Resistors play an indispensable role in the world of electronic circuits, they are not only the "regulator" of current, but also participate in a variety of complex functions such as signal processing and energy conversion.

Contents directory

> Concept and function of resistors

> What kinds of resistors are there?

> How to calculate the power of the resistor?

> Examples of resistor applications in circuit design

> How to read the resistance value of the color ring resistor?

> In addition to color ring marking, what other marking methods are available for resistors?

A resistor, often referred to simply as a resistor, is an electronic component whose function is to limit or regulate the flow of current in a circuit. It is a basic and indispensable part of circuit design, mainly through the consumption of electrical energy to achieve voltage drop or current control. A resistor works based on Ohm's law, which states that the current passing through a resistor is proportional to the voltage applied to its ends, and the constant of proportionality is the resistance value of the resistor. Resistors can be used as voltage dividers, shunt, current limiter, and are often used in conjunction with other electronic components to form more complex circuit functions, such as filtering, coupling, feedback, and compensation circuits. The resistance value can be fixed or variable. Fixed resistors have resistance values that remain unchanged after manufacture, while variable resistors (such as potentiometers) allow the user to adjust the resistance values.

The resistance of a resistor is measured in ohms (Ω), and its performance is also affected by temperature, which is usually described by the temperature coefficient. In practical applications, the choice of resistor also needs to consider its power rating, that is, the maximum power it can withstand without damaging the resistance.

The role of the resistor

The core function of the resistor is to limit excessive current and protect other components from excessive current damage. In addition, it is also used for voltage division, generating specific heat (such as heating elements), acting as a real device for load simulation, combining with capacitors to form filtering networks, and adjusting the bias points of amplifying circuits, showing its diversity and importance in circuit design.

Function details:

1. Current limiting function: This is the most basic function of the resistor, by increasing the resistance value to reduce the size of the current through the circuit, to prevent excessive current damage to the sensitive element.

2. Voltage reduction function: The resistor can reduce the voltage in the circuit through the voltage drop generated by its own resistance value, making it suitable for the working needs of different components.

3. Voltage division function: In the series circuit, the resistor can allocate the input voltage to different parts of the circuit in a certain proportion to achieve voltage division.

4. Bias action: In an amplifier circuit, a resistor can be used to set the bias point of a transistor or other active component, ensuring that it operates at a predetermined current or voltage level.

5. Load function: as a simulation of real load or consumption of unwanted electrical energy, prevent power overload or test circuit output capacity.

6. Impedance matching: Adjust the impedance between the signal source and the load to optimize power transmission and signal quality.

7. Filtering and decoupling: In combination with capacitors or inductors, a filtering network is formed to remove noise from the circuit or provide a stable DC bias voltage to reduce coupling interference.

8. Feedback action: In amplifiers and other feedback circuits, resistors are involved in establishing a negative feedback path, stabilizing magnification and improving circuit performance.

9. Temperature compensation: Special resistors such as thermistors can change resistance values with temperature for temperature compensation or control circuits.

10. Detection and sensing: Resistors can be used to detect current or voltage in a circuit, such as measuring voltage as part of a voltage divider.

11. Control and adjustment: Variable resistors (such as potentiometers) allow users to manually adjust circuit parameters, such as volume control or brightness adjustment.

12. Special applications: Resistors with special properties, such as photoresistors and thermistors, can change the resistance value when the light or temperature changes, for automatic control or sensing systems.

What are the types of resistors?

Resistors can be divided into the following categories according to their structure and function:

Fixed resistance

1. Carbon film resistor: made of carbon material coated on the insulating substrate, low cost, widely used in general electronic equipment.

2. Metal film resistor: The use of metal or metal alloy evaporation deposit on the insulating substrate, with better stability and low noise characteristics, suitable for precision circuits.

3. Metal oxide film resistance: With metal oxide as the resistance layer, it has higher stability and high temperature resistance, suitable for applications with high reliability requirements.

4. Wire winding resistance: Made by winding resistance wire around an insulating skeleton, it can withstand higher power and is suitable for applications requiring high stability and repairability.

5. Solid resistance: divided into inorganic synthetic solid carbon resistance and organic synthetic solid carbon resistance, with small volume, impact resistance and other characteristics.

