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Executive Summary: What is a Phototransistor?A phototransistor is a light-sensitive semiconductor device that converts incident light into electric current while providing internal gain amplification. Unlike simple photodiodes, phototransistors utilize a bipolar junction structure (NPN or PNP) to amplify the signal, making them highly effective for optical switching, object detection, and encoding systems in modern 2026 electronics.Ⅰ Introduction to PhototransistorsThe phototransistor is a specialized semiconductor device engineered to detect light levels and modulate the current flowing between the emitter and collector based on the photon intensity it receives.While both phototransistors and photodiodes serve as optical sensors, the phototransistor distinguishes itself through high sensitivity attributed to the internal gain of its bipolar transistor architecture. As of 2026, this intrinsic amplification makes phototransistors the preferred choice for applications requiring robust signal detection without complex external amplification circuitry.Ⅱ Video Tutorial: How Phototransistors WorkVisual learners can understand the practical operation of light detection in the following tutorial.Phototransistor Tutorial Phototransistor Video Description:A comprehensive tutorial demonstrating how to utilize phototransistors for precise light detection in circuit design.  Ⅲ What Is a Phototransistor?A phototransistor is an electronic switching and current amplification component that operates by converting photon energy into electrical signals. When light strikes the exposed base-collector junction, a reverse current flows proportional to the luminance intensity.Widely used to convert light pulses into digital electrical signals, these components are powered by light interactions rather than solely electrical bias at the base. They offer high gain and low cost, making them ubiquitous in 2026 consumer electronics. Figure 1: Phototransistor SymbolFunctionally, phototransistors share similarities with photoresistors (LDRs), but with a key distinction: phototransistors generate current and voltage through the photovoltaic effect and amplification, whereas LDRs only change resistance.Transistors with the base terminal exposed are chemically doped to maximize light sensitivity. Photons striking the depletion layer generate electron-hole pairs, activating the transistor just as a base current would in a standard BJT. Silicon-based photosensors typically respond to visible and near-infrared radiation (approx. 400nm to 1100nm). Ⅳ How are Phototransistors Constructed?The phototransistor's structure is specifically optimized for photo-applications by maximizing the area of the base-collector junction. While ordinary bipolar transistors exhibit some photosensitivity, phototransistors feature significantly larger base and collector areas to capture maximum light flux.Figure 2: Construction of a PhototransistorⅤ Semiconductor Material EvolutionHistorical phototransistors utilized a homo-junction structure, fabricated entirely from germanium or silicon. In contrast, modern 2026 phototransistors often employ type III-V semiconductor materials, such as gallium arsenide (GaAs), to target specific wavelengths and increase efficiency.Key structural variations include:NPN Topology: The most popular configuration due to the higher mobility of electrons compared to holes.Heterostructures: Utilizing different materials on either side of the PN junction to enhance conversion efficiency.Mesa Structure: A common physical layout for optimized light absorption.Schottky Junctions: Occasionally used for the collector to improve switching speeds.To ensure optimal sensitivity, the emitter contact is frequently offset, preventing it from blocking light from reaching the active region. Ⅵ How Does a Phototransistor Work?A phototransistor operates by using light to control the flow of current, effectively replacing the base current of a standard transistor with photon energy.Biasing: The collector is biased positively relative to the emitter (in NPN), creating a reverse-biased Base-Collector (B-C) junction.Injection: Light strikes the B-C junction, generating electron-hole pairs.Amplification: The movement of these carriers constitutes a base current, which the transistor amplifies by its gain factor (hFE).Typically, the physical base terminal is left unconnected (floating), as the device is controlled entirely by incident light. Ⅶ Key Electrical CharacteristicsSince phototransistors are essentially Bipolar NPN Transistors with an exposed junction, their V-I characteristics resemble a standard BJT family of curves, but with Light Intensity (mW/cm²) replacing Base Current (IB).Dark Current: When no light is present, a minuscule leakage current flows from collector to emitter. In high-precision applications, minimizing this Dark Current is crucial.Light Current: As light intensity increases, the base current rises, triggering the amplification process. Figure 3: Reverse Bias Configuration The collector current characteristics curve below demonstrates the linear relationship between light intensity and output current in the active region.Figure 4: Collector Current vs. Irradiance Ⅷ Selection Criteria & PropertiesWhen selecting a component for 2026 designs, engineers must evaluate specific properties to ensure the device matches the optical environment.Critical Datasheet Properties:Peak Wavelength: The specific color of light (e.g., 850nm IR vs. 560nm Visible) the device is most sensitive to.Linearity: How accurately the output follows the input light intensity.Sensitivity: The ratio of output current to incident light power.Response Time: The rise and fall time, which determines the maximum data rate (typically slower than photodiodes).Acceptance Angle: The field of view from which the sensor can detect light. Ⅸ Common Types: BJT vs. FETPhototransistors are primarily categorized by their internal transistor architecture:BJT Phototransistor: The standard type. In darkness, it leaks only ~100 nA. Under illumination, it can conduct up to 50mA. This high current handling capability distinguishes it from photodiodes.Photo-FET (Field Effect Transistor): Utilizes light to generate a gate voltage that controls the drain-source current. Photo-FETs offer extremely high input impedance and are more sensitive to weak light signals, though they are less common in general switching applications. Ⅹ Practical Circuit Examples (2026 Applications)The primary goal of phototransistor circuits is to generate a usable output voltage from light-induced current. Unlike photodiodes which often require Transimpedance Amplifiers (TIA), phototransistors have built-in gain, allowing for simpler circuit designs.Common Configurations:Common-Emitter (Inverting): Output voltage drops as light increases.Common-Collector (Non-Inverting): Output voltage rises as light increases.Figure 5: Basic Amplifier Configurations 10.1 Step-by-Step Circuit Implementations 1. Light Operated Relay (Automatic Day Switch)Mechanism: When light strikes phototransistor Q1, it conducts, supplying base current to the driver transistor Q2. Q2 then activates the mechanical relay, turning on the connected load. 2. Darkness Operated Relay (Night Light)Mechanism: By inverting the logic, the relay activates only when light is absent. In darkness, the phototransistor turns off (high resistance), allowing the bias resistor to trigger Q2. 3. Light Interruption Alarm (Security System)Mechanism: This circuit functions as a tripwire. Under normal conditions (laser/light hitting sensor), the phototransistor pulls the SCR gate LOW (off). When the beam is broken by an intruder, the gate voltage rises, latching the SCR and sounding the alarm until manually reset. Ⅺ Datasheet Specifications to WatchTo ensure system reliability, consult the following parameters in manufacturer datasheets:Collector Current (IC): Maximum current the device can handle (typically 1mA - 50mA).Dark Current (ID): Leakage current in total darkness (lower is better for precision).Peak Wavelength (λp): The wavelength of maximum sensitivity.VCE(sat): Collector-Emitter saturation voltage.Rise/Fall Time (tr/tf): Critical for optical data transmission applications.Power Dissipation (Ptot): Thermal limits of the package. ⅻ Pros and Cons AnalysisSelecting the right optical sensor requires balancing sensitivity, speed, and cost.AdvantagesDisadvantagesHigh Gain: Produces higher current output than photodiodes, reducing the need for external amplifiers.Limited Voltage: Cannot withstand high voltages compared to Thyristors or Triacs.Cost-Effective: Inexpensive to manufacture and integrate into ICs.Slower Speed: Slower response time (lower bandwidth) compared to PIN photodiodes.Simplicity: Can drive small relays or logic gates directly in simple circuits.Temperature Sensitivity: Dark current increases significantly with temperature fluctuations. XIII Modern Applications in 2026Due to their versatility, phototransistors are integral to many modern technologies:Optocouplers (Optoisolators): Protecting low-voltage logic circuits from high-voltage spikes in power supplies.Optical Encoders: Used in robotics and motors to detect position and speed.Object Detection: Proximity sensors in smartphones and automated manufacturing lines.Safety Systems: Smoke detectors and light curtain barriers for industrial machinery.Remote Control Receivers: IR detection for consumer electronics (though often integrated with demodulators). XIV Comparison: Photodiode vs. PhototransistorWhile both detect light, their use cases differ based on speed and sensitivity needs.FeaturePhotodiodePhototransistorOutputLow Current (µA)High Current (mA) - AmplifiedResponse SpeedVery Fast (Nanoseconds)Moderate (Microseconds)ApplicationsFiber Optics, High-Speed DataRemote Controls, Light Switches, EncodersNoiseLow NoiseHigher Noise levels XV Frequently Asked Questions1. What type of device is a phototransistor?A phototransistor is a bipolar semiconductor device. It functions as a transistor where the base current is generated by incident photons striking the exposed semiconductor junction, rather than an electrical connection.2. What is the main difference between a standard transistor and a phototransistor?Physically, the primary difference is the packaging. A phototransistor has a transparent lens or window to allow light to reach the junction, and it often lacks an external base pin. Electrically, it is controlled by light intensity rather than input current.3. Is a phototransistor considered a sensor?Yes, it is a discrete photosensor. It detects the presence and intensity of light and converts it into a measurable electrical signal.4. How do you test if a phototransistor is working?You can test it using a multimeter or a simple circuit:Connect the phototransistor in series with a resistor and LED to a power source (checking polarity).Expose the sensor to light; the LED should brighten.Cover the sensor; the LED should dim or turn off.5. Which is better: Photodiode or Phototransistor?Neither is universally "better"; it depends on the application. For high-speed data (like fiber optics), a photodiode is superior. For switching and sensing without extra amplifiers, a phototransistor is more efficient due to its internal gain.{ "@context": "https://schema.org", "@graph": [ { "@type": "Article", "headline": "Phototransistors: The Ultimate 2026 Guide", "datePublished": "2021-12-02", "dateModified": "2026-01-07", "description": "A comprehensive guide to phototransistors, covering construction, working principles, circuit diagrams, and 2026 applications.", "image": "https://www.kynix.com/editor_u/image/20211202/2021120216390176.jpg", "author": { "@type": "Organization", "name": "Kynix Electronics" } }, { "@type": "FAQPage", "mainEntity": [ { "@type": "Question", "name": "What type of device is a phototransistor?", "acceptedAnswer": { "@type": "Answer", "text": "A phototransistor is a bipolar semiconductor device where the base current is generated by incident photons striking the exposed junction." } }, { "@type": "Question", "name": "What is the difference between a transistor and a phototransistor?", "acceptedAnswer": { "@type": "Answer", "text": "The main difference is that a phototransistor has an exposed optical window and is controlled by light intensity, whereas a standard transistor is controlled by electrical current at the base pin." } }, { "@type": "Question", "name": "Is a phototransistor a sensor?", "acceptedAnswer": { "@type": "Answer", "text": "Yes, a phototransistor is a discrete photosensor that converts light intensity into an electrical signal." } }, { "@type": "Question", "name": "Which is better: Photodiode or Phototransistor?", "acceptedAnswer": { "@type": "Answer", "text": "Photodiodes are better for high-speed data applications, while phototransistors are better for switching and sensing applications requiring higher sensitivity and gain." } } ] }, { "@type": "HowTo", "name": "How to Build a Simple Light Interruption Alarm", "step": [ { "@type": "HowToStep", "name": "Setup the Phototransistor", "text": "Connect the phototransistor to a pull-down resistor to create a voltage divider." }, { "@type": "HowToStep", "name": "Connect the SCR", "text": "Connect the output of the phototransistor junction to the Gate of an SCR (Silicon Controlled Rectifier)." }, { "@type": "HowToStep", "name": "Align the Light Source", "text": "Point a laser or light beam directly at the phototransistor. This keeps the SCR gate low (Off)." }, { "@type": "HowToStep", "name": "Trigger the Alarm", "text": "Interrupt the light beam. The phototransistor turns off, voltage spikes at the SCR gate, latching the alarm on." } ] } ]}

2026 Executive Summary: Reading SMD Resistor CodesHow do you read SMD resistor codes? For standard 3-digit codes, the first two numbers are significant digits, and the third is the multiplier (10^x). For 4-digit codes (precision), the first three are significant. The EIA-96 system uses a two-digit code and a letter multiplier. This authoritative guide covers all calculation methods, updated for 2026 industry standards.What are SMD Resistors? (2026 Overview)SMD Resistor, also known as a Chip Resistor, is a surface-mount passive component essential for modern high-density electronics. Manufactured by sintering metal powder and glass glaze on a ceramic substrate, these components offer superior resistance to humidity, high temperatures, and vibration compared to legacy through-hole parts. As of 2026, they are the industry standard for everything from AI hardware to smartphones. While different resistors feature varied specifications, the critical question remains: how are these microscopic resistance values marked and decoded? Figure 1. Structure of SMD ResistorsⅠ How to Read Resistor Markings: 4 Key MethodsTo master resistor identification, one must understand the four global standards used to denote resistance values. These methods are governed by IEC 60062 standards:1. Direct Marking MethodThis method prints the actual numbers and unit symbols directly on the resistor surface. The allowable error (tolerance) is expressed as a percentage. If no deviation is marked, the standard tolerance is typically ±20%.2. Text Symbol MethodThis approach uses a combination of Arabic numerals and text symbols to indicate the nominal resistance and tolerance. The number preceding the symbol represents the integer value, while the number following represents the decimal. Tolerance characters are standardized: D (±0.5%), F (±1%), G (±2%), J (±5%), K (±10%), M (±20%).3. Digital Method (Most Common for SMD)This method uses a 3-digit or 4-digit code. Read from left to right, the initial digits represent the significant figures (effective values), and the final digit is the exponent (multiplier), indicating the number of zeros to add. The unit is always Ohms (Ω).4. Color Code Marking MethodWhile rare on modern SMDs (except MELF packages), color bands are the standard for through-hole resistors. The bands represent values and multipliers:Black (0), Brown (1), Red (2), Orange (3), Yellow (4)Green (5), Blue (6), Violet (7), Gray (8), White (9)Tolerance: Gold (±5%), Silver (±10%), Colorless (±20%)Figure 2. Universal Resistor Color Code DiagramReading Tip: For a four-band resistor, the last band (usually gold/silver) is the tolerance. The first two bands are digits, and the third is the multiplier. For five-band precision resistors, the first three are digits, the fourth is the multiplier, and the fifth is the tolerance. Ⅱ Calculating SMD Resistor Values (Step-by-Step)2.1 Understanding Character Code MarkingsVideo: SMD Resistor Coding ExplainedMarking chip resistors requires a compact system due to the component's microscopic size. While large packages may use full numbers, 0603, 0805, and 1206 packages use coded systems. Here is the 2026 standard breakdown for decoding these values:The 3-Digit System (Standard Tolerance ±5%):1. The first and second digits represent the significant resistance figures.2. The third digit is the multiplier (10^x).Decoding Guide by Third Digit:• Ends in 0: No extra zeros. Example: 100 = 10 Ω.• Ends in 1: Add one zero (x10). Example: 101 = 100 Ω.• Ends in 2: Add two zeros (x100). Example: 102 = 1,000 Ω (1 kΩ).• Ends in 3: Add three zeros (x1,000). Example: 103 = 10,000 Ω (10 kΩ).• Ends in 4: Add four zeros. Example: 104 = 100 kΩ.• Ends in 5: Add five zeros. Example: 105 = 1 MΩ.• Ends in 6: Add six zeros. Example: 106 = 10 MΩ.The 4-Digit System (Precision Tolerance ±1%):For higher precision, three significant digits are used. Example: 1001 means 100 + one zero = 1000 Ω (1 kΩ).Note: Ultra-small packages like 01005, 0201, and 0402 are physically too small for markings. These must be measured with a multimeter or tracked via reel tape labeling.2.2 Real-World Calculation ExamplesCase 1: 3-Digit Code (±5% Tolerance)This uses two significant digits followed by a multiplier.Calculation: 153 → 15 followed by 3 zeros → 15,000 Ω = 15 kΩDecimal Values: "R" represents the decimal point. Code 6R8 → 6.8 ΩCase 2: 4-Digit Code (±1% Tolerance)Common on packages like 0805, 1206, and 2512. The first three digits are significant.Calculation: 2372 → 237 followed by 2 zeros → 23,700 Ω = 23.7 kΩDecimal Values: 3R24 → 3.24 ΩCase 3: EIA-96 System (The "Cryptic" Code)Used for 1% tolerance resistors on small 0603 packages where 4 digits won't fit. This system uses a two-digit code (referencing a lookup table) and a letter multiplier.Format: [Code] [Letter]Example Multipliers: Y=0.01, X=0.1, A=1, B=10, C=100, D=1000, E=10000.E-96 Series Standard Resistance Lookup Table (Partial)ValueCodeValueCodeValueCode100011471721533102021501822134105031541922635107041582023236110051622123737113061652224338115071692324939118081742425540121091782526141124101822626742127111872727443130121912828044133131962928745137142003029446140152053130147143162103230948 ValueCodeValueCodeValueCode316494646568181324504756669882332514876771583340524996873284348535116975085357545237076886365555367178787374565497280688383575627382589392585767484590402595907586681412606047688792422616197790993432626347893194442636497995395453646658097696EIA-96 Calculation Examples:Code 29B: Lookup "29" in table → Value 196.Multiplier "B" → x10.Result: 196 × 10 = 1.96 kΩCode 10X: Lookup "10" in table → Value 124.Multiplier "X" → x0.1.Result: 124 × 0.1 = 12.4 ΩCase 4: The Underlined Code (Special 0603 Case)Sometimes you see a standard 3-digit code with a line under it on an 0603 package. This usually indicates the manufacturer uses the E-24 series values (loose tolerance) rather than E-96, but the calculation is standard.122 = 12 × 100 = 1.2 kΩ680 = 68 × 1 = 68 Ω (Note: 680 does not mean 680 ohms here, it means 68 and zero extra zeros). Ⅲ How to Identify Damaged SMD Resistor Values?When a resistor is burned or the marking is unreadable, use these four forensic engineering methods to deduce the value:1. Parallel Circuit ComparisonPCB designs, especially in power supplies and audio amplifiers, often use symmetrical channels. • Example: In an LCD backlight driver, if the resistor in Channel A is burnt, check the corresponding position in Channel B. Often R17 = R51, or R23 = R48. Measure the intact sibling component to find the value.2. Circuit Context Analysis (Pull-Up/Pull-Down)For Microcontroller (MCU) circuits, resistors connected to GPIO pins are typically "pull-up" or "pull-down" resistors used to stabilize logic levels.