Introduction
Resistors are one of the most fundamental components used in printed circuit boards (PCBs). They are passive devices that resist the flow of electrical current in a circuit. PCB Resistors allow circuits to be controlled, divided, stabilized, interfaced, and more. They are available in various resistance values, sizes, power ratings, and tolerances to suit different circuit requirements.
The resistance value (measured in ohms) represents the amount of opposition to the current flow. Resistors obey Ohm’s law, which states:
V = IR
Where V is voltage in volts, I is current in amps, R is resistance in ohms
So for a given voltage, higher resistance leads to less current flow. Resistors convert electrical energy into heat, which must be dissipated. They are purely passive devices that cannot amplify or increase power.
Why Use Resistors in PCBs?
Here are some of the main applications of resistors in PCB circuits:
Current Limiting – Resistors prevent damage by limiting current to delicate LEDs, integrated circuits, and other components.
Voltage Division – Resistors connected in series form voltage dividers to obtain lower voltages from the supply.
Pull Up/Down – Used with digital logic circuits to ensure a known logic voltage level when an input is not driven.
Biasing – Provide proper DC bias voltage or current for transistor amplifiers and other analog circuits.
Feedback – Critical for op amps, ADCs, DACs, and other analog circuits to control gain and response.
Pulse Shaping – Combine with capacitors to form RC timing circuits for pulse generation.
ESD Protection – Protect against electrostatic discharge damage.
Heaters – Wirewound power resistors convert electrical energy into heat.
In summary, resistors allow electronic circuit adjustment, stabilization, and protection. Modern PCBs would not function without them.
Types of PCB Resistors
An enormous range of resistor types are available to suit varied applications. They can be categorized based on their construction, material composition, size, and other factors.
Here are some of the most common types of PCB resistors:
Carbon Composition Resistors
Carbon composition resistors are the oldest type of resistor. They are made from a solid cylindrical rod of ceramic and carbon particles. Key characteristics of carbon composition resistors:
- Low cost but poor tolerance (±10%) due to temperature effects
- Used where precision is not critical
- Available in 1/4 to 2-watt power ratings
- Commercial grade stability
- Noisy compared to other types
- Prone to voltage coefficient issues
Carbon Film Resistors
Carbon film resistors have a thin carbon deposition film on a ceramic rod. They demonstrate better performance than carbon composition:
- Improved tolerance (±5%) and stability
- Low-temperature coefficient of resistance
- Available in 1/8 to 1-watt power ratings
- Very low inductance and capacitance
- Low noise operation
- Withstand voltage surges better
Metal Film Resistors
For better precision and stability, metal film resistors are used. They are constructed by depositing a thin nickel-chrome or tin oxide film onto a ceramic substrate. Key features of metal film resistors:
- Much better tolerance (±1%) and temperature stability
- Lower noise than carbon composition and carbon film
- Available from 1/10 watt up to 1-watt ratings
- Capable of handling higher peak voltages
- Low voltage coefficient for good stability
Metal Oxide Film Resistors
Instead of metal films, metal oxide film resistors utilize metal oxide materials like tin oxide. This provides further improvements:
- Excellent tolerance down to ±0.5%
- Extremely stable over time and temperature
- Low thermal EMF for reduced noise
- High-frequency capabilities up to GHz range
- High reliability and pulse handling capacity
- Used in high-precision test equipment
Wirewound Resistors
Wirewound resistors obtain resistance by winding resistive wire around a ceramic core. Key characteristics include:
- High power handling, up to 100 watts or more
- Shallow resistance values are achievable
- Inductive, unsuitable for high-frequency usage
- Commercial tolerance around ±5%
- Often enclosed in aluminum shells or ceramic casings
- Prone to thermal EMF effects under shock/vibration
Foil Resistors
For shallow resistance values below 1 ohm, foil resistors are used. They consist of a thin foil of metal alloy bonded to a ceramic substrate.
Features include:
- Resistance values as low as milliohms
- High precision of ±0.005% tolerance available
- Low TCR for excellent stability
- High current carrying capacity
- Fully welded construction with no spiraled wires
- Often found in aerospace applications
Ceramic Composition Resistors
Ceramic composition resistors are made from a ruthenium oxide glaze layer fused onto a ceramic body. They deliver superb performance:
- Extremely tight tolerance down to ±0.02%
- Ultra-low temperature coefficient
- High stability over time and under environmental stress
- Withstand repeated thermal shocks without drift
- Used in high-reliability aerospace and military applications
Fusible Resistors
Fusible power resistors contain a built-in fusible link that will burn open in case of excessive current flow. Under normal conditions, they behave like any other fixed resistor. The key features of fusible resistors include the following:
- Provide overload protection to circuits
- Available in 1 to 5-watt power ratings
- The fuse link opens safely before the main resistor body overheats
- It can be manually reset by replacing the fuse wire
- Often used in power supply and power distribution circuits
Trimmer Resistors
Trimmer resistors have an adjustable resistance value obtained by mechanical means – either a screw or sliding contact. This allows “fine-tuning” of the resistance during circuit calibration and testing.
