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TO-92 front view[1]
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The TO-92 is a widely used style of semiconductor package mainly used for transistors. The case is often made of epoxy or plastic, and offers compact size at a very low cost.

History and origin

[edit]

The JEDEC TO-92 descriptor is derived from the original full name for the package: Transistor Outline Package, Case Style 92.[1] The package is also known by the designation SOT54. By 1966 the package was being used by Motorola for their 2N3904 devices among others.[2]

Construction and orientation

[edit]
Size comparison of BJT transistor packages, from left to right: SOT-23, TO-92, TO-126, TO-3

The case is molded around the transistor elements in two parts; the face is flat, usually bearing a machine-printed part number (some early examples had the part number printed on the top surface instead). The back is semi-circularly-shaped. A line of moulding flash from the injection-moulding process can be seen around the case.

The leads protrude from the bottom of the case. When looking at the face of the transistor, the leads are commonly configured from left-to-right as the emitter, base, and collector for 2N series (JEDEC) transistors, however, other configurations are possible, such as emitter, collector, and base commonly used for 2S series (Japanese) transistors or collector, base, and emitter for many of the BC series (Pro Electron) types.

If the face has a part name made up of only one letter and a few numbers, it can be either a Japanese or a Pro Electron part number. Thus, "C1234" would likely be a 2SC1234 device, but "C547" is usually short for "BC547".

TO-92 packages with pre-bent wires; on the left to emulate a TO-18 footprint (SO-97 in BS 3934, 10A3 in DIN 41868)

The leads coming out of the case are spaced 0.05" (1.27 mm) apart. It is often convenient to bend them outward to a 0.10" (2.54 mm) spacing to make more room for wiring.[3] Units with their leads pre-bent may be ordered to fit specific board layouts, depending on the application. Otherwise, the leads may be bent manually; however, care must be taken as they can break easily, as with any other device that is manually configured.

The physical dimensions of the TO-92 housing may vary slightly depending on the manufacturer, however, the 1.27mm lead spacing must be respected.

Advantages

[edit]
  • Transistors of this type can be made very inexpensively and take up very little board space. Most models are readily available in large quantities from wholesale distributors.
  • They are easy to find in small electronics stores because of their wide usefulness, making them a popular choice for hobby work and prototyping.

Disadvantages

[edit]

The main disadvantage of this style of case is the lack of heat sinking.

  • Transistors and ICs of these types cannot handle as much power as higher-power equivalents, such as the TO-220, and can burn out quickly if they dissipate excessive power.
  • There is no standard pinout for the TO-92. The American BJTs use the E-B-C pinout while their Japanese counterparts use the E-C-B pinout and some RF devices use the B-E-C pinout.[citation needed]

Voltage and current

[edit]

Although TO-92 devices are mainly used in low-voltage / low-current (<30 V; <1 A) applications, high-voltage (600 Volt Vce) and high-current (5 A Ic) devices are available. Nominal maximum power dissipation is less than one watt (600 mW).

Variants

[edit]

For diodes or integrated circuits with two connections (e.g. temperature sensors) the middle lead is either not connected or omitted entirely.

Comparison between the E-Line/Miniplast package and the TO-92 package

In the late 1960s, Ferranti introduced a smaller package with a compatible footprint, called "E-Line".[4][5] This package was later standardized as a British Standard (but not by JEDEC) and remained in production with Ferranti Semiconductors' successor companies (Plessey, Zetex Semiconductors, Diodes Incorporated[6]). In East Germany the E-Line package was known as the "Miniplast" package and widely used by Kombinat Mikroelektronik Erfurt.

Standards

[edit]
Standards organization Standard Designation for
TO-92 E-Line/Miniplast
3-lead 2-lead
IEC IEC 60191[7] A68
DIN DIN 41868[8] 10B3[a]
EIAJ / JEITA ED-7500A[9][10] SC-43A
British Standards BS 3934[11] SO-94[b]
Gosstandart GOST 18472—88[12] KT-26[c] — KD-129[d]
Rosstandart GOST R 57439—2017[13]
Kombinat Mikroelektronik Erfurt TGL 200-8380[14] — — L2
TGL 11811[7] — L3 —
TGL 26713/07[7] F2 F3 F4
  1. ^ DIN 41868 also defines variants with the leads bent to emulate the footprint of other packages: 10A3 (TO-18 footprint).
  2. ^ BS-3934 also defines variants with the leads bent to emulate the footprint of other packages: SO-95 (TO-5 footprint), SO-96 (for flat mounting), and SO-97 (TO-18 footprint).
  3. ^ Russian: КТ-26
  4. ^ Russian: КД-129

