Which Power Devices?
The two recommanded power transistors are chunky devices in a TO-247 package. These are
the TIP35C for the NPN, and the TIP145 or TIP147 for the Darlington PNP.
The TO-247 version of the TIP147. Note that the mounting hole is insulated from the metal parts of the
case.
Although they are directly soldered onto the circuit board they are actually mounted onto the
heatsink plate to which the whole power supply is attached. To prevent the metal part on the
underside of the power device from making electrical contact with the heatsink you need to
use an insulating pad. This allows the heat to travel through but not any electrical current.
These will be sold as TO-247 (or TO-3P) insulating pads. You need one pad per transistor but
it may be best to order a couple of extras since they can damaged if not handled carefully.
Obsolete versions of both power devices are still available in the older TO-218 package. This
package is similar in appearance to a larger version of the popular TO-220 package with an all
metal mounting tab. Try to avoid these in this project as they are more complex to mount onto
the heatsink as the fixing screw, washer and nut need to be insulated from the metal tab.
Special mounting kits are available for these older devices but they do take more time to
assemble.
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