Over the years DigiKey technician have answered many thousand questions regarding transistors. From these, there are a few questions that stand out including telephone inquiries about transistors such as the 2N3904, 2N2222, and 2N3055. This post will attempt to answer some of your questions about those common transistors.
Transistors have been with us for a long time. Like a good pair of shoes, designers have naturally selected a few transistors with desirable traits. These transistors could be called “jellybean” components as they are common and generally offered by multiple manufactures. They are unlikely to become obsolete for a few decades.
The following graphic show the relationship between a few of these jellybean transistors along with recommendations for increasing power.
For each listed transistor there is a complementary PNP device. This table contains each transistor with select parameters:
| NPN | PNP | VCE Max | Ic (Max) | PD (Max) |
|---|---|---|---|---|
| 2N3904BU | 2N3906BU | 40 V | 0.2 A | 0.625 W |
| TIP41BG | TIP42BG | 80 V | 6 A | 62 W |
| BDX33C (Darlington) | BDX34C (Darlington) | 100 V | 10 A | 70 W |
| TIP35CG | TIP36CG | 100 V | 25 A | 125 W |
Notes:
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The maximum power dissipation assumes the transistor is attached to an ideal heatsink. This will not be achievable for most designs. Instead, you should design with a wide safety margin. For example, the TIP41BG is rated for and ideal 62 W, a more realistic expectation is 5 to 20 W with a moderate to large heatsink.
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The part numbers for the listed transistors often includes a suffix. As a rule of thumb, this suffix is not important, provided the transistor is operated well within its design maximum parameters. However, care must be taken as the performance demands on the transistor are increased. Be sure to consult the datasheet for your specific transistor. As an example, the generic TIP41 transistor is has a VCE max of 40 V while the TIP41BG can withstand 80 V
No discussion about jellybean transistors would be complete without mentioning the venerable 2N2222 and the 2N3055. These were once the first choice for many common projects. You will often see these classic components mentioned in old articles. The 2N2222 is easy to identify as a small metal can transistor. Likewise, the larger 2N3055 is easy to identify. Today, these iconic components have fallen out of favor as the previously mentioned components can be manufactured at a lower cost.
Many of the old designs featuring the 2N2222 can be modified to use the 2N3904. Once again, we must carefully investigate the circuit operation. Most designs operate the transistor well within the design maximum specification. However, some designs push the 2N2222 to its limits. You will find that the metal can 2N2222 is able to dissipate more power than the 2N3904. Also, the 2N2222 transistor is capable of delivering nearly 4x the current of a 2N3904. One substitution for the classic 2N2222 transistor is the PN2222ABU. With regards to the 2N3055, the previously mentioned TIP35CG is generally a solid substitute. The only stipulation is that proper heatsinks and thermal insulators must be used.
At the time of this writing, DigiKey offers over 100,000 unique transistor part numbers. Obviously, the seven transistors mentioned in this article will not cover all your design needs. Yet, they serve as a solid starting point.
If you have any questions about transistors, substitutions, or purchasing I encourage you to visit the DigiKey Tech Forum. I hope to see you there.
Do you agree with this list of generic transistors?
What are your go-to parts?
Best Wishes,
APDahlen