Properties of trimmer resistors:
- Variable over a limited range, typically 20-30%
- Set to exact value using a screwdriver or small tuning tool
- Sealed construction protects from environmental contamination
- Available in through-hole and SMD packages
- Widely used in analog or precision circuit trimming
Thermistors
Thermistors are resistors that utilize semiconductor materials with a significant temperature coefficient of resistance. It means their resistance value varies significantly with temperature changes.
Thermistor characteristics:
- Used for temperature measurement, compensation, and control
- NTC types have a resistance that decreases with rising temperature
- PTC types increase resistance as temperature increases
- Response time ranges from seconds to minutes
- The resistance versus temperature relationship is highly nonlinear
Varistors
Varistors are voltage-dependent resistors made from specially formulated metal oxide materials.
Their essential function is transient voltage suppression:
- Resistance decreases sharply with applied voltage
- Absorb and limit transient voltage spikes during ESD or surges
- Used to protect sensitive electronics from damage
- Bidirectional operation for AC or DC circuits
- Energy handling capacity specified by Joule rating
- Response time in nanoseconds
Photoresistors
Photoresistors, also known as light-dependent resistors (LDR), have a resistance that decreases when exposed to light. They are constructed from semiconductor materials like cadmium selenide or cadmium sulfide. Key features:
- Resistance can change by orders of magnitude between dark and lit conditions
- Excellent sensitivity to visible and infrared wavelengths
- Used in electronics using light sensing and activation
- Slow response time makes them unsuitable for fast optical signals
- The package must allow light exposure to the photo-sensitive area
Magneto resistors
Magneto resistors are resistors made from materials whose resistance changes with a magnetic field. Some types include:
- Permalloy magneto resistors using nickel-iron films
- Gallium-magnesium-arsenide semiconductors
- Giant magnetoresistive multilayer devices
- Tunnel magneto resistors utilizing magnetic tunnel junctions
Properties that make magneto resistors useful include:
- Detect magnetic field strength and orientation
- Provide electrical isolation from sensed media
- Rugged and operate in the wide temperature range
- Used in magnetic field sensors, current sensors, isolators
- Can exhibit high sensitivity to small magnetic changes
SMD Resistors
Surface Mount Devices (SMD) resistors are designed for surface mount PCB assemblies rather than through hole mounting. They are much smaller and allow very high component density on boards.
Some characteristics of SMD resistors:
- Rectangular, square, or oval-shaped ceramic body with terminals on the bottom
- Terminations are thick film, solder-coated nickel bands
- Marked with a numerical resistance code instead of color bands
- The resistance range is typically 10 ohm to 22 Mohm
- Power rating from 0.05 watt to 1 watt
- Tolerance as low as ±0.1%
- Sizes specified in imperial (01005) or metric (0402) codes
The tiny size of SMD resistors enables fitting more components into increasingly smaller PCBs. Automated assembly and cost savings are additional benefits over through-hole resistors.
Resistor Marking Codes
Since resistors are tiny, their resistance values and tolerances must be indicated using color bands, alphanumeric codes, or other markings printed directly on the body.
Color Bands
The color band system has been used since the 1920s to denote resistor values.
- Three-band resistors have two significant color bands for the numerical resistance and a third multiplier band
- Four band resistors add a fourth tolerance color band for greater precision
- Five-band and six-band resistors also exist for high stability requirements
SMD Marking Codes
Due to their small size, SMD resistors use compact numerical resistance and multiplier codes instead of color bands.
Some typical schemes are:
- Three-digit code – first two digits denote resistance, 3rd digit is a multiplier
- Four-digit code – first three digits for resistance, 4th digit is a multiplier
- EIA-96 system – 2 digits for resistance, letter denotes multiplier
So 472 would equal 47 x 100 = 4.7KΩ. And 15R2 = 15 x 0.01 = 150 ohm.
Laser trimming of the resistive layer is also used to achieve very tight tolerance SMD resistor values.
How to Select the Right Resistor
Choosing suitable resistors for a particular PCB application requires considering several important factors:
Resistance Value
The optimal resistance depends on the circuit design requirements and is calculated using Ohm’s law. Standard EIA decade values are commonly chosen.