6. Metal glass glaze resistance: made of metal glass glaze material, with good high frequency performance and stability.

7. Metal nitriding film resistance: has good long-term stability, suitable for harsh environments.

Variable resistance

1. Potentiometer: A device that adjusts the resistance value, often used for volume control, signal regulation, etc.

2. Trimmer resistor: A small adjustable resistor, usually used to precisely adjust circuit parameters, which may not change after adjustment.

3. Slide wire resistor: Adjust the resistance value by changing the position of the contact point, suitable for a wide range of resistance regulation.

Special resistance (sensitive resistance)

1. Thermistor: The resistance value changes significantly with temperature, and is divided into positive temperature coefficient (PTC) and negative temperature coefficient (NTC).

2. Photoresistor: The resistance value changes when the light intensity changes, often used in light switches and automatic control systems.

3. Varistor: When the voltage reaches a certain threshold, the resistance value drops rapidly, which is used for overvoltage protection.

4. Magneto resistor: A change in the resistance value caused by a change in the magnetic field, used in magnetic sensors and magnetic recording equipment.

5. Force resistor: Pressure change causes resistance value to change, used for pressure measurement.

6. Humidity-sensitive resistor: Resistance changes due to changes in ambient humidity. It is used for humidity measurement and control.

Other categories

• Precision resistance: with very low resistance deviation and temperature coefficient, suitable for high-precision requirements such as measurement and calibration.

• High voltage resistor: Designed to withstand high voltage without electrical breakdown, commonly found in high voltage power supplies and measuring equipment.

• High frequency resistors: Designed for high frequency circuits with low parasitic capacitance and inductance to ensure signal integrity.

• Chip resistors: Resistors in the form of miniaturized SMD (Surface mount device) for automated assembly.

Each type of resistor has its specific application scenario, and designers should choose the appropriate resistance type according to the specific needs of the circuit.

How to calculate the power of the resistor?

The power of a resistor can be calculated by the following three formulas, which are essentially basic expressions of electrical power, but are chosen to be used according to different known conditions:

1. P = UI
   This is the most straightforward formula for calculating power, where P represents power (in watts, W), U represents voltage (in volts, V), and I represents current (in amperes, A). If you know both the current passing through the resistor and the voltage at both ends of the resistor, you can calculate the power directly using this formula.

2. P = U²/R
   This formula can be used when you only know the voltage at both ends of the resistor and the resistance value of the resistor, where R represents the resistance value of the resistor (in ohms, Ω).

3. P = I²R
   If you know the current through the resistor and the resistance value of the resistor, but do not know the voltage, then this formula is very suitable.

The specific steps are as follows:

• Determine the known conditions: First, determine the data you have at hand, whether it is voltage and current, voltage and resistance, or current and resistance.

• Choose the right formula: Choose one of the above three formulas based on the data you have.

• Plug in the numerical calculation: the known values into the formula for calculation, noting that the unit should be unified (such as voltage with volts V, current with amperes A, resistance with ohms, power with watts W).

• Calculation result: The power value obtained is the electrical power consumed by the resistance.

For example, if the voltage at both ends of a resistor is 20V and the current through it is 0.5A, then its power is calculated as follows:

Alternatively, if the voltage at both ends of the resistor is 20V and the resistance value is 100Ω, the power is calculated as:

For example, if the current through the resistor is 0.5A and the resistance value is 100Ω, the power is calculated as:

Select an appropriate formula based on the actual situation.

An application example of resistor in circuit design

Resistors are widely used in circuit design, and the following are some specific application examples:

LED lights limited current

When the LED (light emitting diode) is connected to the power supply, because its forward voltage is relatively fixed and more sensitive to current, directly connecting the high voltage power supply will cause the LED to burn. Therefore, a resistor is usually placed in series in the LED to limit the current. For example, if the power supply voltage is 5V, and the forward voltage of the LED is 3V, the current that the LED is expected to pass through is 20mA, according to Ohm's law (V = IR), the resistance value required is:

This resistor will ensure that the current flowing through the LED is kept within a safe range, preventing overcurrent damage.