• Common Values: 3.3kΩ, 4.7kΩ, 10kΩ.• Deduction: If the resistor connects a data line to VCC or GND, replacing it with a 10kΩ resistor is a safe starting point for testing.3. Reference Similar SchematicsIf the exact schematic is unavailable, search for schematics of devices using the same main IC. Manufacturers often use the "Reference Design" provided by the chipmaker, meaning the peripheral resistor values will be identical across different brands.4. The Potentiometer Test (Advanced)If all else fails, trace the circuit diagram. Temporarily solder a high-value potentiometer (variable resistor) in place of the damaged part. Power on the device and slowly adjust the resistance while monitoring voltage levels until the circuit functions correctly. Remove the potentiometer, measure its set resistance, and replace it with the closest standard fixed resistor.  Ⅳ Top SMD Resistor Manufacturers (2026 Updated)Reliability is paramount in 2026 electronics. The following brands are currently recognized as Tier-1 manufacturers for automotive, industrial, and consumer electronics:YAGEO: Global leader in chip resistors (acquired KEMET).Vishay: Known for high-precision, military-grade foil resistors.Panasonic: Industry standard for high-reliability automotive parts.KOA Speer: Major supplier for automotive and industrial markets.Bourns: Famous for circuit protection and resistors.TE Connectivity: Specialist in harsh environment resistors.Other Notable Brands: ROHM, Ohmite, Welwyn, TT Electronics, UNI-ROYAL (Uniohm). ⅴ Frequently Asked Questions (FAQ)1. What is an SMD resistor used for?SMD (Surface Mount Device) resistors limit current, divide voltage, and stabilize signal lines in compact electronic circuits. They are essential for miniaturizing devices like smartphones, wearables, and IoT sensors where traditional through-hole components would be too bulky. 2. How do I calculate the value of a 3-digit SMD resistor?Use the formula: [1st Digit][2nd Digit] x 10^[3rd Digit]. For example, "103" means 10 x 10^3 (1000) = 10,000 Ohms or 10kΩ. 3. What does "R" mean in a resistor code like 4R7?The letter "R" represents the decimal point. It is used when the resistance value is too small to use a multiplier code. Therefore, 4R7 equals 4.7 Ohms. 4. What is the difference between 103 and 1002 markings?Both equal 10kΩ, but the marking indicates tolerance. "103" (3-digit) typically indicates ±5% tolerance. "1002" (4-digit) indicates higher precision, typically ±1% tolerance. 5. How do I read the cryptic "01A" or "29B" codes?These are EIA-96 codes for 1% precision resistors on small 0603 parts. You cannot read them directly; you must use an EIA-96 lookup table. The number refers to a value code, and the letter is the multiplier. 6. Why do some SMD resistors have no markings?Resistors in package sizes 0402, 0201, and 01005 are physically too small to print legible text. To identify these, you must measure them with a multimeter or refer to the manufacturer's reel tape packaging. 7. What does SMD stand for?SMD stands for Surface Mounted Device. It refers to the component itself. SMT (Surface Mount Technology) refers to the manufacturing process of placing these components onto a PCB. 8. What materials are SMD resistors made of?Most SMD resistors are "Thick Film" or "Thin Film" types. They consist of a ceramic substrate (alumina) coated with a resistive paste (metal oxides and glass). This is fired in a kiln, laser-trimmed to the exact value, and then coated with a protective layer.{ "@context": "https://schema.org", "@type": "Article", "mainEntityOfPage": { "@type": "WebPage", "@id": "https://www.kynix.com/Blog/How-to-Read-the-Value-of-SMD-Resistor-Example-Explained.html" }, "headline": "How to Read SMD Resistor Codes: The 2026 Guide to 3-Digit, 4-Digit & EIA-96 Markings", "image": "https://www.kynix.com/editor_u/image/20211027/2021102711243403.jpg", "author": { "@type": "Organization", "name": "Kynix Electronics" }, "publisher": { "@type": "Organization", "name": "Kynix Electronics", "logo": { "@type": "ImageObject", "url": "https://www.kynix.com/logo.png" } }, "datePublished": "2021-10-27", "dateModified": "2026-01-08", "description": "Learn how to calculate SMD resistor values using 3-digit, 4-digit, and EIA-96 codes. Includes updated 2026 lookup tables and troubleshooting steps for damaged components.", "articleBody": "SMD Resistor, called Chip Resistor, is one type of resistors..."}{ "@context": "https://schema.org", "@type": "FAQPage", "mainEntity": [{ "@type": "Question", "name": "What is an SMD resistor used for?", "acceptedAnswer": { "@type": "Answer", "text": "SMD (Surface Mount Device) resistors limit current, divide voltage, and stabilize signal lines in compact electronic circuits like smartphones and IoT devices." } }, { "@type": "Question", "name": "How do I calculate the value of a 3-digit SMD resistor?", "acceptedAnswer": { "@type": "Answer", "text": "Use the formula: [1st Digit][2nd Digit] x 10^[3rd Digit]. For example, 103 means 10 x 1000 = 10,000 Ohms (10kΩ)." } }, { "@type": "Question", "name": "What does 'R' mean in a resistor code like 4R7?", "acceptedAnswer": { "@type": "Answer", "text": "The letter 'R' acts as a decimal point. 4R7 represents 4.7 Ohms." } }, { "@type": "Question", "name": "How do I read EIA-96 codes like 01A?", "acceptedAnswer": { "@type": "Answer", "text": "EIA-96 codes require a lookup table. The number represents a significant value, and the letter represents a multiplier. For '01A', 01 is 100 and A is x1, resulting in 100 Ohms." } }]}{ "@context": "https://schema.org", "@type": "HowTo", "name": "How to Read a 3-Digit SMD Resistor Code", "description": "Step-by-step guide to calculating resistance from standard 3-digit markings found on most chip resistors.", "step": [{ "@type": "HowToStep", "name": "Identify the Significant Digits", "text": "Read the first two numbers on the resistor. These are your significant digits (e.g., in '103', the significant digits are '10')." }, { "@type": "HowToStep", "name": "Identify the Multiplier", "text": "Read the third number. This indicates the power of 10 to multiply by (or how many zeros to add). In '103', the multiplier is 3 (10^3 or 1000)." }, { "@type": "HowToStep", "name": "Calculate the Result", "text": "Multiply the significant digits by the multiplier. 10 x 1000 = 10,000 Ohms (10kΩ)." }]}

  Catalog Ⅰ Introduction Ⅱ Resistor network  in Series vs in Parallels 2.1 Resistor in Series  Ⅲ Resistor Circuit in Series vs in Parallels 3.1 Resistor Circuit in Series 3.2 Resistor Circuit in Parallels Ⅳ Equation in Series vs Parallels 4.1 Series Resistor Equation 4.2 Parallel Resistor Equation Ⅴ Examples 5.1 Resistors in Series Example 5.2 Resistor in Parallels Ⅵ Applications Ⅶ Summary 7.1 Resistors in Series Summary 7.2 Resistors in Parallel Summary Ⅷ FAQ Ⅰ Introduction   Individual resistors can be commonly connected to three types of circuits such as series, parallel, or a combination of series and parallel connections to form more complex resistor networks, the equivalent resistance of which is the mathematical combination of the individual resistors connected together.   A resistor is not only a fundmental electronic component that can be applied to convert a voltage to a current or a current to a voltage but it can also be used to place a different weighting on the converted current and/or voltage by correctly adjusting its value, allowing it to be used in voltage reference circuits and applications.    A single equivalent resistor can take place of resistors in series or complicated resistor networks. REQ, or impedance, ZEQ, and regardless of the resistor network's combination or complexity, all resistors follow the same basic rules defined by Ohm's Law and Kirchhoff's Circuit Laws.   Resistors in Series | Electricity and Circuits | Don't Memorise   Ⅱ Resistor network  in Series vs in Parallels     2.1 Resistor in Series   When resistors are daisy-chained together in a single line, they are connected in "Series." Because there is no other way for the current flowing through the first resistor to go, it has to pass through the second, third, and so on. The current that flows through one resistor should flow through the others as well because it can only take one path, so resistors in series have a Common Current flowing through them.   The current flowing through a series of resistors will then be the same at all points in a series resistor network. As an example:       Figure1：Current flowing through a series     In the following example, resistors R1, R2, and R3 are connected in series between points A and B, with a common current, I, flowing through them.         2.2 Resistor in Parallels In contrast to the previous series resistor circuit, the circuit current in a parallel resistor network can take more than one path because there are multiple paths for the current. Parallel circuits are then classified as current dividers.   