Chip resistors, also known as patch resistors or SMD resistors, are resistors specifically designed for surface mount technology (SMT). They are small, lightweight and welded directly to the surface of the circuit board without drilling through the circuit board. Chip resistors usually use numbers and color rings to identify resistance values, which are suitable for high-density assembly and are commonly found in small electronic products such as mobile phones and laptops.

Features:

• Chip resistors are small in size and light in weight, and are more suitable for high density circuit board design than traditional axial lead resistors.

• Designed for automated assembly, it can be welded directly to the board surface without perforation, increasing production efficiency and assembly density.

• Common package sizes follow EIA (American Electronics Industry Association) standards, such as 0201, 0402, 0603, 0805, 1206, and so on, each corresponding to a different length and width.

• It has high accuracy and stability, the error range is usually 1%, 2%, 5%, etc., and shows good stability under temperature changes.

• It can be sintered from metallic glass uranium material at high temperature, or produced by thin film resistance process to ensure high reliability.

• Resistance values and errors are represented by a silk screen numeric code, such as a 3 - or 4-digit code, some of which contain the letter "R" for the decimal point position.

• Including but not limited to glass sealed NTC thermistors, these resistors are sensitive to temperature changes, fast response, high precision, suitable for temperature compensation and control circuits.

Meaning of key performance index of chip resistance and selection basis

Power tolerance:

Meaning: refers to the maximum power that the resistor can withstand in the case of long-term operation without exceeding its maximum operating temperature. Exceeding this power will cause the resistance to overheat, which may cause resistance changes, performance degradation or even damage.

Selection basis: According to the actual working power of the resistor in the circuit design to choose, usually the selected resistance power should be slightly greater than the calculated power demand, leaving a certain safety margin. It is also necessary to consider the ambient temperature of the circuit, because high temperatures will reduce the power tolerance of the resistor.

Accuracy level:

Meaning: Indicates the maximum allowable deviation range between the actual resistance value of the resistor and its nominal resistance value. The smaller the accuracy level, the higher the accuracy of the resistance value.

Selection basis: According to the requirements of the circuit for the accuracy of the resistance value to choose. For example, applications requiring high precision measurements, amplifier bias, or partial voltage ratios may require the selection of resistors with ±1% or higher accuracy; General filtering, current limiting applications may use ±5% or ±10% resistance.

Temperature coefficient (TCR) :

Meaning: Indicates the rate at which the resistance value of a resistance changes with temperature, usually expressed as parts per million per degree Celsius (ppm/°C). For example, a TCR of 100 PPM /°C means that the resistance changes by 100 millionths of its nominal value for every 1°C increase in temperature.

Selection basis: In temperature-sensitive circuits (such as precision measurement, temperature compensation circuits), resistance with small TCR should be selected to reduce the impact of temperature changes on circuit performance. For general circuits, a standard TCR value (such as ±100ppm/°C) may be sufficient.

Power derating rules for resistors in high temperature environments

When using resistors at high temperatures, power derating is a very important consideration to ensure the safety and long-term reliability of the resistors. Here are some key rules and principles for the use of resistance power derating:

Reference Derating curve: Each resistor manufacturer usually provides a power derating graph that shows the recommended maximum power usage of the resistor at different operating temperatures. As the temperature increases, the maximum available power of the resistor gradually decreases until a specific maximum hot spot temperature is reached, at which point the power drops to zero.

Ambient temperature consideration: First determine the maximum temperature of the resistance working environment. The rated ambient temperature (Ts) of most resistors is about 70 ° C, and if the operating temperature will exceed this value, the power derating needs to be performed according to the actual situation.

Derating percentage: A common practice is to use about 60% of the rated power derating when ensuring that the resistance operating temperature is below 70 ° C. If the operating temperature is higher, the power usage ratio needs to be further reduced according to the data or calculation formula provided by the manufacturer.

Calculation method: Use the P=V²/R formula to calculate the actual working power of the resistor, and ensure that this calculated value does not exceed the maximum allowable power determined by the ambient temperature and derating rules.

Impulse current handling: In the case of a large impulse current, it may be necessary to use a winding resistor or other resistance type that can withstand high power pulses, and adjust the power derating strategy according to the specific situation.

