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Nov 26 2018

What is Resistor and Its Function?

Warm hints: This article contains about 5000 words and reading time is about 20 min.


Before we understand resistors, we need to know what a resistor is. The resistance is often abbreviated as R, which is a basic property of a conductor and is related to the size, material, and temperature of the conductor. The basic unit of resistance is ohms, which is represented by the Greek letter "Ω". Ohm's law says that I=U/R, then R=U/I, so there is a definition: when adding one volt to the conductor, one amp is generated. The resistance corresponding to the current. The main function of the resistor is to block the flow of current.

Electronics for beginners - Resistor What it is and how to use it




I What is Resistor?

II Naming Convention of Resistors

III The Type of Resistor

 3.1 Fixed Resistor

 3.2 Rheostat

 3.3 Special Resistors

IV What is the Function of Resistor?

 4.1 Divide Voltage

 4.2 Shunt

 4.3 Impedance Matching

 4.4 Filtering

V Main Characteristic Parameters

VI Methods for Marking Values on Resistors

VII Commonly Used Resistors

I What is Resistor?

Since "the main function of the resistor is to block the flow of current", it can be inferred that the resistor is a "current limiting component." We usually define a resistor as a two-terminal electronic component made of a resistive material and having a certain structural form that can restrict the passage of current in the circuit. The effect of the conductor on the current is called resistance, and is represented by the symbol R. The unit is ohm, kiloohm, and megaohm, which are represented by Ω, KΩ, and MΩ, respectively.

II Naming Convention of Resistors

The model of the resistor in China consists of four parts (not applicable to sensitive resistors)

The first part: the main name, expressed in letters, indicates the name of the product. For example, R represents a resistor and W represents a potentiometer.

The second part: Materials, indicated by letters, indicate what material is used for the resistor body, T-carbon film, H-synthetic carbon film, S-organic solid, N-inorganic solid, J-metal film, Y-nitride film, C-deposited film, I-glass glaze film, X-ray wound.

The third part: classification, generally expressed by numbers, individual types are represented by letters, indicating what type of product belongs. 1-Normal, 2-Normal, 3-UHF, 4-High Resistance, 5-High Temperature, 6-Precision, 7-Precision, 8-High Voltage, 9-Special, G-High Power, T-Adjustable.

The fourth part: serial number, expressed by numbers, indicates different varieties in the same product, to distinguish the product's appearance size and performance indicators, etc. For example: R T 1 1 type ordinary carbon film resistor.

III The Type of Resistor

There are many types of resistors, which are usually divided into three categories: fixed resistors, variable resistors, and special resistors. In electronic products, it is most commonly applied with fixed resistors.

3.1 Fixed Resistor

Fixed resistors are divided into many types by their materials. Common types include RT type carbon film resistors, RJ type metal film resistors, RX type wirewound resistors, and chip resistors that have been widely used in recent years. Its model name is also very regular, R stands for resistance, T-carbon film, J-metal, X-wire winding.

In the old-fashioned electronic products of China, it is often seen that the resistors coated with green paint are RT-type carbon film resistors, which are coated with a layer of carbon on the porcelain tubes to deposit crystalline carbon on the ceramic rod skeleton. Made on. Because of its low cost, stable performance, wide resistance range, low temperature coefficient and low voltage coefficient, it is the most widely used.

The red color resistor is an RJ type metal film resistor. It is plated with a layer of metal on the porcelain tube, and then evaporated to the surface of the ceramic rod skeleton by vacuum evaporation. Since it has higher precision than carbon film resistor, good stability, noise, and small temperature coefficient, it is usually It is widely used by instrumentation and communication equipment.

The RX wirewound resistor is wound from a resistor wire into a resistor and then wound with a high resistance alloy wire on the insulating frame. It is coated with a heat-resistant glaze insulation layer or insulating varnish. It has a low temperature coefficient, high resistance precision, good stability, heat and corrosion resistance, and is mainly used for precision high-power resistors. The disadvantage is poor high-frequency performance. The time constant is large.

Fixed Resistors

Figure1. Fixed Resistors

3.2 Rheostat

The rheostat is also called a potentiometer. The volume potentiometer on the electronic device is a variable resistor. However, it is generally believed that the potentiometers can be manually adjusted, and the rheostat are generally small and are not often adjusted on the board. The rheostat has three pins, and the resistance value between the two pins is fixed, and the resistance value is referred to as the resistance of the rheostat. The value of the resistor between the third pin and any two pins can change as the arm moves. In this way, the voltage or current in the circuit can be adjusted to achieve the effect of the adjustment.


