Overview
Resistors come in many types and are generally classified into three categories: fixed resistors, variable resistors, and special sensor-type resistors.
1. Fixed Resistors
Fixed resistors are manufactured from different materials and processes and can be grouped into several types. The choice of material and structure should be based on the circuit requirements. SMD resistors are the most commonly used type.
Nominal resistance
The value printed on the product, in ohm, kiloohm, or megaohm. Nominal resistance values should conform to standard E-series values multiplied by 10^n (n is an integer).
Tolerance
Tolerance is the maximum allowable deviation of the actual resistance from the nominal value and represents the precision of the product. Do not blindly pursue extremely tight resistor tolerances, because environmental influences can push values outside the nominal tolerance. Focus on reliability test metrics. Avoid selecting tolerance better than 0.1% for general use. Thick film resistors are typically used for tolerances of 1% and above; for better than 1% use thin film resistors.
Rated power
The maximum power the resistor can dissipate under specified ambient temperature and humidity, assuming no airflow, and under continuous load without damage or significant performance change. Select a rated power typically 1 to 2 times the power the resistor will dissipate in the circuit. Common ratings include 0.05W, 0.125W, 0.25W, 0.5W, 1W, 2W, 3W, 5W, 7W, and 10W.
Maximum working voltage
The maximum voltage across the resistor at which it will not overheat or suffer dielectric breakdown under long-term operation. Exceeding this voltage can cause internal arcing, noise, or damage.
Stability
Stability measures how much the resistance changes under external conditions such as temperature, humidity, voltage, time, and load type.
Temperature coefficient (TCR) indicates the relative resistance change per degree Celsius. Voltage coefficient indicates the relative resistance change per volt.
Rated operating temperature
Each resistor type has a specified ambient operating temperature range that should not be exceeded in use.
Resistors with low TCR are typically thin film. Carbon film and ceramic resistors often have negative TCR. For low TCR designs, 10 ppm/°C is a typical target. TCR varies significantly by material.
Selection recommendations for fixed resistors
- For high-frequency circuits, use non-wirewound resistors with low distributed inductance and capacitance, such as carbon film, metal film, or metal oxide film resistors.
- For low-noise, high-gain signal amplifier circuits, select low-noise resistors such as metal film, carbon film, or wirewound resistors. Avoid high-noise composite carbon or organic solid resistors.
- Wirewound resistors offer higher power ratings, low current noise, and high temperature tolerance but are larger in size. They are commonly used in low-frequency circuits, as current-limiting resistors, divider resistors, bleed resistors, or bias resistors for high-power transistors. Precision wirewound resistors are used in attenuators, resistance boxes, computers, and precision instruments.
- The resistor's rated power must meet the circuit's power requirements. Do not arbitrarily increase or decrease resistor power rating. For power-type resistor requirements, choose a rated power 1 to 2 times the actual circuit dissipation.
- Avoid selecting extreme edge specifications within a series, such as the maximum or minimum resistance value for that series.
- Under applied stress, resistance drift should remain within circuit requirements, and aging must be considered. Provide design margin, typically half of the allowed circuit variation. For example, if the circuit allows ±10% variation, select a resistor that varies within ±5%.
- Derating is an important method to improve reliability and lifespan. Power capability depends on package size. Thin film resistors have small power ratings, typically under 1W, and should be derated in use. Different resistor classes have different dielectric and self-healing mechanisms, so recommended derating levels vary, but a common practice is to operate at 0.6 times the rated stress and not exceed 0.75 times.
When ambient temperature exceeds 70°C, further derating from the baseline is recommended.
SMD resistor package size selection
SMD resistor body sizes come in nine common package sizes, each with different rated power. SMD capacitors use the same package size conventions as SMD resistors.
SMD resistor package codes use four digits. The first two digits denote length and the last two digits denote width. Two notations are common: imperial and metric. For example, 0603 in imperial indicates 0.06 inch length and 0.03 inch width; 1005 in metric indicates 1.0 mm length and 0.5 mm width. The industry convention is to use imperial notation. Current minimum is 0201 and maximum is 2512.
2. Variable Resistors
Variable resistors have three terminals: two fixed terminals and one wiper terminal, plus an adjustment knob to move the wiper and change resistance. They are mainly used to vary current and voltage in circuits.
