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Transistor Packages

Common Transistor Packages

Transistors are available in a large variety of packages. The four below are the most common.



Common Transistor Packages:
TO-3, TO-5, TO-92 and TO-220

The first package is called TO-3 and is used for some of the highest power transistors. The case is connected to the collector. The two pins are the base and emitter. It is designed to have a large surface area for mounting to metal heat sinks.1 The metal box or chassis that the circuit is built into or onto is often used for a heat sink. Since this is often grounded, and the transistor collector is usually not grounded, the transistor has to be firmly mounted to the heat sink, but electrically insulated from it. Mounting kits are available with insulators and heat conducting paste to mount these transistors. For FETs, the case is connected to the drain and the two pins are the gate and source.

The second package is the TO-5. It is difficult to see in this picture but there is a tab sticking out of the side of the case. This tab is closest to the emitter lead. The collector is farthest from the tab. The TO-5 package is also used for integrated circuits with up to eight pins. For FETs the tab is closest to the source and the lead farthest from the tab it the drain.

The next package is TO-92. this is a plastic package that doesn't dissipate heat readily and is used for low power transistors. Here is another look:

   

TO-92 flat side
TO-92 rounded side

Pin 1 is usually the emitter, pin 2 is usually the base and pin 3 is usually the collector. For FETs pin 1 is usually the source, pin 2 is usually the gate and pin 3 is usually drain. Check the datasheet to be sure.

The last package is the TO-220. It is for high power transistors but does cannot get rid of heat as well at TO-3.  In the view above the left pin is the base connection and the right pin is the emitter connection. The center pin and the heat sinking tab are connected to the collector. Like the TO-3, the TO-220 is usually mounted on metal heat sinks while electrically insulated from them. The popular 2N3055 high power transistor comes in both TO-3 and TO-220 packages. In the TO-3 it is typically rated at 115 watts. In the TO-220 it is typically rated at 75 watts. For FETs the left pin is the gate, the center pin is the drain and the right pin is the source.

1
 A heat sink is a piece of metal, often aluminum and often with fins to give it a large surface area to dissipate heat into the air.


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