Figure2. Rheostat

3.3 Special Resistors

A photoresistor is a component whose resistance value changes according to changes in external light intensity (light and dark). The stronger the light, the smaller the resistance value, and the weaker the light value is.

If the two pins of the photoresistor are connected to the test leads of the multimeter, and the resistance of the photoresistor is measured by the R×1k block of the multimeter under different illumination, we will find that the photoresistor is moved from the darker drawer. In the sun or on the light, the multimeter reading will change, that is, in the complete darkness, the resistance of the photoresistor can reach several megaohms or more (the multimeter indicates that the resistance is infinite, that is, the pointer does not move); and under strong light The resistance can be reduced to several thousand ohms or even less than one thousand ohms. Using this feature, we can make a variety of light-controlled small circuits through the photoresistor. For example, street lights that are automatically controlled by light-controlled switches on the street, one of the important components is the photoresistor (or Photosensitive triode, a functionally similar semiconductor component with amplification). The photoresistor is made by depositing a layer of cadmium sulfide (CdS) film on a ceramic pedestal, and is actually a semiconductor component. The lights in the voice-activated corridor will not light during the day because the photoresistor is working.

The thermistor is a special semiconductor device whose resistance value changes with the change of the surface temperature. It was originally used to make electronic equipment work properly at different ambient temperatures, called temperature compensation. The CPU temperature measurement and over-temperature alarm function of the computer motherboard are the thermistors that are utilized.

IV What is the Function of Resistor?

Resistance, in daily life, it is usually written R, and the unit is ohm. The resistor is a current limiting component. The obstacle is large, we say that its resistance is large, and vice versa. The resistance does not disappear because there is no current flowing through the conductor. The resistance is an inherent property of a conductor. Even if no current flows through the conductor, its resistance exists. 


Figure3. Resistor

4.1 Divide Voltage

When a resistor and another component, such as a bulb, are in series in the circuit, the current flowing through the resistor and the bulb is the same, and the sum of the respective voltages of the resistor and the bulb is equal to the total voltage across the resistor and the bulb as a whole. At this time, the resistor functions as a voltage divider.

4.2 Shunt

When a resistor and another component such as a bulb are connected in parallel in the circuit, the voltage across the resistor is the same as the voltage across the bulb, and the sum of the current flowing through the resistor and the current flowing through the bulb is equal to the total current flowing through the resistor and the bulb. At this time, the resistor functions as a shunt.

Shunt Resistor

Figure4. Shunt Resistor

4.3 Impedance Matching

Impedance matching refers to the process of adapting the load impedance to the internal impedance of the excitation source in order to obtain an operating state of maximum power output during signal transmission. One of the methods is to achieve the impedance change by changing the impedance. In this case, the resistor plays its impedance matching function.

4.4 Filtering

In the RC charging and discharging circuit composed of a resistor and a capacitor in series, the switch S is initially connected to point B. As shown in the following figure, there is no charge on the capacitor C, and the voltage across the two ends is zero. Then, the switch S is placed at point A, and the power is turned on. The capacitor is charged through the resistor R. When the charge across the capacitor increases to the circuit balance, the power supply no longer charges the capacitor. After the switch S is placed at point B, the capacitor begins to discharge, and the charge at both ends is gradually reduced to zero. When the battery is no longer discharged, the switch S is placed at point A to start charging. In this infinite loop of charging and discharging, we call the function of the resistor R a filtering effect.

Harmonic Filter Resistor

Figure5. Harmonic Filter Resistor

V Main Characteristic Parameters

(1) Nominal Resistance: The resistance value indicated on the resistor. 

(2) Allowable Deviation: The percentage of the difference between the nominal resistance and the actual resistance and the nominal resistance is the resistance deviation, which indicates the accuracy of the resistor. The correspondence between the allowable error and the accuracy level is as follows: ±0.5%-0.05, ±1%-0.1 (or 00), ±2%-0.2 (or 0), ±5%-I, ±10%-II, ± 20%-III level.

(3) Power Rating: The maximum power allowed to be dissipated by the resistor for long-term operation under normal atmospheric pressure of 90-106.6KPa and ambient temperature of -55°C to +70°C. Wirewound resistor rated power series is (W): 1/20, 1/8, 1/4, 1/2, 1, 2, 4, 8, 10, 16, 25, 40, 50, 75, 100, 150, 250, 500 non-wire wound resistor rated power series is (W): 1/20, 1/8, 1/4, 1/2, 1, 2, 5, 10, 25, 50, 100.

(4) Rated Voltage: The voltage converted from resistance and rated power.

(5) Maximum Working Voltage: The maximum continuous working voltage allowed. When operating at low air pressure, the maximum operating voltage is low.  

