Transistor Package TO-3

TO-3 Package

Transistor Package TO-3 Case style [TO-204AA]


TO-3 Transistor Package Shape, Dimensions and Transistor Pinout
TO-3 Metal Can, Flange Mount

TO-3 Outline
Diamond Shape Case

Case dimensions are in the table below.
Provided as inches and millimeters.
With minimum & maximum tolerances.



TO-3 Package Description:

TO-3 Transistor Package photograph
2N5302

Note the location of the Base and Emitter terminals in the graphic above, the collector is attached to the metal body of the TO-3 package to dissipate heat faster. The picture to the left shows a military JAN qualified 2N5302 transistor, while the picture below shows a commercial 2N3055 transistor in a TO-3 package. This is a through-hole device also called a TO-204, and is similar to a TO-23 package, but having different physical dimensions.


Case mount. A type of package which provides a method of readily attaching one surface of the semiconductor device to a heat dissipater to achieve thermal management of the case temperature. The case of this device is connected to the Collector. The TO-3 case style may also be called a flange mount device.

Case temperature. Case temperature is that temperature measured at a specified point on the case of a semiconductor device. In most cases the junction temperature is hotter than the case temperature.

TO-3 Transistor Package photograph
TO-3 Metal Case




TO-3 Transistor Types [3-Pin];
NPN Power Transistor 2N3055, 2N5685, 2N5686, 2N6306, 2N6308, 2N6249, 2N6250, 2N6251, 2N6274, 2N6277, 2N6032, 2N6033, 2N6676, 2N6678, 2N6546
NPN High-Power Transistor 2N3715, 2N3716, 2N3771, 2N3772, 2N5038, 2N5039, 2N5302, 2N5303
NPN Power Darlington Transistor 2N6058, 2N6059, 2N7569, 2N7570, and 2N7571
PNP High-Power Transistor 2N4399, 2N5745
PNP Power Transistor 2N5683, 2N5684, 2N6378 and 2N6379, BU208
PNP Power Darlington Transistor 2N6286, 2N6287, 2N6648, 2N6649, 2N6650

Normal TO-3 heatsinks are a Black Anodized Aluminum Alloy with 2 mounting holes. Un-coated heat-sinks are also available. The two larger diameter holes are used for the TO3 terminals, made larger so the pins don't make contact with the metal heat-sink.
The output of the semiconductor function is tied to the case, which normally makes contact with the metal heat-sink. The semiconductor output would be the collector terminal for a transistor or the drain terminal for a FET. How to Temperature Derate a 2N3055 transistor.

TO3 Metal Heatsink
TO3 HeatSink

Note that there are also a large number of diodes that use the TO-3 [TO-204AD] package, a few are listed below. The examples represent diode voltage regulators, which are really just a Zener diode in a TO-3 case. These examples have an internal jumper that connects pins 1 and 2 together [which are the two terminals of the TO-3 package]. So the diode connects between pin 1 [and 2] of the package and the case. Some devices connect the Anode to the case [suffix B] and the cathode to the terminals, while other may have the Cathode connected to the case [suffix RB] and the Anode connected to the terminals. Normally a diode is offered as both types, R or RB, and that as a general rule the diode numbers listed only differ by the Zener voltage.





TO-3 Diode examples;
IN2804, IN2805, IN2806, IN2807, IN2808, IN2809, IN2810, 1N2811, 1N2813, 1N2814, 1N2816, 1N2818, 1N2819, 1N2820, 1N2822, 1N2823, 1N2824, 1N2825, 1N2826 IN2827, 1N2829, 1N2831, 1N2832, 1N2833, 1N2834, 1N2835, 1N2836, 1N2837, 1N2838, 1N2840, 1N2841, 1N2842, 1N2843, 1N2844, 1N2845, 1N2846, 1N4557, 1N4558, 1N4559, 1N4560, 1N4561 1N4562

Any device listed is provided as an example semiconductor that used a TO-3 package. No attempt has been made to determine if the component is in production or stocked by a distributor. Also any of the devices may also be available in another package type, and may either have a different part number or a variation of the part number shown.

BJT & Diode Package Outline
Diamond Base outline


Black anodize diamond shaped basket TO3 heat sink
TO-3 Heatsink, Solder tabs



TO-3 Transistor Physical Dimension Table
TO 3 Dimension Data


Related Transistor topics:
4-Pin TO-3 Package Dimensions
8-Pin TO-3 Package Dimensions
Types of Transistor cases
Transistor Derating Guidelines
Manufacturers of FETs and BJTs
Heat Sink Manufacturers


Device Code BU208
TO-3 Transistor Package photograph
BU-208 TO-3 Case

Lead-Free version are also available.
Pb-free versions use different part numbers.
Indicating Pb vs. Non-Pb versions.


TO-3 Transistor Socket, wire lugs
TO-3 Socket
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