Common components in a TO-92 package

[edit]

Common transistors:

Other common components:

References

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

TO-92

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from Grokipedia
The TO-92 is a three-lead, plastic-encapsulated, single in-line through-hole package designed for low-power discrete devices, including bipolar junction transistors, field-effect transistors, diodes, silicon-controlled rectifiers, and triacs. It features a cylindrical body approximately 5 mm in diameter with a flat index for component orientation and lead spacing of 2.54 mm, making it suitable for automated insertion and on printed circuit boards. The package conforms to the TO-226AA standard and is typically molded from resin for electrical insulation and mechanical protection. Introduced as a compact alternative to earlier metal-can packages, the TO-92 has become a staple in manufacturing since the due to its low cost, reliability, and versatility in handling moderate dissipation typically 625 mW at 25 °C ambient without heatsinking, up to around 1 W with proper heatsinking. Key variants include straight-lead forms for axial insertion and formed-lead (such as Z-bend) configurations for improved board stability. Packaging options for production include bulk (up to 2,500 units per box), tape-and-reel (2,000 units per reel), and ammo packs to facilitate high-volume assembly processes while minimizing handling damage. Widely applied in , automotive systems, power supplies, and prototyping, the TO-92 enables efficient switching, amplification, and regulation in circuits operating at voltages up to several hundred volts and currents below 1 . Its design supports lead-free plating and halogen-free molding for environmental compliance, ensuring compatibility with modern RoHS standards. Despite the rise of surface-mount technologies, the TO-92 remains prevalent for its ease of manual assembly and robust performance in non-extreme environments.

History and Development

Origins in Semiconductor Packaging

The TO-92 package, formally designated as the Transistor Outline Package, Case Style 92, was developed specifically for encapsulating low-power discrete semiconductors such as bipolar junction transistors and diodes, providing a compact, cost-effective plastic housing suitable for through-hole mounting. This standardization emerged as part of the broader Outline (TO) series, which defined consistent physical outlines to facilitate and assembly in electronic devices. Following , the Joint Electron Device Engineering Council (), originally formed in 1944 as the Joint Electron Tube Engineering Council (JETEC) and restructured in 1958 to encompass solid-state devices, took on a pivotal role in the U.S. by establishing uniform standards for component packaging and specifications. These efforts addressed the growing need for among manufacturers, reducing fragmentation in the rapidly expanding post-war electronics sector and enabling scalable production of transistors and integrated circuits. JEDEC's focus on outline registrations, including the TO series, helped solidify a common framework that supported the transition from vacuum tubes to solid-state technology. The TO-92 package saw its first commercial application in 1966 when introduced it for the NPN general-purpose transistor, signifying the onset of mass production for this versatile low-power device. This debut highlighted the package's advantages in affordability and ease of handling compared to earlier metal-can enclosures, quickly establishing it as a staple for consumer and industrial electronics. In European markets, the package was alternatively referred to as SOT54 under Pro-Electron and EIAJ , reflecting parallel standardization efforts.

Early Adoption and Evolution

The TO-92 package saw rapid adoption in the late 1960s among U.S. manufacturers like and for general-purpose s, driven by the need for robust, low-cost plastic encapsulation amid the burgeoning . This shift from metal-can packages to plastic ones enabled more compact and affordable designs, aligning with the era's explosive growth in production. The package's inclusion in 's 1966 Semiconductor Data Book marked a pivotal milestone, featuring it as Case 29 for devices such as the NPN and facilitating global standardization efforts. Demand surged due to the proliferation of , particularly portable radios and televisions, where the TO-92's small footprint and ease of assembly proved ideal for mass-market applications. Japanese manufacturers, including , quickly incorporated similar plastic packages into their production lines during this period, contributing to the package's international uptake as radios became ubiquitous household items. By the end of the decade, the TO-92 had become a staple for low-power amplification and switching in these devices, supporting the transition from to transistors. Through the and , the TO-92 evolved primarily through refinements in molding compounds, enhancing moisture resistance, stability, and overall reliability without altering the core design. In 1970, Dexter Chemical introduced the first transfer-moldable specifically for TO-92 packages, allowing for optically clear encapsulation and improved efficiency. Subsequent advancements in the focused on low-alpha emission and higher conductivity formulations, addressing reliability concerns in increasingly dense circuits, though the package remained through-hole oriented until the rise of surface-mount alternatives in the late .