Heat dissipation design: Improving the heat dissipation design of the circuit board can also help improve the power tolerance of the resistance in high temperature environments, such as increasing the heat sink and optimizing the layout to facilitate air circulation.

Pulse power processing: For pulse operating conditions, it is necessary to consider single pulse or multi-pulse power, different types of resistance have different processing methods, may need to refer to the specific model of pulse power specifications.

Selection of special application resistors, such as applications and requirements for low resistance values and high power resistors

Applications for high power resistors with low resistance values usually involve situations where large currents need to be handled or where low voltage drops need to be achieved in the circuit. This type of resistor is characterized by a very low resistance (usually in the milliohm or microohm class), but can withstand high power without overheating damage. The following are some typical applications and corresponding requirements:

1. In applications such as power electronic converters, battery management systems (BMS), and DC bus current monitoring, low-resistance high-power resistors are used as galvanometers to measure the high current passing through while minimizing the voltage drop caused by the resistance to ensure system efficiency.

2. In electric vehicles, elevators, servo drives and other devices that require rapid deceleration or energy recovery, low-resistance high-power resistors are used to consume the energy returned by the motor, prevent overvoltage, and protect other components in the circuit.

3. In switching power supplies, inverters, and UPS systems, low-resistance high-power resistors are used as buffer resistors or filter resistors to help absorb transient current, smooth current waveforms, and improve system stability and efficiency.

4. In some industrial heating or temperature control applications, low-resistance high-power resistors are used as heating elements because of their ability to generate large amounts of heat quickly.

When choosing this type of resistor, the following key requirements should be considered:

• Power rating: Must be high enough to withstand the heat generated by a large current passing through the resistance, usually ranging from tens of watts to several kilowatts.

• Resistance stability: The resistance value changes in the operating temperature range to ensure the accuracy of the measurement or the stability of the circuit.

• Thermal characteristics: including thermal resistance (that is, the ability of the resistor to conduct heat to the surrounding environment) and temperature coefficient, these parameters affect the performance of the resistor when heating.

• Heat dissipation design: Because high power generates a lot of heat, heat dissipation design is extremely important and may need to be combined with heat sinks, fans or liquid cooling systems.

• Packaging and installation: Low-resistance high-power resistors tend to be large, and it is necessary to ensure that the packaging form and size of the selected resistors meet the requirements of the board design and installation space.

• Reliability and life: In continuous high-current applications, the long-term stability and life of the resistance is particularly critical, and it is necessary to choose resistors made of high-temperature and anti-aging materials.

How to determine and select the appropriate patch resistance specifications

Determining and selecting the appropriate patch resistance specification requires consideration of several key factors:

1. Resistance value: According to the requirements of the circuit design to determine the required resistance value. This is usually determined by the calculation or design specifications of the circuit, such as voltage division, bias, current limiting, etc. While confirming the resistance value, it is also necessary to consider the accuracy of the resistance value (error rate), the common ones are ±1%, ±2%, ±5%, ±10%, etc. Precision circuits usually require higher accuracy. Use the numerical representation of the patch resistance to identify the resistance value, such as three-digit representation (the first two digits are significant digits, the third digit is a power of 10), four-digit representation (the first three digits are significant digits, the fourth digit is a power of 10), etc.

2. Power: Calculate the power the resistor is expected to consume in the circuit and select a resistor with a rated power higher than the actual power consumed. It is generally recommended to choose a resistor with a power margin of 1.5 to 2 times to ensure long-term reliability and heat dissipation. Considering the influence of ambient temperature on the resistance power, the rated power of the resistance will be reduced in high temperature environment. Resistors in different package sizes have different maximum power ratings, for example, resistors in 0402 packages are generally less powerful, while resistors in 2512 packages are suitable for high power applications.

3. Package size: When selecting package size, consider the space limitations of the board, the compatibility of automated assembly, and the difficulty of welding. High-density boards tend to use smaller packages, such as 0201 or 0402, while applications with higher power requirements may require larger packages, such as 1206 or 2512. Given the requirements of the SMT process, smaller packages (such as 0201, 0402) require higher assembly accuracy.

4. Environmental factors: If the circuit will operate under extreme temperature, humidity, or vibration conditions, you need to choose a resistor with appropriate environmental adaptability.

