage rating, ESR and frequency response appropriate for the circuit.
• Verify component electrical and thermal ratings match expected operating conditions with margin.
• Follow good layout practices to optimize circuit performance.
• Standard package styles balance size, cost and assembly considerations.
• Identify quality component suppliers to meet sourcing needs and lifecycle requirements.

Proper selection of resistors and capacitors based on critical design parameters results in a robust, reliable circuit that performs its intended function.

Frequently Asked Questions

How do I calculate the power rating needed for a resistor?

Use the formula P=I2R, where I is the current through the resistor and R is the resistance value. Choose a resistor wattage rating that is at least 2-3 times higher than the calculated power to provide a safe margin.

When should I use a ceramic capacitor vs. an electrolytic capacitor?

Ceramic capacitors are preferred for higher frequencies due to low ESR and small sizes. Electrolytic capacitors have higher capacitance values for a given size but limited frequency response. Use electrolytic for large bulk capacitance and ceramics for bypass/decoupling.

How tight should resistor tolerance be for analog circuits versus digital logic?

Precision analog circuits require tight 1% or better tolerance to avoid signal errors. Digital logic can readily tolerate 5% resistors since noise margins are higher. Go with higher tolerance resistors when precision is not required to reduce costs.

What is the meaning of voltage coefficient for capacitors?

Voltage coefficient indicates how much the capacitance value changes with applied voltage. Capacitors with high voltage coefficients will shift value as voltage varies across them. Choose capacitors with low voltage coefficients for stable operation.

Why is thermal management important for high power resistors?

Resistors dissipate power as heat. Excessive temperature buildup can shift resistance value or damage the resistor. High power resistors should be located to promote airflow cooling and have larger pads/traces to dissipate heat into the PCB.