(6) Temperature Coefficient: The relative change of the resistance value caused by the temperature change of 1 °C. The smaller the temperature coefficient, the better the stability of the resistor. The resistance increases with increasing temperature as a positive temperature coefficient, and vice versa.

(7) Aging Coefficient: The percentage of the relative change of the resistance of the resistor under the long-term load of rated power. It is a parameter indicating the length of life of the resistor.

(8) Voltage Coefficient: The relative change of the resistor for every 1 volt change in the specified voltage range.

(9) Noise: An irregular voltage fluctuation generated in a resistor, including thermal noise and current noise. The thermal noise is due to the irregular movement of electrons inside the conductor, causing irregular voltage changes at any two points of the conductor.

VI Methods for Marking Values on Resistors

(1) Straight-marking Method: The resistance value is marked on the surface of the resistor by the number and unit symbol. The allowable error is directly expressed as a percentage. If the resistance is not marked, the deviation is ±20%.

(2) Text Notation: A regular combination of Arabic numerals and literal symbols is used to represent the nominal resistance, and the allowable deviation is also indicated by a literal symbol. The number before the symbol indicates the integer resistance, and the following numbers represent the first decimal resistance and the second decimal resistance. Text symbol indicating allowable error Text symbol D F G J K M Permissible deviation ±0.5% ±1% ±2% ±5% ±10% ±20%

(3) Digital Method: A three-digit digital representation of the nominal value on a resistor. From left to right, the first and second digits are valid values, and the third digit is the index, which is the number of zeros. The unit is Europe. Deviations are usually indicated by text symbols.

(4) Color Scale Method: Mark the nominal resistance and allowable deviation on the surface of the resistor with different colored bands or points. Most of the foreign resistors use the color standard method. Black-0, Brown-1, Red-2, Orange-3, Yellow-4, Green-5, Blue-6, Purple-7, Gray-8, White-9, Gold-±5%, Silver-±10 %, colorless - ±20% When the resistance is four rings, the last ring must be gold or silver, the first two digits are significant digits, the third digit is the power square, and the fourth digit is the deviation. When the resistance is five rings, the distance between the last ring and the front four rings is larger. The first three digits are significant digits, the fourth digit is the power square, and the fifth digit is the deviation.

Marking Convention of Resistors

Figure6. Marking Convention of Resistors

VII Commonly Used Resistors

(1) Potentiometer

A potentiometer is an electromechanical component that relies on the sliding of a brush on a resistor to obtain an output voltage that is related to the brush displacement.

  • Synthetic Carbon Film Potentiometer: The resistor body is made of ground carbon black, graphite, quartz and other materials coated on the surface of the substrate. This process is simple and is the most widely used potentiometer. The characteristics are high resolution, good wear resistance and long life. The disadvantages are current noise, large nonlinearity, moisture resistance and poor resistance stability.
  • Organic Solid Potentiometer: The organic solid potentiometer is a new type of potentiometer. It is a method of heating and pressing to press the organic resistor powder into the groove of the insulator. Compared with carbon film potentiometers, organic solid potentiometers have the advantages of good heat resistance, high power, high reliability and good wear resistance. However, the temperature coefficient is large, the dynamic noise is large, the moisture resistance is poor, the manufacturing process is complicated, and the resistance value is poor. It is used to regulate voltage and current in electronic devices that are miniaturized, highly reliable, and highly wear-resistant, as well as in AC and DC circuits.
  • Metallic Glass Uranium Potentiometer: The metallized uranium resistive paste is coated on a ceramic substrate by screen printing according to a certain pattern and sintered at a high temperature. The characteristics are: wide resistance range, good heat resistance, strong overload capability, moisture resistance, wear resistance, etc. It is a promising variety of potentiometers. The disadvantages are large contact resistance and current noise.
  • Winding Potentiometer: The winding potentiometer is made by using a constantan wire or a nichrome wire as a resistor and winding it around the insulating frame. The characteristics of the winding potentiometer are small contact resistance, high precision and small temperature coefficient. The disadvantages are poor resolution, low resistance and poor high frequency characteristics. Mainly used as voltage divider, varistor, zero adjustment and working point in the instrument.
  • Metal Film Potentiometer: The resistor body of the metal film potentiometer may be composed of an alloy film, a metal oxide film, a metal foil, and the like. It features high resolution, high temperature resistance, small temperature coefficient, low dynamic noise and good smoothness.
  • Conductive Plastic Potentiometers: DAP (diisopropyl phthalate) resistor paste is coated on the insulating body by special process, heated to form a resistive film, or thermoplastically pressed DAP resistor powder into the groove of the insulating substrate. The solid body formed inside serves as a resistor. The characteristics are: good smoothness, excellent resolution, good wear resistance, long service life, low dynamic noise, high reliability and chemical corrosion resistance. Servo systems for space devices, missiles, aircraft radar antennas, etc.
  • Potentiometer With Switch: There are rotary switch potentiometer, push-pull switch potentiometer, push-pull switch potentiometer.
  • Pre-adjustable P[otentiometer: The pre-adjustable potentiometer is in the circuit. Once it is debugged, the adjustment position is sealed with wax and is not adjusted under normal conditions.
  • Straight-slip Potentiometer.  The resistance value is changed by straight-slip method.
  • Double-connected Potentiometers: There are different-axis double-connected potentiometers and coaxial double-connected potentiometers
  • Non-contact Potentiometer: The non-contact potentiometer eliminates mechanical contact, has long life and high reliability, and is divided into photoelectric potentiometer and magnetic sensitive potentiometer. 