Tolerance
Tolerance expresses the accuracy of the resistor’s nominal value. Tighter tolerances of ±1% or better are used for precision circuits. Larger tolerances around ±5% or ±10% are acceptable for non-critical applications to reduce cost.
Power Rating
The power rating must exceed the maximum expected power dissipation through the resistor. Power rating depends on physical size. 1/4 watt (or 250mW) can handle most low-power circuits. Higher wattage may be needed for power supplies or heaters.
Temperature Coefficient
The inherent resistance change per degree of temperature change. Lower coefficients provide better stability in precision circuits. The maximum operating temperature must also be considered.
Voltage Rating
Must exceed maximum voltage dropped across the resistor with a safe margin. Higher voltage ratings are required for power supplies and other high-voltage applications.
Size
Physical dimensions can dictate SMD vs through-hole forms. Height restrictions may apply on dense boards. High-power resistors are larger than low-power ones.
Noise
Low-noise film resistors should be used for high-gain analog circuits and RF/data transmission lines. Wirewounds exhibit some inductance that produces noise.
Response Time
Choose resistors with the appropriate transient response for timing circuits, pulsed operation, and high-frequency applications.
With these factors considered, the optimal PCB resistor type and specifications can be selected.
PCB Resistor Applications
Resistors find ubiquitous use in all kinds of printed circuit board applications, including:
Voltage Dividers
Two or more resistors connected in series across a voltage source form a simple but helpful voltage divider circuit. This generates a lower voltage tap from a higher supply voltage.
Voltage dividers are employed in power supplies, as bias networks for transistor amplifiers, interfaces to ADCs, and many other places where a fraction of a voltage rail is needed. Carefully choosing the proper resistor values results in the desired voltage division ratio.
Current Limiting
Connecting a resistor in series with LEDs, integrated circuits, and other delicate electronic components limits the current flowing through them. The resistor value is calculated to drop excess voltage beyond what the device requires. It prevents damage due to inrush currents, overloads, or short circuits.
Pull Up/Pull Down
Pull-up and pull-down resistors are connected between an integrated circuit logic output and the positive or negative power rail (ground). It ensures a known logic voltage level when the IC output impedes high.
Pull-up/down resistors provide a defined default condition that prevents logic errors and false triggering. They are critical for the reliable operation of all digital logic families.
Biasing Networks
Providing proper DC bias voltage or current levels through resistors allows transistor amplifiers and analog integrated circuits like op-amps to operate in their linear region. Bias resistors help stabilize these active devices at optimal quiescent conditions to meet circuit design requirements.
Feedback Circuits
Resistors are indispensable components within feedback loops for operational amplifiers, ADCs, DACs, and other analog circuits. They control the gain, frequency response, stability, and output impedance per the intended function.
Timing Circuits
Combining resistors with capacitors forms RC timing circuits that generate known time constants. These are used in oscillators, timers, pulse generators, and other circuits requiring delays, waveform shaping, or frequency control.
Termination
Resistors are commonly used to terminate transmission lines and cabling for signals like RF, video, digital data, USB, etc. The termination resistor matches line impedance to minimize signal reflections. It allows maximum power transfer for improved signal integrity.
Surge Protection
Varistors and fusible resistors help protect delicate semiconductor devices from high-voltage transients. They clamp or absorb dangerous voltage spikes before they can damage ICs, chips, and other components. Resistors also help limit current flow during electrostatic discharge (ESD) events.
Temperature Compensation
Thermistors measure temperature changes and appropriately alter their resistance to compensate for variations in other components caused by heating effects. This maintains circuit stability and performance.
Voltage Regulation
In combination with Zener diodes, transistors, and other active components, resistors help build stable DC voltage regulator circuits. The resistors enable adjustable output voltages, limit current flow, and dissipate excess power from the supply.
Filtering
Resistors are combined with capacitors to form low-pass, high-pass, band-pass, and band-stop analog filters. These filter out undesired signals and pass the frequencies of interest. Filtering helps remove noise, harmonics, and interference.
Bleeders
High-value power resistors slowly discharge capacitors after the equipment is powered down. Bleeders safely drain stored energy to prevent shock hazards. They are commonly found in power supplies, defibrillators, and industrial electronics.
Divided Networks
Series-parallel resistor ladder networks provide multiple precise tap voltages from a single reference. It creates a resistive divider for applications like A/D converters, calibration, voltage scaling, etc.
By utilizing the variety of roles that resistors can play, PCB designers leverage them to build robust, reliable, and efficient electronic circuits.
