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 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.
Construction and orientation
The case is molded around the transistor elements in two parts; the face is flat, bearing a machine-printed part number. 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.
If the face has a part name made up of only one letter and a few numbers, it is usually assumed to be a Japanese part number (with the base on the end rather than in the center). Thus, "C1234" would likely be a 2SC1234 device.
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 sightly depending of the manufacturer, however, the 1.27mm lead spacing must be respected.
Pros and cons
- 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.
- 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.
Common transistors using a TO-92 case
- BC548, NPN transistor
- BC558, PNP transistor
- 2N3904, General purpose NPN
- 2N3906, General purpose PNP
- 2N7000, N-Channel Field Effect Transistor (FET)
- BS170, N-Channel FET, similar to 2N7000
- PN2222A, 2N2222 NPN
- 2N2907, PNP
- 2N4401, NPN
- 2N4403, PNP
- 2SC945, NPN
- MPSH10, NPN RF
Additional uses of the TO-92 case
- MK484, AM radio Integrated Circuit
- 78Lxx series voltage regulators
- TCR1AM, Triac
- DS2501-UNW/DS2502, Add-only memory
- DS18B20, Temperature sensor, digital output (1-Wire)
- LM335, Temperature sensor, analog output (+ 10-mV/°C)
Soviet plastic quality issues
They had equivalent transistors, but the cases in the USSR were either epoxy, resin, metal or ceramic due to the crappy nature of Soviet plastic and packs castings. It's resin and epoxy was on occasions gritty, crumbly or perished very quickly in extreme temperature situations. Casing results were also poor and some rather irregular casting shapes thus occurred.