Design and Construction

Physical Structure and Materials

The TO-92 package employs a semi-cylindrical molded resin case that encapsulates the die, offering electrical insulation and mechanical protection. This case typically measures approximately 5 mm in diameter and 4.6 mm in height, featuring a flat face for printing part numbers, date codes, and manufacturer logos, while the opposite side is rounded. Internally, the package is constructed using a stamped metal , typically composed of three tin-plated leads that protrude from the base of the case. These leads have a straight-line spacing of 1.27 mm (0.05 inches) in their unformed state, which is often pre-bent by manufacturers to a 2.54 mm (0.1 inch) pitch to align with standard (PCB) hole spacing for easier insertion and . The primary material for the case is a thermoset resin, which provides robust environmental resistance and is sometimes reinforced with fillers in variants to improve mechanical strength and dimensional stability. Package color is commonly black to minimize light sensitivity in certain devices, though variations exist depending on the manufacturer. For assembly, the TO-92 supports through-hole mounting directly into PCB vias, with the leads soldered on the opposite side. Automated production is facilitated by optional tape-and-reel packaging, where components are aligned in carrier tape with 2,000 units per reel for high-volume pick-and-place operations.

Lead Configuration and Pinouts

The TO-92 package employs a three-lead configuration primarily for bipolar junction transistors (BJTs), with the leads corresponding to the emitter, base, and collector terminals. However, unlike some packages, the TO-92 lacks a universal pinout standard, as it defines only the physical enclosure rather than the internal device wiring. This variability stems from differing regional and manufacturer practices, requiring users to consult specific datasheets for accurate identification. Pin numbering conventionally proceeds from left to right (1, 2, 3) when viewing the package with the flat face toward the observer and leads pointing downward. In the United States, the standard—commonly applied to 2N-series transistors—assigns pin 1 to the emitter, pin 2 to the base, and pin 3 to the collector (E-B-C configuration). Japanese manufacturers, following EIAJ conventions, frequently adopt an emitter-collector-base (E-C-B) arrangement, as seen in 2S-series (e.g., 2SC) devices like the 2SC945, where pin 1 is the emitter, pin 2 the collector, and pin 3 the base. European-style BC-series transistors, such as the BC547, typically use a collector-base-emitter (C-B-E) pinout, with pin 1 as the collector, pin 2 the base, and pin 3 the emitter. These differences reflect historical conventions rather than a deliberate design choice.
Series/StandardPin 1 (Left)Pin 2 (Middle)Pin 3 (Right)Example DeviceSource
EmitterBaseCollectoronsemi 2N3904 Datasheet
EIAJ (Japanese)EmitterCollectorBase2SC945Components101 2SC945 Guide
European (BC)CollectorBaseEmitterBC547onsemi BC550 Datasheet
Lead identification relies primarily on the device's , given the absence of a fixed standard. Some manufacturers incorporate visual aids, such as colored dots or stripes on the package body (e.g., a green dot denoting the emitter in select legacy schemes), to mark pin 1, though these are not universally applied. The flat face of the package serves as the for orientation during or insertion. This lack of poses challenges in prototyping and assembly, as mismatched pinouts from historical regional practices—such as the E-B-C for American 2N devices, C-B-E for European BC types, and E-C-B for Japanese 2S equivalents—can lead to circuit errors if not verified. Engineers must cross-reference series-specific documentation to mitigate risks. The leads, typically formed from tinned wire, are flexible and can be bent for vertical (upright) or horizontal (lying flat) mounting on PCBs, facilitating compact layouts. Bending should occur beyond the package's seating plane (at least 1.5 mm from the body) with a minimum exceeding the lead thickness (approximately 0.43 mm for standard TO-92) to prevent mechanical stress and potential fractures in the wire or encapsulation.

