science

TO-18

TO-18 is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand TO-18 rather than just read about it. In short: In electronics, TO-18 is a designation for a style of transistor metal case. The case is more expensive than the similarly sized plastic TO-92 package.

Key takeaways

TO-18 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-18 to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of TO-18 from memory before moving on to harder problems.

Reference excerpt

In electronics, TO-18 is a designation for a style of transistor metal case. The case is more expensive than the similarly sized plastic TO-92 package. The name is from JEDEC, signifying Transistor Outline Package, Case Style 18.

Construction and orientation The typical TO-18 metal can package has a base diameter of 5.6 mm (0.22 in), a cap diameter of 4.70 mm (0.185 in), a cap height of 4.83 mm (0.190 in). The tab is located 45° from pin 1, which is typically the emitter. The lead diameter is nominally 0.45 mm (0.018 in). The leads are arranged in a circle with a diameter of 2.54 mm (0.100 in). The minimum length of the leads is 12.7 mm (0.50 in). Different manufacturers have different tolerances, and the actual form factor may vary slightly, depending on function.

Uses and variants

The 3-lead TO-18 is used for transistors and other devices using no more than three leads. Variants for diodes, photodiodes and LEDs may have only two leads. Light-sensitive or light-emitting devices have a transparent window, lens, or parabolic reflectors in the top of the case rather than a sealed, flat top. For example, diode lasers such as those found in CD players may be packaged in TO-18 cases with a lens. There are variants with between 2 and 8 leads.

TO-46 / TO-52 The TO-46 and TO-52 packages have 3 leads. These packages differ from all other variants in the height of the cap. Instead of 4.83 mm (0.190 in) the cap height is only 3.30 mm (0.130 in) for TO-52 and 1.90 mm (0.075 in) for TO-46.

TO-72 The package with 4 leads but otherwise with dimensions identical to TO-18, is standardized as TO-72. The fourth wire is typically connected to the metal case as a means of electromagnetic shielding for radio frequency applications.

TO-71 The TO-71 package has 8 leads (up to three of those may be omitted). The minimum angle between two adjacent leads is 45°.

TO-206 TO-206 is intended to replace previous definitions of packages with leads arranged in a circle with a diameter of 2.54 mm (0.100 in). The different outlines are now defined as variants of TO-206: TO-18 is renamed to TO-206-AA, TO-46 to TO-206-AB, TO-52 to TO-206-AC, TO-72 to TO-206-AF. A new package with 3 leads and a cap height of 1.40 mm (0.055 in) (i.e. smaller than TO-46) is added as TO-206-AD. TO-206-AE does not require a minimum diameter of the leads but is otherwise identical to TO-18. The somewhat unrelated TO-58 package is included as TO-206-AG.

National standards

See also Common transistors in a TO-18 package: 2N2222, BC108 family Common integrated circuits in a TO-18 package: ZN414

References

External links

TO-18 Package, EESemi.com

Illustrations

Worked examples

Example 1 — a first encounter with TO-18

Start with the simplest possible case. Write down what TO-18 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-18 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-18 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-18

In research: TO-18 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-18 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-18 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-18 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.

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How to study TO-18 in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what TO-18 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-18 out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is TO-18 in simple terms?

In electronics, TO-18 is a designation for a style of transistor metal case. The case is more expensive than the similarly sized plastic TO-92 package.

Why does TO-18 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-18?

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-18.