Because the supply current can flow through multiple paths, the current may not be the same through all of the parallel network's branches. The voltage drop across all resistors in a parallel resistive network, on the other hand, so it is. Then, parallel-connected resistors have a common voltage across them, as do all parallel-connected elements.       Figure2: Circuit current in a parallel      Ⅲ Resistor Circuit in Series vs in Parallels   3.1 Resistor Circuit in Series         Figure3: Resistor Circuit in series     Because the resistors are linked in series, the same current flows through each resistor in the chain, and the total resistance, RT, of the circuit must equal the sum of all the individual resistors added together. That is            Figure4: resistance     and by taking the individual values of the resistors in our simple example above, the total equivalent resistance, REQ is therefore given as:   REQ = R1 + R2 + R3 = 1kΩ + 2kΩ + 6kΩ = 9kΩ     3.2 Resistor Circuit in Parallels       Figure5: resistor circuit in parallel     The total resistance, RT, of the circuit in the previous series resistor network was equal to the sum of all the individual resistors added together. The equivalent circuit resistance RT is calculated differently for parallel resistors. Instead of the resistances themselves, the reciprocal (1/R) value of each is added together, with the inverse of the algebraic sum giving the equivalent resistance as shown. Instead of the resistances themselves, the reciprocal (1/R) value of each is added together, with the inverse of the algebraic sum giving the equivalent resistance as shown.     Ⅳ Equation in Series vs Parallels 4.1 Series Resistor Equation Because it is the algebraic sum of the individual resistances, the total or equivalent resistance, RT, has the same effect on the circuit as the original combination of resistors. If two equal and of the same value resistances or impedances are connected in series, the total or equivalent resistance, RT, is equal to twice the value of one resistor. That is equal to 2R for two equal resistors in series, 3R for three equal resistors in series, and so on.         Figure6：Series Resistor Equation     If two series resistors or impedances are unequal and of different values, the total or equivalent resistance, RT, is equal to the mathematical sum of the two resistances. R1 + R2 is the answer. The equivalent resistance of three or more unequal (or equal) resistors connected in series is: R1 + R2 + R3 +..., etc.       Figure7：Equivalent resistance     One important thing to remember about resistors in series networks is to double-check your math. The total resistance (RT) of any two or more resistors connected in series is always greater than the value of the chain's largest resistor. In our previous example, RT = 9k, whereas the largest resistor value is only 6k.     4.2 Parallel Resistor Equation         The algebraic sum of the inverses of the individual resistances is the inverse of the equivalent resistance of two or more resistors connected in parallel. If the two parallel resistances or impedances are equal and of the same value, the total or equivalent resistance, RT, is equal to half the value of one resistor. That is R/2 for two equal resistors in parallel, R/3 for three equal resistors in parallel, and so on.       Figure8: Resistances or impedances     Because the equivalent resistance is always less than the smallest resistor in the parallel network, as more parallel resistors are added, the total resistance, RT, will always decrease.     Ⅴ Examples   5.1 Resistors in Series Example Calculate the voltage drops across X and Ya) Without RL connected   b) With RL connected         Figure9: series example     As shown above, the output voltage Vout without the load resistor connected gives us the required output voltage of 6V, but when the load is connected, the output voltage drops to only 4V. (Resistors in Parallel).   Then we can see that a loaded voltage divider network's output voltage changes as a result of the loading effect because the output voltage Vout is determined by the R1 to R2 ratio. However, as the load resistance, RL, approaches infinity (), the loading effect diminishes and the voltage ratio of Vout/Vs is unaffected by the addition of the load on the output. Then, as the load impedance increases, the loading effect on the output decreases.   Attenuation is the effect of lowering a signal or voltage level, so when using a voltage divider network, it is essential to have cautiousness. This loading effect could be compensated for by using a potentiometer instead of fixed value resistors and adjusting the potentiometer accordingly. This method also compensates the potential divider for variations in resistor tolerances.     5.2 Resistor in Parallels   Find the total resistance, RT of the following resistors connected in a parallel network.       Figure10: Total resistance     The total resistance RT across the two terminals A and B is calculated as:       Figure11: Total resistance RT     This reciprocal calculation method can be used to calculate any number of individual resistances connected in a single parallel network. If, on the other hand, there are only two individual resistors connected in parallel, we can use a much simpler and faster formula to find the total or equivalent resistance value, RT, and thus help reduce the reciprocal maths a little.         Figure12: Single parallel network     Ⅵ Applications   Series We've seen how Resistors in Series can be applied to generate different voltages across themselves, and how this genre of resistor network can be used to create a voltage divider network. We can convert an analog quantity being sensed into a suitable electrical signal that can be measured by replacing one of the resistors in the voltage divider circuit above with a Sensor such as a thermistor, light-dependent resistor (LDR), or even a switch.     Parallel The five resistive networks shown above may appear to be different, but they are all arranged as Resistors in Parallel, and thus the same conditions and equations apply.   Ⅶ Summary   7.1 Resistors in Series Summary When two or more resistors are connected end-to-end in a single branch, Reputedly, they are connected in series. Resistors in series carry the same current, but the voltage drop across them is not the same as their resistance values result in different voltage drops across each resistor, as determined by Ohm's Law (V = I*R). Then there are series circuits, which are voltage dividers. Individual resistors in a series resistor network add together to give the series combination's equivalent resistance, (RT). A series circuit's resistors can be swapped without affecting the total resistance, current, or power to each resistor or the circuit.     7.2 Resistors in Parallel Summary   When two or more resistors are connected in such a way that their terminals are connected to the terminals of the other resistor or resistors, they are connected in parallel. The voltage across each resistor in a parallel combination is the same, but the currents flowing through them are not because of their resistance value and Ohms Law. Parallel circuits are then used as current dividers. Reciprocal addition is used to find the equivalent or total resistance, RT, of a parallel combination, and the total resistance value is always less than the smallest individual resistor in the combination. Within the same combination, parallel resistor networks can be swapped without changing the total resistance or total circuit current. Resistors connected in a parallel circuit will continue to operate even if one of them is open-circuited.   Ⅷ FAQ   1. How do you calculate resistors in series? In a series circuit you will need to calculate the total resistance of the circuit in order to figure out the amperage. This is done by adding up the individual values of each component in series. ... To calculate the total resistance we use the formula: RT = R1 + R2 + R3. 2 + 2 + 3 = 7 Ohms. R total is 7 Ohms.     2. Do you add up resistance in series? How do you know if a series resistor is parallel? The trick is to look at the nodes in the circuit. A node is a junction in the circuit. Two resistor are in parallel if the nodes at both ends of the resistors are the same. If only one node is the same, they are in series.   3. Which resistor gets the most current? which resistor has the most current passing through it? the 5-Ω resistor has the most current passing through it, since I = V/R.     4. What is resistor connected in parallel? Resistors are in parallel if their terminals are connected to the same two nodes. The equivalent overall resistance is smaller than the smallest parallel resistor. Written by Willy McAllister.     5. What happens to resistors in parallel? When resistors are connected in parallel, more current flows from the source than would flow for any of them individually, so the total resistance is lower. Each resistor in parallel has the same full voltage of the source applied to it, but divide the total current amongst them.     6. Why do resistors decrease resistance in parallel? Resistors in parallel   In a parallel circuit, the net resistance decreases as more components are added, because there are more paths for the current to pass through. The two resistors have the same potential difference across them. ... The total current in the circuit is the sum of the currents through each branch.            

