TO-3
TO-3 or "metal can" is an older style of electronic component package, commonly used for very large transistors, silicon-controlled rectifiers, and, occasionally, integrated circuits. The component is mounted on a metal plate with a metal can crimped on top of it, providing excellent heat conduction and durability. TO-3 packages usually have three leads, two of which pass through and are insulated from the metal plate, one directly attached to the metal plate. They have two tabs with holes for mounting to a heatsink. Components made in TO-3 dwarf those constructed in TO-220 or TO-92 cases, and can safely dissipate more power.
Typical applications
TO-3 packages are heatsinkable, and thus can be used in projects where a large amount of power is being drawn. The sides of the package are metal tabs with holes used in mounting the component to a heatsink. Thermal compound is also used to provide greater heat transfer.
The case is usually connected electrically to the internal circuitry. A mica insulator may be required to electrically isolate the component from the heatsink if the heatsink is grounded or is mounted to multiple devices. Mica insulators are favorable because they have excellent thermal conductivity while acting as an electrical insulator.
Advantages
- Can be used in high-power and high-current applications where equivalent components of other cases may be susceptible to damage.
- Almost all surfaces are metal for better heat conduction and durability.
- Mounting with the tabs ensures the component to be held firmly in place.
Disadvantages
- More costly than equivalent components of other cases.
- Component size and lead placement makes soldering to a circuit board difficult -- off-board space must be dedicated to the components and/or their heatsinks, thereby increasing production costs.
Common components that use the TO-3 package
Common Voltage Regulators:
Common Transistors:
- 2n3055, NPN power transistor