Types of variable resistors:
- Potentiometer: a three-terminal device. The wiper divides the resistor into two parts, so the output voltage can be adjusted.
- Rheostat: effectively a potentiometer used with only two terminals for precise resistance adjustment.
- Trimmer potentiometer: a potentiometer that is not frequently adjusted and is often set at manufacture or during calibration; adjustment typically requires a screwdriver.
Key parameters and considerations
- Nominal resistance: the resistance between the two fixed terminals.
- Rated power: the power the variable resistor can withstand during normal operation. Exceeding this power can damage the device.
- Linearity (conformance): also called conformity, it measures how closely the actual output characteristic matches the theoretical function. It is expressed as the maximum deviation relative to the applied total voltage and reflects the potentiometer's accuracy.
- Resolution: determined by theoretical precision. For wirewound and linear potentiometers, resolution is the resistance change caused by moving the wiper one turn expressed as a percentage of total resistance. For potentiometers with nonlinear functions, resolution is variable and is often defined as the average resolution over the steepest slope region of the function curve.
- Wiper noise: unique to variable resistors. During adjustment, noise can occur due to poor resistance distribution, mechanical mismatch, or contact resistance between wiper and resistive element. Wirewound potentiometers also exhibit step noise and shorting noise when the wiper shorts adjacent turns; these are proportional to current through the winding, winding resistance, and contact resistance.
- Mechanical life: also called wear life, expressed as the number of reliable wiper operations under specified test conditions, often measured in cycles. Mechanical life varies greatly with design, materials, and manufacture.
Selection guidance
- For high-gain amplifier circuits, choose variable resistors with low noise, such as carbon film or metal film potentiometers.
- In high-frequency circuits, distributed parameters should be minimized. For operation at hundreds of MHz, wirewound potentiometers are usually unsuitable; metal oxide film potentiometers are preferred. For ultra-high-frequency applications, special UHF carbon film potentiometers are recommended.
- For circuits with moderate to high working frequencies, wirewound variable resistors can be considered when their distributed parameters are acceptable. For circuits below about 50 kHz, distributed parameters of resistors have minimal impact, so wirewound potentiometers are acceptable.
- When replacing potentiometers, consider sealing type and environmental temperature variation; for large temperature swings choose high-precision trimmers with appropriate sealing.
3. Special Sensor-Type Resistors
3.1 Thermistors
Thermistors are temperature-dependent resistors.
PTC thermistors
PTC thermistors act like a resettable fuse for overcurrent protection. Unlike a one-time fuse, a PTC is self-resetting, which is useful where fuse replacement is undesirable. PTCs are safety-related components and often must meet standards such as UL1439.
Under overcurrent, the PTC heats rapidly, its resistance increases sharply, effectively opening the circuit, reducing current, and cooling the device so resistance returns to a low state. PTCs are therefore well suited for short-duration overcurrent protection.
NTC thermistors
NTC thermistors have a negative temperature coefficient. Made from metal oxides such as manganese, cobalt, nickel, and copper oxides, they decrease in resistance as temperature rises and are widely used for temperature sensing, control, and temperature compensation.
CTR thermistors
CTR devices exhibit a negative resistance-change transition at a certain temperature and are used in temperature control alarms.
Thermistor parameters
- Hold current: the maximum current at which a PTC remains in its low-resistance state. Hold current decreases with increased ambient temperature.
- Trip current: the current at which the PTC enters high-resistance state to provide protection.
- Rated voltage: the maximum voltage the PTC can withstand. Exceeding this may cause breakdown and permanent damage. When a PTC trips, it may be exposed to the full supply voltage; select PTCs with rated voltage higher than supply voltage. Common practice is to derate to about 80%; for example, for a 12 V supply, choose a PTC rated above 15 V. Consider surge protection on input ports since surge current times PTC resistance produces surge voltage that must not exceed the PTC rated voltage.
- Rated current: the maximum short-circuit current the PTC can withstand at rated voltage without damage.
- DC resistance: the PTC's resistance in the low state, which will introduce voltage drop in the circuit and should be considered in the design.