Introduction As we all know, Resistors play a important role in limiting current in the circuit. Among then, pull-up resistors and pull-down resistors are often mentioned and frequently used in electronics. The pull-up is to clamp the uncertain signal to a high logical level through a resistor, which acts as a current limiter; while the pull-down resistor clamps the uncertain signal to a low logical level. Because there are only two states of high level and low level in digital circuits, it is uncertain at the initial stage of digital signals. Pull-up/ Pull-down Resistor - Explained ( with calculation ) Catalog Introduction Ⅰ Why Pull-down and Pull-up Resistor? Ⅱ Pull-up & Pull-down Resistor Circuits Ⅲ What the Role of Pull-up and Pull-down Resistor? Ⅳ Pull-up & Pull-down Resistor Applications Ⅴ How to Select Pull-up & Pull-down Resistors? Ⅵ FAQ Ⅰ Why Pull-down and Pull-up Resistor? Pull-up and pull-down resistors are often applied when interfacing a switch or some other input with a microcontroller or other digital gates. That is, in the initial stage of digital circuit power-on, because the high logical level and low level of the output state are uncertain, in order to make the circuit state normally, a pull-up resistor or pull-down resistor is needed to stabilize the uncertain circuit state. The low logical level is connected to GND inside the IC, and the high level is connected to the super resistance inside the IC.The pull-up resistor connects with the status port of the power supply. Simply put, the high voltage is applied to this point, where the potential will increase. The pull-down resistor means that the resistor is connected to the negative pole, and there is also the case of digital grounding. When the input port signal changes due to different circuit forms, the change will be fed back to the output port, so that the output port acquires a state that should have been completed, but the input port has no signal at this time and keep the original state.According to the above understanding, many people may feel awkward. Take an example from daily life, when you use the key to open the door, people enter but the door is not closed, at this time, you can add a switch to make the door close automatically. Figure 1. Schematic of Pull-up Resistor at Positive Input The above schematic diagram explains why the positive pole and the input terminal resistor can high the level. The two resistances of the port are assumed to be equivalent. We can get that the voltage of the port is 2.5V according to Ohm's law. By connecting the pull-up resistor (red part), the voltage of the port rises at this time, calculate the port voltage. Among them, 10K is connected in parallel with the later connected 1K, and the resistance must be greater than or equal to 1K, which is equivalent to the series relationship between 1K and the 10K resistor below, but the passing current is actually the same. Finally, the voltage of the two 10K resistors increases, and the terminal voltage also increases.The pin connected to the IC and power (or ground) is not necessarily a pull-down resistor. When this happens, many people may think that the red part of the figure is also a pull-down resistor. However, it is not connected in series with any pin or ground. In fact, it is used for circuit startup resistor, not pull-up/pull-down resistor. For the pull-up/pull-down resistors, it is only for the input port and the output port. Although some circuits will connect the pull-up and pull-down resistors to the redundant ports for stability, not all the resistors are connected to one pin of the IC all the time, and the other pin is connected to power or ground to represent the pull-up and pull-down resistors.   Ⅱ Pull-up & Pull-down Resistor Circuits Look at the following analyses to figure out what are pull-up resistor and pull-down resistor in circuits. Pull-up resistors are used to ensure that a wire is pulled to a high logical level in the absence of an input, while pull-down resistors ensure the voltage between VCC and a microcontroller pin is actively controlled. Just check the details below. Figure 2. OC(TTL) Circuit，OD(COMS) Circuit When the I/O port of the IC is in high level, the impedance between the node and GND is very large, which can be understood as infinite. At this time, it is connected to VCC through a pull-up resistor (such as 4.7K ohm, 10K ohm resistor), and the voltage divider of the pull-up resistor is almost negligible. When the I/O port node is in low level, it can be directly connected to GND. At this time, VCC and GND are connected through the pull-up resistor, and the current passing through is very small, which can be ignored.The level value are relative to the ground level, so you should refer to the ground level value. See if those pins are connected to the ground, it has nothing to do with whether they are connected to peripheral devices.Connect a 10K ohm or 4.7K ohm pull-up resistor between the node and +5V to pull up the potential of this node. Often this node requires a single-chip microcomputer or other controller to control it (and this node is connected to I/O). If you simply want to make this node a high level, and the output impedance is very large, you can directly connect the power supply, but if the microcontroller wants to make this node low, that is, the node is grounded inside the microcontroller, so that the 5V power supply and the ground are short-circuited.In addition, when this node is required to be at a high level, the impedance between this node and the ground is generally very large. For example, with an impedance of 100K ohms, when connect a 10K ohm pull-up resistor, the voltage at this point is 100KΩ/(100K +10K)*5V=4.5V, so it can also get a high level.When the node is required to be low level, just connect it to the ground, and there is a 10K resistor between the power supply and the ground, so that it will not be short-circuited. When it is low, there is a loop formed by a load between the power supply and the ground. Sometimes this node will be connected with a resistor in series. Because the current flows to the place with low impedance, the current will flow to the ground through the resistor connected to the power supply instead of Flow to this resistance connected to the node, because the resistor connected to this node has a high impedance, so the potential at this point is in low level.It can be considered that, for the I/O port of the IC, controlling the high and low levels inside the IC is equivalent to controlling the O/O port to be connected to its internal GND or a very large resistor, such as 100K ohms. When the I/O port is the low level (0V), inside the IC, the pin that controls the O/O port of the IC chip is connected to GND.When the I/O port is at a high level, such as 5V, the I/O port pin is connected to a very large resistor in the chip, such as 100K ohms, and sometimes another one is connected in series at the I/O node. A resistor with a small resistance value, such as 68 ohms, because the current flows to a place with low impedance, when the I/O port and GND inside the chip are connected to a low level, the pull-up resistor and the GND inside the chip form a loop.At this time, the current at the I/O port node will flow to the GND inside the chip, because a small resistance resistor is connected in series at the node, which is high resistance relative to GND, so the current will not flow through this series resistor.Using a pull-down resistor, when the I/O port is in a high-impedance state, the pull-up resistor can keep it in a high-level state. That is, when the I/O port is in the high-impedance state, using a pull-down resistor to connect this port to GND. The high-impedance state has a large resistance value, which can be understood as disconnection, in fact, it is actually a large resistor inside the chip. The resistors are connected and pulled to the ground, so there is no current and the level value is 0. It can only work unless a high level value is given to this pin. Figure 3. Pull-up and Pull-down Resistor in MCU   Ⅲ What the Role of Pull-up and Pull-down Resistor? As for the purpose of pull-up & pull-down resistors, generally speaking, the pull-up resistor increases the current, and the pull-down resistor is used to absorb the current.1) Increase the voltage level.When the TTL circuit drives the CMOS circuit, if the output high level of the TTL circuit is lower than the lowest high level of the CMOS circuit, then it is necessary to connect a pull-up resistor to the output terminal of the TTL to increase the value of the output high level. The OC gate circuit must add a pull-up resistor to increase the high-level value of the output.2) Increase the drive capability of the output pin.In order to enhance the drive capability of the output pins, pull-up resistors are often used on some single-chip pins.3) The N/A pin (the pin not connected) should be anti-static and anti-interference.On the CMOS chip, in order to prevent damage caused by static electricity, the unused pins cannot be left floating. Generally, a pull-up resistor is connected to reduce the input impedance, provide a leakage path, and improve the anti-electromagnetic interference ability of the bus. Because the pin is left floating, it is easier to receive electromagnetic interference from the outside world.4) Resistance matchIn the long-line transmission, the resistance mismatch can easily cause the reflected wave interference. In addition, the pull-down resistor makes the resistance match, which can effectively suppress the reflected wave interference.5) Preset space state/default potentialPull-up or pull-down resistors are connected to some CMOS input terminals to preset the default potential. When these pins are not used, these input terminals are pulled down to low level or pulled up to high level. The state when idle on the bus such as I2C is obtained by the pull-up and pull-down resistors.6) Improve the noise tolerance of the chip input signal.If the input terminal is in a high-impedance state, or in a floating state, a pull-down or pull-down resistor needs to be added at this time, so as to avoid the random level. Similarly, if the output terminal is in a passive state, a pull-down or pull-down resistor needs to be added. For example, the output terminal is only the collector of a transistor, thereby improving the noise tolerance of the chip input signal and enhancing the