5. Cost and availability: The price and market availability of different specifications of resistors will also vary, and these commercial factors need to be considered when choosing.

6. Other characteristics such as temperature coefficient and frequency characteristics (especially important for high frequency applications) are also factors that may need to be considered when selecting.

What is the difference between chip resistance and plug-in resistance

In contrast to chip resistors, plug-in resistors (PTH) are traditional resistors that need to be installed through holes in the board, with one end welded to the top layer of the PCB and the other end welded to the bottom layer or extended out of the board through pins. This type of resistance has a large volume, good high temperature resistance, and is commonly found in circuits that require high mechanical strength or need to withstand large currents, such as power supplies and industrial control equipment. The main differences between chip resistance (chip resistance, SMD resistance) and plug-in resistance (PTH resistance) are as follows:

1. Mounting method: The chip resistor adopts surface mount technology (SMT) and is welded directly to the surface of the circuit board without passing through the hole on the circuit board. The plug-in resistor needs to be inserted into the circuit board through the hole and welded from the other side.

2. Size and weight: The chip resistance is small, light weight, easy to install high-density, space saving, suitable for miniaturization and portable electronic products. Plug-in resistors are relatively large and heavier.

3. Welding convenience: Chip resistors are easy to weld and disassemble, no holes are required, and less solder is used. The welding and disassembly of plug-in resistors are more complicated, especially the multi-pin components, which are easy to damage the circuit board.

4. High frequency performance: Chip resistors generally have superior high frequency characteristics because of their short leads and small parasitic inductance and capacitance.

5. Production efficiency: Chip resistors are suitable for automated production, which can increase production speed and reduce costs. The assembly process of plug-in resistor is complicated and the degree of automation is low.

6. Environmental adaptability: Plug-in resistors can generally withstand larger currents and work in harsher environments, and have relatively good stability, suitable for high-power applications or occasions with high stability requirements.

About interchangeability:

In theory, any resistance as long as the resistance value is the same, you can achieve the basic resistance function in the circuit. However, the electrical and physical characteristics of chip resistors and plug-in resistors are different in design, and direct interchange needs to be cautious, and the following factors need to be considered:

• The design of the circuit board must support the selected resistance type, that is, whether there is a corresponding pad design.

• The power level of the replacement resistor must meet the requirements of the original design to avoid overheating.

• Especially for designs with limited space, it may not be possible to accommodate larger plug-in resistors.

• In high-frequency applications or circuits with high stability requirements, it is not appropriate to replace them at will, so as not to affect the circuit performance.

• If the product line is a fully automated SMT line, only patch resistors can be used.

Can the chip resistors be replaced with normal resistors or resistors in different packages?

In some simple or low-requirement applications, ordinary resistors can be used as temporary or emergency substitutes for chip resistors, but in applications where design and performance are critical, patch resistors of the same specification and package should be replaced as far as possible to ensure the normal operation and long-term stability of the circuit.

Replacement note:

• Ensure that the resistance value of the replacement resistor is the same or close to the resistance value of the original chip resistor. The difference in resistance values may change the behavior of the circuit, affecting the function and performance of the circuit.
• When replacing ordinary resistors, their rated power should be at least equal to or greater than the power of the original chip resistance to avoid overheating and possible damage.
• In high frequency circuits, pins of ordinary in-line resistors introduce additional inductance, which may affect the signal integrity of the circuit. Therefore, in high-frequency applications, it is best to use equally packaged patch resistors to maintain the electrical performance of the circuit.
• While resistors in different packages can theoretically physically adapt and perform basic resistance functions, this may require additional welding skills (such as vertical welding) and may not be suitable for all board layouts. In the long term, improper size or packaging may affect the stability and reliability of the board.
• For precision circuits, the temperature coefficient of the resistance also needs to be considered to ensure that the temperature stability of the replacement resistance is close to that of the original resistance.
• In high-speed digital or high-frequency analog circuits, the parasitic capacitance and inductance of the resistor are considered, and these factors are often optimized in chip resistors to reduce the impact, while ordinary resistors may not have such characteristics.
• For resistors with specific functions, such as thermistors, varistors, safety resistors, etc., they cannot be directly replaced with ordinary resistors, because they have special nonlinear characteristics and protection functions.