Compared with fuses, PTCs typically have much lower rated voltage and current capacities and higher DC resistance, and when protecting a circuit they can allow milliamp-level leakage current. Fuses provide a physical open circuit with essentially no leakage when blown.
Thermistor selection tips
- Focus on voltage and current withstand capabilities.
- To reduce cost, select elements with high Curie temperature and small size, and keep sensitive components at least 5 cm away from the PTC where possible.
- For temperature measurement, select appropriate B-value and match the T-R curve to the measurement range.
- For circuit protection, ensure the minimum resistance in the low state does not compromise circuit safety.
3.2 Metal Oxide Varistors (MOV)
MOVs are clamping devices similar in concept to Zener diodes and TVS diodes and are used to protect circuits from transient overvoltage events such as surges.
MOV parameters
- Nominal voltage: must be selected accurately. If too high, the MOV will not protect; if too low, the MOV may false-trigger or be destroyed.
- V1mA: the voltage across the MOV when 1 mA flows through it.
- Leakage current Ir: typically measured at 83% of the nominal voltage.
- Rated operating voltage: the maximum continuous voltage the MOV can withstand while remaining high impedance. MOVs are specified differently for AC and DC applications; DC-rated MOVs are not always suitable for AC.
For AC applications the rated voltage is given as Vrms or Vm. The device in the illustration operates normally at 130 V rms; exceeding that voltage may cause action or damage.
- Clamping voltage: MOVs clamp transient overvoltages by dropping in impedance and conducting large current, but the residual voltage is typically 2 to 3 times the rated operating voltage. Ensure the clamping voltage does not exceed the maximum withstand voltage of downstream protected components. If necessary, use multi-stage protection such as an upstream MOV followed by a series resistor and a TVS to further reduce residual voltage.
- Surge current capability (maximum pulse current): the peak pulse current the MOV can handle without the V-I characteristic changing more than ±10% for specified surge waveform and number of pulses. Surge capability is related to energy dissipation; excessive energy can overheat and destroy the MOV. Surge ratings are commonly specified for an 8/20 μs waveform with defined pulse counts and amplitudes.
MOV selection guidance
- Choose based on the maximum transient surge current expected, with margin to spare.
- Ensure the MOV clamping voltage is below the maximum transient tolerance of downstream circuitry.
- MOVs age; account for environment, test standards, and pulse count when applying derating curves.
- For communications or low-power circuits, consider MOV capacitance and leakage current to avoid degrading normal operation.
- MOVs have larger parasitic capacitance and slower response than TVS diodes, so they may be unsuitable for high-speed signal lines or very fast ESD pulses.
3.3 Photoresistors (LDR)
Photoresistors change resistance with incident light intensity by exploiting the photoconductive properties of semiconductors. Resistance decreases with increasing illumination and increases when illumination decreases.
Photoresistor characteristics
- Photocurrent and light resistance: under a fixed applied voltage, current under illumination is photocurrent. The ratio of applied voltage to photocurrent is the light resistance, often specified at a given illuminance such as 100 lx.
- Dark current and dark resistance: under the same applied voltage with no illumination, the current is dark current and the ratio is dark resistance (0 lx).
- Maximum working voltage: the maximum voltage the device may sustain at rated power.
- Sensitivity: relative change between dark resistance and light resistance.
- Rated power: the allowable power dissipation, which decreases as temperature rises.
Photoresistor selection
Select the spectral response appropriate to the application. For visible-light automatic control, cameras, and light alarms use visible-spectrum photoresistors. For infrared signal detection, astronomy, or military control systems use infrared-sensitive devices. For UV detection use UV-sensitive photoresistors.
3.4 Force-Sensitive Resistors
Force-sensitive resistors convert mechanical force into an electrical signal using the piezoresistive effect of semiconductor materials. They are used in load cells, torque sensors, accelerometers, semiconductor microphones, and various pressure sensors.
Characteristics
- Temperature coefficient: percentage change in resistance per 1°C temperature change.
- Sensitivity coefficient: relates strain to resistance change, typically ΔR/R = k ΔL/L, where k is sensitivity.
- Sensitivity temperature coefficient: percentage reduction in sensitivity per 1°C increase.
- Temperature zero drift: the percentage change in zero output per 1°C change in ambient temperature relative to rated output.
