TO-92

Key takeaways

TO-92 belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect TO-92 to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of TO-92 from memory before moving on to harder problems.

Reference excerpt

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

Construction and orientation

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".

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. 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 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 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.

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

In the late 1960s, Ferranti introduced a smaller package with a compatible footprint, called "E-Line". 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). In East Germany the E-Line package was known as the "Miniplast" package and widely used by Kombinat Mikroelektronik Erfurt.

Standards

Common components in a TO-92 package Common transistors:

BC548, general purpose NPN transistor BC558, general purpose PNP transistor 2N3904, general purpose NPN transistor 2N3906, general purpose PNP transistor 2N7000, N-channel FET PN2222A, a "plastic" version of the original 2N2222 NPN transistor which came in a TO-18 package Other common components:

MK484, AM radio integrated circuit 78Lxx series, voltage regulator integrated circuits TL431, voltage regulator integrated circuit

References

External links

TO-92 Package, EESemi.com [1] Essay about TO92 housings (German) by Dipl.-Ing. Bernd Wiebus at German national library server.

Illustrations

Worked examples

Example 1 — a first encounter with TO-92

Start with the simplest possible case. Write down what TO-92 claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to TO-92 before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about TO-92 ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of TO-92

In research: TO-92 appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses TO-92 in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: TO-92 is common in secondary-school and first-year university syllabi. It links to neighbouring topics Semiconductor packages, so understanding it makes those chapters shorter.
In everyday life: Look for TO-92 outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study TO-92 in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what TO-92 means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain TO-92 out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is TO-92 in simple terms?

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.

Why does TO-92 matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study TO-92?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on TO-92.