Divider circuit

Divider circuits are useful in situations where a lower voltage is needed from a voltage source. For example, if you have a 12V power supply, but a certain part of the circuit needs only 5V operating voltage, you can divide the voltage by two resistors in series. If you choose two resistors with equal resistance values, such as 10kΩ, then the partial voltage point (that is, the voltage at both ends of the second resistance) will be 6V, but this is not the 5V we need. In fact, in order to get to 5V, the resistance ratio needs to be adjusted, such as using a 20kΩ and a 10kΩ resistor in series, so that the voltage drop on the 10kΩ resistor will be 1/3 of the total voltage, that is, about 4V, which is closer but still higher than the target. In order to get an accurate 5V, it is necessary to calculate the appropriate resistance value.

filter

In filter circuits, resistors are usually used in combination with capacitors or inductors to form low-pass filters, high-pass filters, or bandpass filters. For example, a simple RC low-pass filter consists of a resistor and a capacitor, where the resistor is in parallel with the capacitor. In this circuit, the combination of resistance and capacitance determines the cut-off frequency, and signals below this frequency are allowed to pass, while signals above this frequency are attenuated. This circuit is often used at the output end of audio amplifiers to remove high-frequency noise, or to smooth out ripples in power supply circuits.

Bias in the amplifier circuit

In transistor amplification circuits, resistance is used to set the static operating point (bias point) of the transistor. For example, in a common-beam amplifier circuit, the base resistance and collector resistance work together to provide a stable bias current to the base of the transistor, ensuring that the transistor remains linearly amplified as the input signal changes. The base resistance limits the current flowing into the base, while the collector resistance works with the load resistance to affect the gain and output swing of the amplifier.

These are just a few examples of the applications of resistors in circuit design, in fact, from simple voltages to complex signal processing circuits, resistors are an integral part.

How to read the resistance value of the color ring resistor?

The recognition of resistance value of color ring resistance is one of the basic skills of electronic beginners. The resistance value of reading the color ring resistance needs to be determined according to the number and sequence of color rings. The common color ring resistance is divided into four rings and five rings, and sometimes three or six rings of resistance can be seen. Here are the basic steps for identifying four - and five-ring color ring resistors:

Tetracyclic resistance

1. The first two rings represent the first and second significant digits of the resistance value, respectively. Refer to the color ring color code table, brown is 1, red is 2, orange is 3, yellow is 4, green is 5, blue is 6, purple is 7, gray is 8, white is 9, and black is 0.

2. Third ring: represents the power of 10 after the number needs to be multiplied, that is, the magnitude of the resistance value. For example, black represents 10^0100 (i.e., multiplied by 1), brown represents 10^1101 (i.e., multiplied by 10), red represents 10^2102 (i.e., multiplied by 100), and so on.

3. The fourth ring: indicates the allowable error of the resistance. Gold represents ±5%, silver represents ±10%, colorless (sometimes shown as white, but the actual meaning is different) represents ±20%, while other colors such as brown, red, etc., indicate a smaller error, such as ±1%, ±2%, etc.

Pentacyclic resistance

1. First three rings: three significant digits representing the resistance value respectively.

2. Fourth ring: With the fourth ring resistance, representing the power of 10 after the number needs to be multiplied.

3. The fifth ring: indicates a more accurate error level, usually more accurate than the four-ring resistance, the common error level is ±1%, ±2%, ±0.5%, ±0.25%, etc., the meaning of gold and silver is unchanged, but there may also be green representing ±0.5%, blue representing ±0.25% and so on.

Identification step

• Determine the color ring order: Start at one end of the resistor, usually away from the end of the resistor or the notched end.

• Apply color ring code: Interpret the meaning of each color ring in turn according to the above rules.

• Calculate the resistance value: Multiply the significant number of the first few rings by the corresponding power of 10 to obtain the resistance value.

• Consider the error: Finally, determine the allowable error range of the resistance value according to the error loop.

For example, the color ring order of a five-ring resistor is green, blue, red, black, and brown, which is interpreted as:

• Green represents 5 (the first significant digit),

• Blue represents 6 (the second significant digit),

• Red represents 2 (the third significant digit, x 100),

• The black represents multiplying by 10^0100 (i.e., multiplying by 1, which does not change the order of magnitude here because the first three have already determined the order of magnitude),

• Brown represents an error of ±1%.