anti-interference ability through a pull-up resistor or pull-down resistor. Figure 4. Pull-up/ Pull-down Resistor   Ⅳ Pull-up & Pull-down Resistor Applications When to use pull-up or pull-down resistors? Look at the following cases explained.1) If a pull-up & pull-down resistor is used for the input signal pin, the usual function is clamping the signal to a certain level to prevent the signal line from appearing in an uncertain state. In practical applications, the 10K ohm resistor is the most used pull-up resistor. Whether to use a pull-up resistor or a pull-down resistor depends mainly on the needs of the circuit system itself. For example, for a highly effective enable control signal, we hope that the circuit system be in an invalid state after power-on, and then a pull-down resistor will be used.Assuming that the enable signal is used to control the motor, if it is left floating, the signal line may be triggered falsely to a high level by other noise interference after power-on (or during operation), resulting in undesired rotation of the motor, and a pull-down resistor can be added at this time. Correspondingly, for the active-low reset control signal (RST#), if we want to be in an inactive state after power-on reset, a pull-up resistor should be used.2) Most chips with logic control functions (such as single-chip microcomputers, FPGAs, etc.) will integrate pull-up or pull-down resistors. Users can choose whether to turn on or not according to their needs. STM32 microcontroller GPIO mode includes pull-up or pull-down.3) According to the resistance value of the pull-up resistor, we can also divide it into strong or weak pull-up/down. The pull-up resistors integrated in the chip are usually weak pull-up (larger resistance), the smaller the pull-up resistance, the stronger the level capability (strong pull), and the stronger the ability to resist external noise (that is, if the unwanted interference noise is to change the strong pull signal level, the required energy must be strengthened accordingly ), but the smaller the pull-up resistance, the greater the corresponding power consumption, because the normal signal requires more energy to change the state of the signal line. In terms of energy consumption, both pull-up /down resistors are the same.4) There is no strict definition of how many ohms are the boundary between strong pull and weak pull. Generally, the pull-up resistors we use are weak pulls, so we can still use external control signals to pull up/down the signal lines as needed.The extreme of the strong pull resistance is the zero, that is, the signal line can directly connected to the power supply or ground.5) There are more knowledge points involved when the pull-up resistor is used as an output (or input and output), but the essential function is also to clamp the level. The most common output pull-up resistor appears in the open collector (OC) Or open drain (OD) structure pin.6) The current sink capability and current source capability are also called the drive capability of the chip pins. For any given chip, the pin drive capability is limited. If the load driven by the pin is large, it may cause the output level to be incorrect (the predetermined level cannot be output).7) OC (OD) pin output structure is different (OC structure exists in the transistor, and OD structure exists in the field effect transistor FET). The output of most comparator chips is an OD/OC output structure, and the signal pins of many chips or modules that feed back the system status are also in this structure, so that users can pull up the level to the corresponding level according to the actual needs of the circuit system. With the power supply voltage VCC, the level conversion can be omitted. Figure 5. Pull up Resistor with Example   Ⅴ How to Select Pull-up & Pull-down Resistor? When select pull-up & pull-down resistors, you can consider the following three aspects:1) Considering power saving, sink current capability of the chip should be large enough, the resistance is large and the current is small.2) It is necessary to ensure sufficient drive current, so the resistance is small and the current is large.3) For high-speed circuits, excessive pull-up resistors may have smooth edges.Considering the above three points comprehensively, the resistance value is usually selected between 1K and 10K. The same principle applies to pull-down resistors.   Ⅵ FAQ 1. What is pull-down and pull-up resistor?A pull-up resistor connects unused input pins (AND and NAND gates) to the dc supply voltage, (Vcc) to keep the given input HIGH. A pull-down resistor connects unused input pins (OR and NOR gates) to ground, (0V) to keep the given input LOW.   2. What is difference between pull up and pull-down resistor?A pull-up resistor connects unused input pins (AND and NAND gates) to the dc supply voltage, (Vcc) to keep the given input HIGH. A pull-down resistor connects unused input pins (OR and NOR gates) to ground, (0V) to keep the given input LOW.   3. When to use pull-up or pull-down resistors?Pull-up and pull-down resistors are often used when interfacing a switch or some other input with a microcontroller or other digital gates. Most microcontrollers have built-in programmable pull-up and/or pull-down resistors, so fewer external components are needed.   4. What is the function of a pull-up resistor?In electronic logic circuits, a pull-up resistor or pull-down resistor is a resistor used to ensure a known state for a signal. It is typically used in combination with components such as switches and transistors, which physically interrupt the connection of subsequent components to ground or to VCC.   5. What is the purpose of pull-down resistor?What is Pull-down Resistors. Similarly to pull-up resistors, pull-down resistors ensure the voltage between VCC and a microcontroller pin is actively controlled when the switch is open. However, instead of pulling a pin to a high value, such resistors pull the pin to a low valued instead.   6. How do you calculate pull-down resistors?To calculate the pull-down resistor value, it's slightly different from the pull-up resistor value. Knowing that current is 100uA, we'll take 0.5v as our pull-down voltage since the input is 0.8v. Thus, applying our R = V/I once again, but this time we don't have to minus, so our formula remains constant.   7. Why does I2C need pull-up resistor?As discussed in the I2C Basics module, the resistors that are commonly seen on I2C circuits sitting between the SCL and SDA lines and the voltage source are called pull up resistors. ... A pull up resistor is used to provide a default state for a signal line or general purpose input/ouput (GPIO) pin.   8. Which port has no built in pull-up resistor?Input/Output (I/O) pin − All the circuits within the microcontroller must be connected to one of its pins except P0 port because it does not have pull-up resistors built-in.   9. What is pull up and pull down in Arduino?Introduction: Understanding the Pull-up/Pull-down Resistors With Arduino. ... With a pull-up resistor and with the button unpressed you make a logic state ON and with the button pressed you make a logic OFF. With a pull - down resistor and a pressed button you make an ON logic state and OFF logic state when its unpressed.   10. What happens if the pull up resistor for an I2C signal is too small?Too small of a value will once again prevent the output drivers from sinking enough current to pull the pin all the way down to 0.

Introduction A resistor is a passive two-terminal electrical component. After it is connected to the circuit, the resistance is fixed, which can limit the current through the branch connected to it. On one hand, the resistance that cannot be changed is called a fixed resistor, on the other hand, the resistances of potentiometers or variable resistors are changeable. The main physical characteristic of a resistor is to transform electrical energy into thermal energy. It can also be said that it is an energy-consuming element, because internal energy is generated when current passes through it. Figure 1. Use Resistor in Circuit Catalog Introduction Ⅰ Functions of Resistor Ⅱ Three Basic Principles for Resistor Selection Ⅲ The Role of Resistors in Transistor Circuits 3.1 Why Should a Resistor Be Added to the Base of the Transistor? 3.2 Pull-down Resistor in Transistor Circuits Ⅳ FAQ Ⅰ Functions of Resistor In short, the function of resistance is to limit current, divide current, divide voltage, and convert electric energy into internal energy (heating) in the circuit. According to Ohm's law, through calculations, resistors in parallel and series connections can be used to achieve the desired current and voltage. Also there are different resistors and switches combined to produce voice-activated switches, photosensitive switches, infrared switches and so on. How to Use Resistors in circuits? 1) Limit CurrentIn order to prevent the components connected in series from being burnt out by the excessive current and to ensure the normal operation of the electrical appliances, a variable resistor can usually be connected in series in the circuit.2) Current DiversionThe resistor is connected in parallel to the component or circuit that needs to be shunted, and the voltage does not change. The function of this resistor is to divide current.3) Voltage DiversionGenerally, electrical appliances are marked with a rated voltage value. If the power supply is higher than it, the electrical appliance cannot be directly connected to the power supply for a normal operation. In this case, a resistor with suitable resistance can be connected in series in the circuit to share a part of the voltage, therefore the electrical appliance can work at the rated voltage. At this time, the role of the resistor is to divide the voltage.4) Provide Bias VoltageIn the transistor circuit, the resistor is connected between the base of it and the working voltage. At this time, the power supply provides a bias voltage to the base through the resistor, and the resistance can determine the bias voltage. The role of the resistor in the circuit at this time is to provide a bias voltage.5) Negative FeedbackUsed in the resistance between the base and collector of the transistor, then the feedback branch of the negative feedback circuit is formed in the circuit. At this time, the resistor plays a negative feedback role in the circuit.6) OscillationResistor and capacitor form an RC circuit, which can be combined in parallel and in series.7) Damping EffectConnecting a resistor in parallel in the LC resonant circuit can reduce the Q value, at this time, resistor plays a damping effect.8) DecouplingThe use of resistors in multi-stage amplifier circuits can prevent harmful low-frequency interference, which play a decoupling effect.9) Convert Electrical Energy into Internal Energy (Heating)When the current passes through the resistor, it will convert all (or part) of the electrical energy into internal energy, which will generate heat. This principle is often used in electric stoves and heaters in our lives.10) Convert Current into VoltageWhen current flows through the resistor, a voltage will be generated across the resistor. As shown in the figure below, the collector load resistor R2 plays this role, converting the current flowing through the resistor R2 into a voltage and outputting it from U0. Figure 2. Resistor Circuit   Ⅱ Three Basic Principles for Resistor Selection 1) Choose resistors that are manufactured by a certification body that implement high-level standards.2) Choose resistors produced by manufacturers with functional advantages, quality advantages, efficiency advantages, price advantages, and service advantages.3) Choose a manufacturer that can meet the above-mentioned requirements in the model catalog.   