Case styles

The silicon chip inside the case determines most of the primary characteristics. The case we put it in has a small influence. If we put it in a small surface mount case we loose overall power capability. The smaller case just canít dissipate the power a larger or metal case can. The SOT-23 case may be limited to a 200 mW. The same chip put into a TO-92 case may be capable of 500 mW. In a TO-18 or TO-5 case it may be capable of 1 Watt. Our maximum Voltage and Current capabilities are the same but the operating range is limited to higher voltage at lower current or higher current at lower voltage combinations.

Pinout of various cases take on a few standards, but donít assume they are always the same.







There is some degree of standard in pinouts for each case, but donít expect all devices to be standard. When in doubt consult the manufacturerís data sheet.


SMD cases


One of the more popular Surface Mount cases these days. The case is about 2 mm by 2.8 mm (0.08 by 0.10 inches) with a few variations. These cases are so small there is not enough room for a complete part number. A two or three character code is all that is printed on the case.

Typically there are three to six leads on the case. Up to eight leads can be found with slight variations in the case. The case gives no indication of what type of device is inside. It may be a diode, two or more diodes, a single transistor or it may be an IC.







As small as the SOT-23 case is the SC70 is similar in appearance but even smaller, around 1.5 mm by 2 mm. It takes some degree of familiarity to eyeball the difference between an SOT-23 and an SC70 case on a board.






These are notably larger than the SOT-23 and are comparable in power to TO-92 cased devices. Currents of 1 Amp are common.






Used as motor drivers and voltage regulators we often find SOT-223 case devices on our boards. The most notable difference if you have a hard time eyeballing the difference between 5 mm (SOT-89) and 6 mm (SOT-223) is how the leads come out of the package. The SOT-89 mounts flat and the leads are even with the bottom of the case while SOT-223 leads come out a bit higher and bend down to make contact with the board.

There are numerous other SMD cases we wonít commit the space to go into here. Just be aware that there are more things between Heaven and Earth than are apparent in these philosophies.


Leaded cases for transistors

TO-3 (TO-204AA)

These are the diamond shape cases with two leads sticking out the bottom. Not in use too much any more these are finding their way out of applications.

The drawing shown is but one variation of a TO-3 case. Many variations are around. It could be a diode, a transistor or an IC just as easy. You canít tell one part from another without being familiar with the part numbers.


These are a smaller version of the TO-3 case. Even less often found these days.














TO-5 / TO-39

These are small cylindrical metal cases about 0.35" (9 mm) in diameter. The two cases are very similar in size and are treated as one for the purposes of this paper. The leads are in a triangle on about 0.200" at the widest part.
















These are smaller cylindrical metal cases about 0.23" (5.8 mm) in diameter. The leads are in a triangle on about 0.100" at the widest part.








































There are many variations similar to this case. The case is a bit smaller than the TO-18 and is black plastic. The leads may be in a triangle shape or in line. This is probably the most popular case style in use for leaded small signal devices. The leads are on 0.050" centers and need to be spread a bit to fit in the 0.1" inch centers for breadboards.

The leads may be straight or formed.


TO126 (TO-225AA)

This has been a popular case for medium power SCRís and Triacs in slot machines. A bit lower in power than the more popular TO-220 types, but more power handling capability than TO-92 cases.


In between the TO-126 and the TO-220 we have the TO-202. These are also used for medium power SCRs, Triacs and transistors of all kinds. As you can see in the picture there are many variations on the pinout just for bipolar transistors. Usually the tab is the same as pin 2, but not always. When the tab is used as the Collector it makes a handy test and troubleshooting point. On NPN devices that drive to a positive voltage through a load you can ground the Collector tab and check the wiring and connected device.























This is probably the most popular leaded package for power devices. The tab may be metal and the same connection as pin 2, or it may be isolated, or even a completely plastic case (TO-220AA variation). Typical current is 1 Amp to 10 Amp range. Typical voltage is around 100 V or less. Again, as you can see from the drawing, pinout has many variations. It may be a transistor of any kind or even an IC.




A bit larger than the TO-220 package is the TO-127. There are quite a few packages in this physical size. Some have a metal tab. Some are all plastic. We will cover a few. Transistors of this size are higher power devices like Horizontal Output Transistors and higher wattage Switch Mode Power Supply Regulators. Not everything this size is a transistor. It could also be diodes or an IC.









A slight bit bigger than the TO127 the TO218 is also used for HOT and SMPS designs. Here we are looking at ratings up to 1,000 Volts and dozens of Amps.

These are similar in size to the TO-3P case style. Referenced to TO-3 because it can be physically formed to fit into a TO-3 socket.

Is this the largest transistors get? In the gaming industry, perhaps. In real life of other industries, certainly not. Elevator controls use transistors rated in thousands of Volts at hundreds of Amps and are close in size to a VCR tape.