Therefore, the resistance value of this resistor is 56×100=5600 ohms, that is, 5.6 kilohm, and the error is ±1%.

What are the methods for marking resistors?

In addition to color ring marking, resistors also have direct marking method, text symbol method, digital method, color marking method and alphanumeric coding.

1. Direct marking method

Direct marking method is directly on the surface of the resistor with numbers and unit symbols (such as Ω, kΩ, MΩ) clearly marked the resistance value and allowable error. This method is intuitive and easy to identify quickly. For example, a resistor printed directly with "47kΩ ±5%" means that its resistance is 47 kilohm, with an error of ±5%. The direct marking method is usually used on resistors with large volume and sufficient surface space.

2. Word symbol method

The word symbol method combines Arabic numerals with specific letters to indicate the resistance value of a resistor. In this method, the letters before and after the number have a specific meaning, the front number represents the significant number of the resistance value, and the following number and/or letter may represent the value after the decimal point and the multiplier factor. In addition, the text symbol method is also used to indicate the allowable error, for example, D, F, G, J, K, M and other letters represent the error of ±0.5%, ±1%, ±2%, ±5%, ±10%, ±20%, respectively. For example, "2R2K" might indicate a resistance value of 2.2kΩ, with the error rating indicated by a separate letter.

3. Digital Method

The digital method is mainly used in miniaturized resistors, especially SMD resistors, because these resistors are small and not suitable for direct printing of large amounts of text. The digital method uses three digits to represent the resistance value, where the first two digits represent the significant value, and the third digit represents the power of 10, in ohms by default. For example, "103" means a resistance of 10×10³Ω=10kΩ. Error levels are usually not marked directly on the resistor, but follow industry standards, default to ±5% or ±10%, and special error levels are indicated by additional identification or manufacturer documentation.

4. Color coding

The color coding method is to indicate the resistance value and the allowable error by painting different colored rings or points on the resistor. Four-ring resistors are the most common, starting from the end near the resistance body, the first and second rings indicate the significant number of the resistance value, the third ring is the multiplier (based on a power of 10), and the fourth ring indicates the degree of error. Five-ring resistors have one more ring to indicate higher accuracy, usually by adding a finer indication of the error level. Each color corresponds to a fixed number or magnification, such as brown for 1, red for 2, black for 0, gold for ±5%, silver for ±10%, etc. The color coding method is widely used in the world, especially for small resistors that cannot be printed directly.

5. Alphanumeric coding

Alphanumeric encoding is a less common method of resistance marking, mainly used in some specific occasions or in older electronic devices. This encoding method usually combines letters and numbers to represent the resistance value of the resistor, but it is not as standardized and common as the direct notation method, the letter symbol method, or the color ring method. Therefore, different manufacturers or application areas may have different coding rules, which makes alphanumeric coding methods may be relatively complex and not uniform in recognition.

Nevertheless, it is possible to outline a more traditional or context-specific understanding of alphanumeric coding, but please note that in practical application, reference should be made to the product-specific data book or standard:

• Digit part: The significant digit part of the resistance value that usually represents the resistance of the resistor, similar to the number representation in the literal notation method.

• Letter part: May be used to denote multipliers (similar to the multiplier ring in the color ring), or suffixes of resistance values, with specific letters representing specific resistance ranges or error levels. For example, some older systems may use letters such as "A", "B", "C" to distinguish different resistance series or error levels, but the specific meaning of these codes needs to be consulted according to the manufacturer's regulations.

Due to the lack of a uniform standard for alphanumeric coding, it is not as intuitive and easy to use globally as direct notation, word symbol, or color ring. In modern electronic design, this type of encoding has become very rare, and in most cases has been replaced by more intuitive and precise labeling methods. Therefore, if you encounter an alphanumeric encoded resistor, the best practice is to find the specific data sheet for that resistor or contact the manufacturer for accurate resistance values and parameter information.

In the following articles, INFINITECH will further explain the common faults of resistors, detection tips, selection guidelines, and methods of replacing resistors and other practical knowledge.