Ⅲ The Role of Resistors in Transistor Circuits 3.1 Why Should a Resistor Be Added to the Base of the Transistor? First of all, we must understand the basic principle of the transistor. It is a current-controlled element, which is different from the MOFET, a voltage-controlled element. The transistor has three working areas: cut-off area, amplification area and saturation area. Take NPN transistor as an example, the voltage difference（UBE）of BE is about 0.6V (the actual size depends on the model of the component). When UBE＜0.6V , the transistor is off; when UBE=0.6V, the transistor is in the amplification or saturation region. Figure 3. Schematic Diagram of the NPN Transistor Current When the transistor is in the amplification area, the added resistance between the base and VCC is a bias resistance. The following explains why the base should be added when the transistor is used as a switch. What is the difference between transistor and MOSFET circuits when adding a resistor.The following figure is the most commonly used circuit diagram of NPN transistors. The common input terminal is the I/O port of the microprocessor (microcontroller, DSP, ARM, etc.). Figure 4.  NPN Transistor Take the microcontroller I/O port with 0/5V input as an example. Why must a resistor be connected in series with the base? Can it work without a resistor? Here the resistor is a current control element. When the transistor is in an amplified or saturated state, the voltage of the UBE is 0.6V, and the base current can be calculated according to the input voltage U. The calculation formula is Ib=(U-0.6 )/R1. It can also be seen from the formula that if the current limiting resistor R1 is not connected, when the input voltage is greater than 0.6V, the base current will be very large to burn the tube.Moreover, the resistor cannot be used casually. It needs to be calculated according to the input voltage and the characteristics of the tube. For example, the amplification factor β of the transistor is 50, the maximum current of the collector is 500mA, and the input voltage is 5V. If the design requires the transistor to be in a saturated state, then Ic=500mA, Ib=Ic/β=10M=mA, where the current-limiting resistance R1=(5V-0.6V)/Ib=430Ω. If it is required to input 5V, the collector current is about 200mA, then Ib=Ic/β=200mA/50=4mA can be calculated, finally the current-limiting resistance R1=(5V-0.6V)/Ib=1075Ω (1K can be selected Standard resistance). Note: The above figure is used to explain the example, but it is not very reliable. A more reliable connection method should be to connect a large resistor (such as 10K, or 20K) between the base and the ground. When there is no input, pull the base down quickly to ensure that the tube is in a stable cut-off state.If the NPN transistor in the figure above is replaced with an N-channel MOS tube, the principle is the same. When a high level is input, the tube is turned on, and when a low level is input, the tube is turned off. Figure 5. MOSFET Circuit Since the MOSFET is a voltage-controlled device, the current of the gate (G) is very small and can be ignored, so it can work normally without connecting the resistor R1.The figure after remove the resistor is shown in the below: Figure 6. MOSFET Circuit without Resistor Note: In actual applications, a resistor is generally connected in series to improve reliability. The product reliability is very important. Without current-limiting resistor, when the MOS is   damaged by voltage breakdown, the components on the control terminal will be affected easily, especially the processor, is easily damaged by high current. 3.2 Pull-down Resistor in Transistor Circuits 🔺For TransistorsThe transistor is a current-type driving component, so a current-limiting resistor is connected to the base, generally less than 10K, and the typical values are 3.3K, 4.7K, 5.1K, 6.8K, etc. What is the function of this pull-down resistor?The following figure shows the transistor 8050 switching circuit. The transistor will be turned on when the I/O port outputs a high level, and the transistor will not be turned on. If the I/O port does not output a high level, the base will always be pulled low without a 68K pull-down resistor, that is to say it is in the cut-off state. The circuit may be in an unstable state, especially when it is initialized at the moment of power-on. It is easy to generate noise and easily cause the transistor to malfunction, especially for some general input/output ports. Therefore, this resistor is actually a bias resistor, which makes the base to be pulled down when there is no driving signal, making the circuit more reliable. Figure 7.  Pull-down Resistor in Transistor Circuit Although the pull-down resistor can make the circuit more reliable, tit cannot be too large or too small. If the resistance too large, the base current will not be enough to drive the transistor. On the contrary, if it is too small, the bias voltage will be less than the transistor conduction voltage. In general, this resistance is not more than 100K.Sometimes we see that a capacitor is connected in parallel with this resistor. In fact, this is generally designed in high-speed signal switching circuits. Adding a capacitor can improve performance, as shown below: Figure 8.  RC Circuit 🔺For MOSFETUnlike transistors, MOS transistors are voltage-controlled components, which are driven by voltage. We all know that there is parasitic capacitance between the two pins of MOS transistors. In fact, the key of MOS transistors’ conduction is the charging and discharging of capacitors. Therefore, for the N-type MOS, it will be turned on when Vgs is greater than a certain value, but for the P-type MOS, it will be turned on when the value of Vgs is less than a certain value.Therefore, due to the capacitive effect between the three pins, when the MOS is constantly turned off, the parasitic capacitance voltage can be properly discharged, which is similar to the role of a bleeder resistor and is a kind of protection for the MOS. Figure 9.  MOSFET Circuit Ⅳ FAQ 1. What is the function of the resistor?A resistor has the ability to reduce voltage and current when used in a circuit. The main function of a resistor is to limit current flow. Ohm's law tells us that an increase in a resistors value will see a decrease in current.   2. How do resistors work?A conductor has low resistance, while an insulator has much higher resistance. Devices called resistors let us introduce precisely controlled amounts of resistance into electrical circuits. ... A resistor works by converting electrical energy into heat, which is dissipated into the air.   3. Why do you need resistors?A resistor controls the flow of the electrical current within a circuit. ... Resistors are essential to many electoral circuits, and they can be applied to a myriad of different applications. Protect against voltage spikes. Resistors also protect components against voltage spikes.   4. What role do resistors play in electronic devices?A resistor is a passive two-terminal electrical device that resists the flow of current. It is probably the simplest element in an electronic circuit. It is also one of the most common components as resistance is an inherent element of nearly all electronic circuits. They are usually color-coded.   5. What is a good example of a resistor?A few examples include limiting electric current, voltage division, heat generation, matching and loading circuits, gain control, and setting time constants. They are commercially available with resistance values over a range of more than nine orders of magnitude.   6. What happens if I use a higher ohm resistor?The cases where using a higher value resistor will damage a circuit exist, but are a bit less usual than the cases where it may simply produce a weaker result than desired, or a different frequency response than desired.   7. What is a resistor simple explanation?A resistor is an electrical component that limits or regulates the flow of electrical current in an electronic circuit. Resistors can also be used to provide a specific voltage for an active device such as a transistor. ... The most common type in electronic devices and systems is the carbon-composition resistor.   8. What happens when a resistor blows?Blowing Up a Resistor. By applying too high a voltage to a resistor, the resistor will draw too much current. This causes excessive power to be dissipated in the resistor which makes it go up in flames and a cloud of smoke as this video shows.   9. How is a resistor connected in a circuit?Resistors are said to be connected in “Series”, when they are daisy chained together in a single line. Since all the current flowing through the first resistor has no other way to go it must also pass through the second resistor and the third and so on.   10. Do resistors change voltage?The larger the resistor, the more energy used by that resistor, and the bigger the voltage drop across that resistor. Ohm's Law can be used to verify voltage drop. In a DC circuit, voltage equals current multiplied by resistance. V = I R.