Electrical and Thermal Specifications

Voltage, Current, and Power Ratings

The TO-92 package is commonly used for low-power bipolar junction transistors (BJTs), where typical electrical ratings are designed for safe operation in general-purpose amplification and switching applications. For standard NPN or PNP devices, the collector-emitter voltage (V_CEO) is generally rated up to 30-40 V, while the continuous collector current (I_C) reaches up to 100-200 mA. Some high-voltage variants in the TO-92 package, such as the MPSA92 PNP transistor, extend V_CEO to 300 V with I_C up to 500 mA, and devices like the MJE13001 NPN can achieve V_CEO ratings up to 400 V (with V_CBO up to 600 V), though with lower continuous I_C around 200-250 mA. Breakdown voltages for standard TO-92 BJTs include an emitter-base (V_EBO) of approximately 5-7 V and a collector-base (V_CBO) of 50-80 V. These limits protect the device from under reverse bias conditions, with V_EBO typically measured at a small emitter current (e.g., 10 µA) and V_CBO at a collector current of 10-100 µA. Power dissipation (P_D) in TO-92 under linear operation is calculated as PD=VCE×ICP_D = V_{CE} \times I_C, where V_CE is the collector-emitter and I_C is the collector current; this represents the heat generated when the device is not fully saturated or cut off. The maximum P_D is specified at 500-625 mW for an ambient temperature of 25°C, with applied above this temperature (typically 5 mW/°C for ambient mounting) to maintain below 150°C. For example, the NPN has a V_CEO of 40 V, I_C of 200 mA, and maximum P_D of 625 mW at 25°C, illustrating device-specific variations influenced by die size, materials, and intended use. These ratings ensure reliable performance but require careful to avoid exceeding limits, with thermal referenced in related specifications.

Heat Dissipation and Thermal Limits

The TO-92 package features a junction-to-ambient thermal resistance (θ_JA) typically in the range of 200–250 °C/W when operated without a heatsink, which confines its suitability to low-power applications with dissipation limited to a few hundred milliwatts. This high thermal resistance arises from the package's compact plastic encapsulation, which inherently limits convective and conductive to the surrounding environment. Devices housed in TO-92 maintain a maximum (T_J) of 150–175 °C to ensure reliable operation, with required to avoid exceeding this threshold as ambient temperature rises. For instance, power dissipation (P_D) is commonly derated by 5 mW/°C above an ambient temperature of 25 °C, allowing a baseline P_D of around 625 mW at but reducing it progressively at higher ambients to keep T_J within limits. Heat dissipation in the TO-92 primarily occurs via conduction through the plastic case and the protruding metal leads to ambient air or a connected PCB, though the package's small dimensions foster inefficient transfer and localized hotspots, especially in the base-collector region of bipolar junction transistors where power is generated. Thermal limits can be somewhat alleviated using clip-on heatsinks or PCB copper pads to enhance surface area for , yet these approaches provide only modest improvements and remain ineffective for sustained power exceeding 500 mW owing to the plastic's low conductivity and poor junction-to-case coupling.

Advantages and Limitations

Key Benefits for Electronics Design

The TO-92 package provides substantial cost-effectiveness for electronics design, attributable to its straightforward plastic molding and encapsulation process that minimizes material and manufacturing expenses. This low cost makes it particularly suitable for high-volume production in consumer electronics, where economic efficiency is paramount without compromising basic functionality. Its compact dimensions, featuring a typical mounted footprint of approximately 5 mm × 5 mm when leads are formed for PCB integration, enable efficient use in space-limited environments such as prototypes and legacy circuit boards that require minimal board real estate. The small form factor supports dense component placement while maintaining compatibility with standard through-hole assembly techniques. The through-hole lead configuration of the TO-92 enhances ease of use by allowing straightforward hand-soldering, breadboarding, and prototyping, which is especially beneficial for hobbyists, educators, and rapid development cycles. Furthermore, its widespread availability from numerous semiconductor vendors, including ON Semiconductor and , bolsters reliability and reduces procurement risks in design projects. The package's versatility extends to housing both NPN and PNP bipolar junction transistors, as well as small integrated circuits like low-dropout voltage regulators, with embossed or printed markings on the body facilitating quick pin identification and orientation during assembly.