Selection guidance
Consider ambient and in-circuit temperatures. Use bridge circuits or strain gauge compensation techniques to correct for temperature effects, otherwise measurement accuracy will be affected. Ensure the sensor meets circuit requirements for force range, measurement accuracy, and nominal resistance.
3.5 Gas-Sensitive Resistors
Gas-sensitive resistors change resistance when gas molecules are adsorbed on a metal oxide semiconductor surface causing oxidation or reduction reactions. Commonly made from tin dioxide, they are classified as n-type and p-type. The representative designation is R or RG.
Characteristics
- Sensitivity-temperature characteristic: conductivity changes little at room temperature but changes significantly at elevated temperatures, so many gas sensors require heating.
- Resistance-concentration characteristic: sensitivity varies by gas; these sensors can be highly responsive to ether, ethanol, hydrogen, and certain hydrocarbons.
- Heater power: product of heater voltage and heater current.
- Operating voltage: voltage across the sensor under working conditions.
- Sensitivity: response of resistance to gas concentration under optimal conditions, often expressed as a ratio of voltage across the load resistor before and after exposure.
- Response time: time to reach a specified change in voltage across the load resistor after exposure to target gas.
- Recovery time: time to return to a specified voltage after removal of the gas.
Application and selection
- N-type sensors are typically used for detecting methane, carbon monoxide, natural gas, town gas, LPG, acetylene, hydrogen, and similar gases; resistance decreases when these gases are present.
- P-type sensors are used for oxygen, chlorine, carbon dioxide, and certain oxidizing gases; resistance also decreases when these gases are present.
3.6 Humidity-Sensitive Resistors
Humidity-sensitive resistors change resistance with humidity and are composed of a hygroscopic sensing layer or film, electrode leads, and an insulating substrate. Commonly used as humidity sensors.
Characteristics
- Relative humidity (RH): the ratio of actual water vapor density to the saturation density at the same temperature, expressed as percent RH.
- Humidity-temperature coefficient: change in humidity indication per 1°C when ambient humidity is constant.
- Sensitivity: the resolution when detecting humidity.
- Measurement range: the humidity range the device can measure.
- Hysteresis: electrical parameter lag during moisture adsorption and desorption cycles.
- Response time: speed of resistance change when the environment's humidity changes rapidly.
Selection guidance
Choose the sensor type based on the application. For high-humidity detection in appliances such as washing machines and dryers, lithium chloride sensors may be suitable. For moderate humidity monitoring in air conditioners or humidistats, ceramic humidity sensors are common. For meteorological monitoring, dew detection, or imaging equipment, polymer or selenium film sensors are options.
3.7 Magnetoresistive Resistors
Magnetoresistive resistors change resistance when external magnetic field strength or direction changes, allowing precise detection of relative magnetic displacement. They are used for magnetic field strength detection, position/velocity sensing in motors, and related control systems.
Characteristics
- Magnetoresistance ratio: resistance at a specified magnetic flux density divided by resistance at zero magnetic flux density.
- Magnetoresistance coefficient: resistance at a specified magnetic flux density relative to nominal resistance.
- Magnetoresistive sensitivity: relative rate of resistance change with magnetic flux density.
Applications
- Magnetic sensors for measuring static and alternating magnetic fields, residual magnetism testing, and navigation instruments.
- Speed sensors for digital tachometers and frequency meters.
- Displacement and angular displacement sensors, including micro-displacement sensors used in industrial robotics.
- Non-destructive testing sensors for ferromagnetic materials.
- Variable resistors, non-contact potentiometers, and high-performance magnetic switches for positioning and control.
Summary: How to choose resistors in circuits
- Select resistor type and tolerance to meet circuit design requirements.
- Choose resistance values that meet circuit needs; nominal market values may not exactly match calculated values, so prefer standard nominal values. Common tolerances are ±5% to ±10%. For precision instruments or special circuits, select precision resistors.
- Ensure rated power exceeds actual dissipation in the circuit. A typical practice is to design using 70% of the rated power as the maximum operating dissipation.
- Ensure the actual working voltage across the resistor is below its maximum working voltage. A common practice is to design at 75% of the maximum working voltage as an upper limit.
- Ensure stability, frequency response, and noise characteristics meet circuit requirements.