CatalogⅠ What is a Ballast Resistor ?Ⅱ Types of Ballast Resistors2.1 Fixed Resistors2.2 Self-Variable ResistorsⅢ The Working Principle of  Ballast ResistorsⅣ Ballast Resistor – Uses and Applications4.1 Ballast Resistor in Fluorescent Lamps4.2 Ballast Resistor for Automotive Applications4.3 Ballast Resistor in a LED CircuitⅤ Symptons of Failture Ballast-ResistorⅥ Frequently Asked Questions about Ballast resistors Introduction  “Ballast” is generally considered as “something that provides stability in English dictionary.  ” Thus, when we refer to an electrical ballast, we are referring to an electrical device that is important in maintaining the stability of the electrical circuit. However, you might be wondering how it provides stability. Depending on the circuit in which they are used, some electric ballasts limit current while others limit voltage. By doing so, they reduce the risk of overvoltage or overcurrent in the circuit, thereby improving system stability. The complexity of electric ballast varies greatly. It can be as simple as a resistor, capacitor, inductor, or a combination of these, or as complex as the electronic ballast found in fluorescent lamps.  Ⅰ What is a Ballast Resistor ? A ballast resistor is an electronic component that is usually used to regulate the current in a circuit. Some devices, such as fluorescent lamps, can exhibit negative resistance characteristics, and an increase in current will result in a decrease in voltage. Negative resistance can cause damage to the power supply or equipment. Ballast resistors are usually connected in series with the negative load, taking advantage of the fact that all components in the series circuit receive the same current. Some ballasts are just series resistors, while others use capacitors and more complex components. Figure1: complex components Ⅱ Types of Ballast ResistorsA resistor is a current-limiting electronic component that can reduce circuit voltage and current. Ballast resistors are classified into three types: fixed, variable, and reactive.2.1 Fixed ResistorsThis genre of ballast resistors have a fixed resistance. A high resistance value is primarily taken considered for most applications. This genre of ballast resistor is commonly used in simple circuits with low-powered loads such as neon or LED lamps. This fixed resistor is also applied to control the ventilation fan speed. It employs a fixed ballast resistor with two center taps. The fan speed selector switch detours portions of the ballast. Therefore, the entire ballast resistor is suitable for full speed, while no section of the ballast resistor is suitable for the low speed.2.2 Self-Variable ResistorsThese ballast resistors have the property of resistance in response to changes in current, such as an increase in current increasing resistance and a decrease in current decreasing resistance. Incandescent lamps frequently use these ballast resistors. As the current through the lamp increases, the ballast resistor heats up, and the resistance rises with the temperature, as does the voltage drop across the resistor. When the current is down, the temperature of the ballast resistor decreases as well, as does its resistance and thus the voltage drop. The benefits of using this type of ballast resistor are that it provides more precise current control than a suitable fixed resistor. Another benefit is that the power lost in the resistive ballast is decreased because a smaller section of the overall power is lost in the ballast when compared to a fixed resistor. Ⅲ The Working Principle of  Ballast Resistors When 220V 50HZ AC power is applied to the switch closed circuit, the current flows through the ballast, and the lamp filament starter heats the filament（The starter was turned off in the beginning. The gas arc discharge in the jumping bubble in the starter caused the bimetallic sheet to heat and deform due to the application of an AC voltage greater than 190V, and the two electrodes were close together to form the filament heated by the passage.) Because there is no arc discharge when the two electrodes of the starter are close together, the bimetal cools and the two poles separate. As the ballast is inductive, when the circuit is suddenly interrupted, 600V is generated at both ends of the lamp for about 1ms. The pulse voltage is -1500V; the exact voltage value depends on the type of lamp. When the lamp discharges, the voltage at both ends drops immediately. At this point, the ballast limits the lamp current on the one hand while also supplying power on the other. There is a phase difference of 55° to 65° between the voltage and the working current of the lamp in order to keep the secondary starting voltage of the lamp stable.Figure2:  the working principle of  a ballast resistor Due to its simple structure, inductive ballast, as the first type of ballast to work with fluorescent lamps, has a relatively large market share. However, on account of its low power factor, poor low-voltage startup performance, heavy energy consumption, stroboscopic and many other shortcomings , its market is gradually being replaced by electronic ballasts. The energy consumption of inductive ballasts: 40W (lamp tube power) + 10W (inductive ballast self-heating consumption) is equal to the total power 50W consumption of the whole set of lamps and lanterns. An electronic ballast is a converter that converts low frequency alternating current power to high frequency alternating current power. Its basic operation is as follows: after passing through a radio frequency interference (RFI) filter, full-wave rectification, and a passive (or active) power factor corrector (PPFC or APFC), the industrial frequency power supply becomes a DC power supply. The output of high-frequency AC power of 20K-100KHZ is added to the LC series resonant circuit connected to the lamp via the DC/AC converter to heat the filament, but the lamp is "discharged" into the "on" state and then enters the light-emitting state. At this point, the high-frequency inductor limits the current increase, ensuring that the lamp tube receives the voltage and current required for normal operation. Various protection circuits, such as abnormal protection, surge voltage, and current protection, temperature protection, and so on, are frequently constructed to improve reliability. Ⅳ Ballast Resistor – Uses and Applications Ballast resistors are most commonly used to adjust the current to a negative resistance load. They can also be applied in other contexts. Fixed ballast resistors are common in low-power devices like light-emitting diodes (LEDs) and neon lights. LEDs are a positive resistive load that can benefit from ballast resistors. 4.1 Ballast Resistor in Fluorescent Lamps Fluorescent lighting, as we all know, is a popular and efficient lighting system. However, there is a disadvantage to using this type of lighting system. When directly connected to a voltage source, it heats up very quickly. This situation is due to the lamp's uncontrollable current draw as soon as its operation. A ballast resistor, connected in series with the lamp, is applied to the circuit to prevent overheating caused by excessive current draw. Therefore, the function of ballast resistor is to regulates the current and reduces the voltage. However,  for the lamp to light up, an arc has to be formed between its two electrodes. This necessitates a high starting voltage that is nearly equal to the supply voltage. The ballast resistor provides the required voltage during startup, and then immediately after an arc is established, it reduces the voltage while also regulating the current flow. Figure3: application in fluorescent lamp 4.2 Ballast Resistor for Automotive Applications Ballast resistors are generally included in the ignition kits of automotive machines such as automobile engines. Such devices are often called Ignition Ballast Resistors because of their application. The application of this device reduces the possibility of coil failure. It is connected between the ignition coil's primary voltage source and the coil stud. This connection helps to reduce the coil voltage and coil current, so the coil does not get as hot as it would without it, extending the coil's life. However, a high voltage equal to the primary voltage source is required to start the ignition engine. As a result, a jumper wire is frequently connected to the ballast resistor. This jumper wire provides the voltage required to start the engine.  Figure4: application in automotives 4.3 Ballast Resistor in a LED Circuit If the source voltage in an LED circuit is greater than the rated voltage of an LED lamp, the LED may be damaged. It is strongly advised to connect a ballast resistor in series with the lamp to avoid this case. By connecting the ballast resistor in this manner, the voltage across the LED is down to a tolerable level. The circuit diagram for the same is shown in the figure below.  Figure5: application in leds  The following formula is the value of resistance of the ballast :R = (E – Vf)/IFWhere: R= resistance of the ballast resistorE = voltage sourceVf = forward voltage of the LEDIF =Forward current of the LED. So, let's say you have an LED with a voltage rating of 4 volts, a forward current of 10 mA, and a voltage source of 6 volts. This means that the voltage across the LED should be 4 or less than 4 volts. As a result, the resistance of the ballast resistor should be R= (6-4)/0.010 = 200 or greater. Now that we've covered the applications briefly, let's look at the different types of ballast resistors on the market. Ⅴ Symptons of Failture Ballast-Resistor A ballast resistor is a device in your car that limits the amount of current flowing through an electric circuit. Because they did not have the benefit of circuit boards like most modern vehicles, ballast resistors are commonly found in older vehicles. Normal wear and tear can damage the ballast resistor over time, so there are a few things to look for if you suspect a bad or failing ballast resistor needs servicing. 1. Vehicle starts, then immediately goes out The most obvious symptom is that the vehicle starts but then dies as soon as you remove the key. If this occurs, Your Mechanic's experts will be able to measure the voltage coming from the ballast resistor and determine if it needs to be replaced. They will inform you of the condition of your ballast resistor once they have read the voltage. 2. Not starting at all The vehicle will not start if the ballast resistor is not functioning properly. Because it is an electrical system, it is best left to the professionals. The only way to get the vehicle running again is to replace the ballast resistor. 3.  Don’t jump the resistor Some people attempt to jump the resistor, which means that the ballast resistor is ignored and the extra current is routed to the points. The points are not designed to withstand the additional voltage, causing them to wear out and fail prematurely. This will result in a much more extensive repair than if the ballast resistor was replaced at the start. Furthermore, because you are tampering with electricity, it can be dangerous, especially if you don't know what you're doing. 4.  Let the vehicle beIf your ballast resistor is out of work, your vehicle will not start and you will need to have it towed to a mechanic's shop. Because the professionals at Your Mechanic make house calls, you will be able to save money on towing. Furthermore, because the vehicle will not start, it is not a dangerous situation as long as you leave it alone. Do not attempt to bypass the ballast resistor and do not continue to try to start the engine. Allow the professionals to repair it so you can get back on your way. Ⅵ Frequently Asked Questions about Ballast resistors 1. Is a ballast resistor necessary?　If the coil required a ballast resistor when it was used with points, then you must still use the resistor. If it didn't, then no resistor is necessary. The ballast resistor keeps the engine running by preventing the engine from receiving full amperage from the ignition once started.　 2. Why do ignition coils need ballast resistor?　In simple terms, the ballast resistor in a Mopar limits the amperage, or current flow, through the coil while the engine is running, thereby extending the life of the coil and breaker points of the distributor. 3. How do you know if a ballast resistor is bad?To test your ballast resistor you need an ohm meter or multimeter set to ohms. Remove the connectors from both sides of the resistor. The ohms should read between 1.8 and 5 ohms. You should be getting 9 volts to the positive side of the coil. 4. What causes a ballast resistor to fail?The resistance through the ballast resistor varies with current flow which varies with engine rpm. This naturally causes the resistor to expand and contract from heat. That's why they eventually fail. 5. Can a resistor wire go bad?　　The only way the wire will "go bad" is if it gets a break in it so it has infinite resistance (an open circuit). 6. Which circuit the ballast resistor is used?　　Ballasts can also be used simply to limit the current in an ordinary, positive-resistance circuit. Prior to the advent of solid-state ignition, automobile ignition systems commonly included a ballast resistor to regulate the voltage applied to the ignition system. 7. What is inside of a ballast?　A magnetic ballast (also called a choke) contains a coil of copper wire. The magnetic field produced by the wire traps most of the current so only the right amount gets through to the fluorescent light. That amount can fluctuate depending on the thickness and length of the copper wire. 8. What's the difference between a ballast and non ballast coil? Basically, a non-ballast coil is designed to produce full spark output with 12 volts on the input (+ terminal). A ballast coil is designed to produce the same spark output, but with only 6 to 9 volts on the input.