Drawbacks in Performance and Use

The TO-92 package's high junction-to-ambient thermal resistance (θ_JA), typically around 200 °C/, severely limits its power handling capability to less than 1 , often derating to 625 mW at 25°C ambient . This thermal inefficiency restricts its use in power-intensive applications and can result in overheating or failure in environments with elevated or poor unless supplemented with external cooling solutions. Pinout variability in TO-92 transistors arises from the absence of a universal standardization for lead assignments across manufacturers and regions, which frequently causes assembly errors during prototyping and production. Engineers must consult specific datasheets for each device to confirm pin configurations, such as emitter-base-collector ordering, thereby extending design verification time and raising the risk of circuit malfunctions if overlooked. The mechanical fragility of the TO-92's thin leads, measuring 0.41 to 0.51 mm in diameter, makes them susceptible to bending or breakage during manual handling, insertion, or lead forming processes. Assembly guidelines recommend using jigs to minimize stress on the leads during bending and advise against excessive force to prevent damage, rendering the package less suitable for high-volume automated surface-mount technology (SMT) lines without specialized adapters or modifications. Additionally, the through-hole design of the TO-92 contributes to risks in contemporary , as it occupies more board space and complicates automated assembly compared to surface-mount devices (SMDs), which are preferred for high-density printed circuit boards (PCBs) to achieve compact layouts and improved . This format limits its integration in future high-volume, miniaturized products, prompting designers to seek SMD alternatives for long-term viability.

Variants and Form Factors

Standard TO-92 Configurations

The standard TO-92 configuration, also known as TO-226AA (Case 29-11), consists of a cylindrical or body approximately 5 in diameter and 4.5 in height, featuring three straight axial leads with a minimum length of 12.7 for vertical through-hole mounting on printed circuit boards. This baseline design supports low-power devices like transistors and voltage regulators, with leads spaced 1.27 apart at the package base to align with standard PCB hole patterns. A prevalent variation in standard TO-92 configurations is the horizontal lead bend, where the leads are factory-formed at 90-degree angles (as in Case 135AR) to position the package flat against the PCB, commonly employed in space-constrained audio amplification and switching circuits for improved heat distribution and board layout efficiency. Marking on the flat face of the TO-92 adheres to industry practices, embossing the device part number (e.g., 2N3904), a polarity indicator such as a stripe or line identifying the collector pin, and a date code (e.g., YWW format for year and week of manufacture) to ensure traceability and orientation during assembly. Dimensional tolerances across manufacturers permit variations of ±0.25 mm in critical features like body diameter and lead spacing, maintaining compatibility for interchangeable use in designs without requiring PCB modifications.

Specialized and Alternative Variants

The TO-92S, also referred to as the E-Line or Miniplast package, is a compact variant of the TO-92 developed by for applications requiring reduced vertical space while preserving compatibility. It features a body height of approximately 3.7 mm, enabling use in denser European circuit designs where the standard TO-92's taller profile is impractical. Tape-and-reel packaging for these and standard TO-92 components follows the EIA-481 standard to support high-volume pick-and-place automation. High-reliability variants of the TO-92 cater to military and applications, including hermetic versions that provide superior environmental protection against moisture and contaminants. For instance, ' LM135A temperature sensor is offered in a hermetic TO-92 package qualified for military-grade operation over an extended temperature range of -55°C to 150°C. Since the implementation of the RoHS Directive in 2006, TO-92 packages have been produced in lead-free configurations to comply with restrictions on hazardous substances, using tin-based finishes and non-lead solders for environmental compatibility.

Standards and Specifications

JEDEC and Core Packaging Standards

The TO-92 package is formally designated under as TO-226, with the TO-226AA variant specifying the primary plastic-encapsulated configuration for three-lead through-hole components. This standard outlines the package's physical dimensions, including a maximum body diameter of 5.20 mm (0.205 inches), lead diameters ranging from 0.407 to 0.533 mm (0.016 to 0.021 inches), and a body length between 4.58 and 5.33 mm (0.170 to 0.210 inches), ensuring interoperability across manufacturers. Lead finish requirements emphasize tin or equivalent for , with tolerances for lead spacing and straightness to facilitate automated assembly. JEDEC provides non-binding pinout guidelines for transistors in the TO-226AA package, recommending pin 1 as emitter, pin 2 as base, and pin 3 as collector, particularly for JEDEC-registered 2N-series devices to promote consistency in circuit design. Lead coplanarity tolerances are specified at ±0.25 mm to ensure reliable insertion into printed circuit boards without excessive bending. Reliability testing for TO-92 packaged components aligns with JESD22 standards, including mechanical shock per JESD22-B104 (Condition C: 1500 g peak acceleration, 0.5 ms duration, half-sine pulse) to simulate handling and operational stresses. Vibration resistance is evaluated under JESD22-B103 (variable frequency from 10 Hz to 2000 Hz at 20 g acceleration) to assess structural integrity during transportation or use in vibrating environments. Moisture resistance testing follows JESD22-A101 for steady-state temperature humidity bias or equivalent, ensuring encapsulation integrity against environmental exposure. The TO-226 standard was initially registered in the 1960s as part of 's transistor outline series, with the archival TO-92 document reflecting early specifications, and minimal updates in the to incorporate lead-free lead finishes per JESD97 for RoHS compliance.