Figure7. Potentiometer

(2) Solid Carbon Resistors  

A solid resistor is made by mixing a carbonaceous granule, a filler, and a binder.

Features: low price, but its resistance error, noise voltage is large, stability is poor.

(3) Wirewound Resistors

It is made of a high-resistance alloy wire wound on an insulating skeleton, and is coated with a heat-resistant glaze insulating layer or an insulating varnish. The winding resistance has a low temperature coefficient, high resistance precision, good stability, heat and corrosion resistance, and is mainly used for precision high-power resistors. The disadvantage is that the high-frequency performance is poor and the time constant is large.

(4) Thin Film Resistors

A certain resistivity material is vapor-deposited on the surface of the insulating material by evaporation. Mainly as follows:

  • Carbon Film Resistors: Crystalline carbon is deposited on a ceramic rod skeleton. Carbon film resistors are the most widely used resistors due to their low cost, stable performance, wide resistance range, low temperature coefficient and low voltage coefficient.
  • Metal Film Resistors: The alloy material was vapor-deposited on the surface of the ceramic rod skeleton by vacuum evaporation. The metal film resistor has higher precision than the carbon film resistor, good stability, noise, and small temperature coefficient. It is widely used in instrumentation and communication equipment. 
  • Metal Oxide Film Resistor: A layer of metal oxide is deposited on the insulating rod. Since it is an oxide itself, it is stable at high temperatures, has thermal shock resistance, and has high load capacity.
  • Synthetic Film Resistance: The conductive composition suspension is applied to the substrate, so it is also called the film resistance. Because its conductive layer has a granular structure, it has high noise and low precision, and it is mainly used to manufacture high voltage, high resistance, small resistors.

(5) Metal Glass Uranium Resistor

The metal powder and the glass uranium powder are mixed and printed on the substrate by screen printing. Moisture resistant, high temperature, low temperature coefficient, mainly used in thick film circuits.

(6) Chip Resistor SMT

The chip resistor is a form of metallic glass uranium resistor. Its resistor body is a highly reliable bismuth series of glass uranium material which is sintered at a high temperature, and the electrode is made of silver-palladium alloy slurry. The utility model has the advantages of small volume, high precision and good stability, and since it is a chip component, the high frequency performance is good.

(7) Sensitive Resistors

Sensitive resistors are resistors whose characteristics are sensitive to temperature, voltage, humidity, light, gas, magnetic field, pressure, etc. The symbol of the sensitive resistor is to add a diagonal line to the symbol of the ordinary resistor, and mark the type of the sensitive resistor, such as: t. v.

  • Varistors: There are mainly SiC and zinc oxide varistors, and zinc oxide has more excellent properties.
  • Humidity Resistor: It consists of a moisture sensitive layer, an electrode and an insulator. The humidity sensitive resistor mainly includes a lithium chloride moisture sensitive resistor, a carbon humidity sensitive resistor, and an oxide humidity sensitive resistor. Lithium chloride humidity sensitive resistors decrease in resistance with increasing humidity. The disadvantage is that the test range is small, the characteristics are not repeatable, and the temperature is greatly affected. The disadvantage of carbon humidity varistor is that the low temperature sensitivity is low, the resistance value is greatly affected by temperature, and it is used less by aging characteristics. The oxide humidity sensitive resistor has superior performance and can be used for a long time. The temperature influence is small, and the resistance value is linear with the humidity change. There are tin oxide, nickel ferrite, and other materials.
  • Photoresistors: Photosensitive resistors are electronic components whose electrical conductivity changes with the change of light quantity. When a substance is exposed to light, the concentration of carriers increases and the conductivity increases. This is the photoconductive effect.
  • Gas Sensing Resistor: Download the document to the computer to find it easier to use.


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