PCB Resistor Soldering
Leaded resistors are inserted through PCB holes and soldered in place. It securely fastens the component and creates robust electrical connections. Some tips for hand soldering resistors on boards:
- Use appropriate soldering iron temperature (650-700°F for leaded components)
- Apply soldering flux to component leads and PCB pads to aid in wetting
- Ensure resistor leads are fully inserted through board holes before soldering
- Heat both the resistor
Automated Pick and Place Assembly
High-volume PCB manufacturing utilizes specialized pick-and-place machines for the SMD population. It automates the assembly process for incredible speed, precision, and reliability.
Some features include:
- High-speed vacuum nozzles pick components from feeders or tapes
- Machines place thousands of parts per hour with an accuracy of up to 0.01mm
- On-head optics align components and verify the part presence
- Eliminates human errors and variability in placement
- Reduces manufacturing defects compared to manual assembly
- Custom nozzle, feeder, and tape sizes for any component package
- The software integrates with CAD data for program generation
- Reduces overall assembly cost and time to market
Advanced pick-and-place systems enable complex, compact PCBs’ extremely dense component loading.
Embedded Thin Film Resistors
Due to parasitic inductance and capacitance issues, discrete resistors are unsuitable for specialized applications like RF and microwave boards. Instead, embedded thin film resistors can be laser trimmed directly onto the inner board layers.
Benefits include:
- Tight resistance tolerance down to ±1%
- Excellent high-frequency characteristics beyond the GHz range
- Stable resistance over temperature fluctuations
- A thin profile does not take up vertical space on boards
- Compatible with standard PCB laminates like FR-4
- Suitable for both digital and high-speed analog signals
- Improves signal integrity by eliminating discrete components
- Reduces assembly cost by integrating passive parts in the PCB
Embedded resistors continue advancing PCB miniaturization and performance, especially for high-frequency needs.
PCB Resistor Testing
Verifying the correctness of resistors on assembled boards is critical to reliability. Some essential testing methods include:
- In-circuit testing checks resistors while installed on boards
- Continuity testing to confirm connections and detect open circuits
- Voltage measurements across resistors validate proper voltage drops
- Automated optical inspection looks for visible defects and anomalies
- X-ray inspection reveals hidden issues like cracks or voids
- Stress testing under temperature cycling, vibration, and power cycling qualification
- Applying ESD strikes checks robustness and surge handling
Thorough testing of resistors at the bare board and assembled board stages catches faults before shipment. It prevents premature field failures.
PCB Resistor Failure Modes
Despite their simplicity, resistors can still exhibit failures from factors like overstress, contamination, wear out, or improper design. Some standard failure modes to be aware of:
- Overheating due to excessive power dissipation
- Open circuits due to lead breakage or internal fuse blow
- Resistance value shift from thermal or mechanical stress
- Intermittents from cracked resistor bodies or elements
- Shorting or arcing from conductive debris
- Soldering damage like pad lifting or heat shock
- Parameter drift over time and environmental exposure
- Mechanical shock/vibration inducing open contacts
- Flaking of resistor film material leads to increased resistance
- Resistor power rating insufficient for application needs
Understanding how resistors can fail and accounting for reliability risks during design can reduce potential field failures.
Conclusion
Resistors are indispensable components that enable the proper functioning of almost every electronic printed circuit board, thanks to their ability to control current flow and voltage drops. From high-precision applications to basic pull-up needs, resistors empower PCBs through their versatility, low cost, and reliability.
Electrical engineers can fully leverage these fundamental passive devices to build high-performance, long-lasting circuit boards, and systems by selecting appropriate resistor types, sizes, and characteristics using the guidelines provided. With ongoing advances in materials and fabrication, resistors will remain a pillar of electronics designs well into the future.
Frequently Asked Questions (FAQs)
What are pull-up and pull-down resistors?
Pull-up and pull-down resistors are connected to digital logic outputs to ensure a known voltage level when the output is in a high impedance state. Pull-ups connect to positive supply while pull-downs connect to the ground.
What is a variable resistor?
Variable resistors have an adjustable resistance value. Examples are potentiometers and rheostats with a sliding contact. Trimming resistors can also be varied by mechanical screw turning or laser trimming.
How do you read resistor color bands?
The first two color bands give resistor digits, the third is multiplied, fourth is tolerance. For example, red-red-orange-gold equals 2,2 x 1000 = 2.2K ohm with 5% tolerance.
Why use surface mount resistors vs through holes?
SMD resistors take up less space, allow higher component density, and support automated assembly. But through holes are easier for prototyping and modifications.
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