International and Manufacturing Standards

The (IEC) standard 60191 provides mechanical outlines for semiconductor device packages, including equivalents to the TO-92 that align with JEDEC specifications to facilitate global export and . This harmonization ensures consistent dimensions and lead configurations across international markets, supporting the TO-92's use in diverse manufacturing environments. In various national standards, the TO-92 package receives equivalent designations that maintain its core features, such as a 1.27 mm lead pitch. For instance, in , DIN 41868 specifies the 10B3 case for semiconductor devices, defining main dimensions compatible with TO-92. Similarly, the UK's BS 3934 designates it as SO-94, outlining dimensions for through-hole mounting in plastic-encapsulated transistors. In , GOST 18472-88 identifies the equivalent as KT-26, preserving the package's compact form and lead spacing for semiconductor applications. Manufacturing processes for TO-92 components adhere to EIA-481 guidelines for tape-and- packaging, which specify an 8 mm tape width and up to 2000 units per to enable automated assembly. Soldering procedures follow IPC-A-610 standards for acceptability of electronic assemblies, permitting hand or with a maximum of 260°C to avoid package damage. measures include solderability testing per J-STD-002, which evaluates the and adhesion of leads and terminations using dip simulation or wetting balance methods to ensure reliable joints. Additionally, TO-92 packages must comply with RoHS Directive 2002/95/EC, effective from July 1, 2006, restricting heavy metals such as lead, mercury, , , polybrominated biphenyls, and to below 0.1% by weight in homogeneous materials.

Applications and Components

Common Components Packaged in TO-92

The TO-92 package is widely used for bipolar junction (BJTs), which are essential for amplification and switching in low-power applications. Among NPN BJTs, the is a general-purpose capable of handling a collector-emitter voltage of 40 V and a collector current of 200 mA, making it suitable for a broad range of tasks. The , often available in its plastic TO-92 variant as the PN2222A, serves as a reliable switching with similar ratings, supporting high-speed operations up to 300 MHz. For PNP BJTs, the complements the with a -40 V collector-emitter voltage and -200 mA collector current, enabling complementary pair configurations in push-pull circuits. The BC557, another PNP option, offers comparable performance with a -45 V rating and is favored for its low noise characteristics in audio applications. Beyond BJTs, the TO-92 package accommodates other transistor types, including small-signal MOSFETs like the , an N-channel enhancement-mode device rated for 60 V drain-source voltage and 200 mA drain current, ideal for logic-level switching. Junction field-effect transistors (JFETs), such as the 2N5457, are also packaged in TO-92 and used in audio preamplifiers due to their high and low noise. Non-transistor components in TO-92 include , exemplified by the LM78L05, a fixed 5 output providing up to 100 mA with a dropout voltage of about 2 . Small-signal diodes, such as the 1N4148 for fast switching, fit the package for RF and needs. Sensors such as the LM35 precision temperature sensor, which outputs 10 mV/°C linearly from 0°C to 100°C, utilize the three-lead TO-92 for direct analog measurement. NTC thermistors in TO-92 encapsulation, often with 10 kΩ at 25°C and a beta value around 3950 K, provide cost-effective sensing in compact designs. Silicon-controlled rectifiers (SCRs) like the 2N5060, rated for 0.8 A RMS on-state current and 30 V blocking voltage, and triacs such as the BT169, with 0.8 A RMS and 600 V blocking, are common for low-power AC control and switching. These components contribute to the TO-92's enduring popularity due to their reliability and low cost. The , introduced in 1966 by , remains a staple for its versatility in both amateur and professional electronics, continuing to be manufactured by multiple vendors without significant changes.

Typical Circuit Applications

The TO-92 package is commonly employed in switching and amplification circuits due to its suitability for low-power discrete components. For instance, the NPN transistor, housed in TO-92, serves as a general-purpose and switch in digital logic gates, where its fast switching times (storage delay time of 35 ns and fall time of 50 ns at 10 mA collector current) enable reliable operation in simple combinational and sequential circuits. In audio amplifiers, the provides small-signal amplification with a current gain (h_FE) of 100–300 at 10 mA and a low of 5 dB at 100 µA, making it ideal for pre-amplifier stages in low-fidelity sound systems. Additionally, it functions as a driver for low-current loads under 100 mA, leveraging its collector current rating up to 200 mA to control electromagnetic relays in automation controls without requiring additional heat sinking. In sensing and control applications, TO-92 components facilitate precise and . The LM35 precision centigrade temperature , available in TO-92, outputs a linear 10 mV/°C voltage and is widely used in temperature monitors for (HVAC) systems, where its ±0.5°C accuracy at 25°C ensures reliable thermal regulation in residential and commercial buildings. Its low quiescent current of 60 µA also supports integration into (IoT) devices for remote temperature sensing, such as in smart thermostats or environmental data loggers operating over -55°C to 150°C ranges. Similarly, the BC547 NPN in TO-92 configuration is applied in simple oscillators, like astable multivibrators, to generate square-wave signals for timing circuits in basic control systems. It is also utilized in pairs to boost current gain for driving higher-load actuators, such as in switches or interfaces requiring sensitivity enhancement without complex ICs. Despite the shift toward (SMT) in high-volume production, TO-92 retains legacy and modern roles in niche electronics. It excels in prototyping on breadboards, where its through-hole leads allow easy insertion and removal for experimentation with components like the in educational kits and hobbyist projects. However, by 2025, its use persists primarily in cost-sensitive repairs of legacy equipment and educational settings, as SMT alternatives dominate for their smaller footprint and automated assembly efficiency.

References

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  28. https://www.digikey.com/en/products/filter/thermal/heat-sinks/to-92/219
  29. https://www.origin-ic.com/blog/2n3904-datasheet-key-specifications-applications-guide/47937
  30. https://nikkoe.com/tcd8550d-transistor-to-92-53.html
  31. https://www.onsemi.com/products/discrete-power-modules/general-purpose-and-low-vcesat-transistors
  32. https://www.ti.com/lit/gpn/LM337L
  33. https://www.idc-com.co.jp/pdf/en/product/Lead%28TO92S%29_E.pdf
  34. https://www.vishay.com/doc/?45242
  35. https://www.newark.com/pdfs/techarticles/onSemi/CASERM-D.pdf
  36. http://www.bitsavers.org/components/ferranti/1983_Ferranti_Quick_Reference_Guide.pdf
  37. https://ww1.microchip.com/downloads/en/DeviceDoc/00000151Q.pdf
  38. https://www.ti.com/product/LM135A
  39. https://www.ti.com/lit/gpn/LM60
  40. https://www.jedec.org/sites/default/files/docs/22b104C_0.pdf
  41. https://www.jedec.org/sites/default/files/docs/22b103b.pdf
  42. https://img1.wsimg.com/blobby/go/fae029f6-27b1-4578-94bc-ae0bbaeebde4/downloads/to-92_datasheet.pdf
  43. https://www.onsemi.com/pub/collateral/brd8011-d.pdf
  44. https://www.onsemi.com/pdf/datasheet/p2n2222a-d.pdf
  45. https://www.onsemi.com/download/data-sheet/pdf/2n3906-d.pdf
  46. https://www.onsemi.com/download/data-sheet/pdf/bc556b-d.pdf
  47. https://www.onsemi.com/products/discrete-power-modules/mosfets/small-signal-mosfets/2n7000
  48. https://www.ti.com/lit/gpn/LM78L
  49. https://www.vishay.com/docs/81857/1n4148.pdf
  50. https://www.ti.com/lit/gpn/LM35
  51. https://www.teamwavelength.com/download/Datasheets/tcs-series-thermistors.pdf
  52. https://www.onsemi.com/pdf/datasheet/2n5060-d.pdf
  53. https://www.nxp.com/docs/en/data-sheet/BT169_series.pdf
  54. https://www.edn.com/2n3904-why-use-a-60-year-old-transistor/
  55. https://www.electronics-tutorials.ws/waveforms/astable.html
  56. https://www.electronics-tutorials.ws/blog/relay-switch-circuit.html
  57. https://www.digikey.com/en/articles/transistor-basics
  58. https://www.digikey.com/en/articles/simply-put-2n3904-npn-transistor
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