Do you have questions about common transistors?
Over the years DigiKey technicians have answered thousands of questions regarding transistors. There are a few questions that stand out including telephone inquiries about transistors such as the 2N3904, 2N2222, and 2N3055. This post serves as a guide identifying and then comparing popular transistors. The information is generally applicable to the hobbyist, student, and experimenter.
What is a jellybean transistor?
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 manufacturers. They are unlikely to become obsolete for a few decades.
This graphic shows the relationship between a few of these jellybean transistors along with recommendations for increasing power.
Complementary NPN and PNP devices
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 metric assumes that 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 ideal TIP41BG is rated for 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 include 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 has a VCE max of 40 V while the TIP41BG can withstand 80 V.
Tech Tip: Pay attention to datasheet specifications, especially when substituting transistors. Some designs conservatively operate the transistor with respect to V_{CE} and power dissipation. Other designs drive the transistor very hard. Selection of a generic transistor for the first scenario is generally acceptable. Use of a generic transistor for the second application is where attention to datasheet specifications become essential.
Surface mount transistor alternatives
The last few decades have seen a transition away from the through-hole components identified in this article to surface mount devices. This provides the hobbyist and student with many alternatives. For example, the 2N3904 die is available in a SOT-23 package such as the MMBT3904. This allows some designs featured in older articles to be miniaturized.
Tech Tip: Be cautious when substituting surface mount transistors for the original through-hole devices. Close examination of the datasheet suggests that power dissipation is a challenge for the surface mount devices.
Classic transistors
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.
Conclusion
At the time of this writing, DigiKey offers over 100,000 unique transistor part numbers. Obviously, the NPN and complementary PNP transistors mentioned in this article will not cover all your design needs. Yet, they serve as a solid starting point - certainly good devices to stock in your parts kit.
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
About the Author
Aaron Dahlen, LCDR USCG (Ret.), serves as an application engineer at DigiKey. He has a unique electronics and automation foundation built over a 27-year military career as a technician and engineer which was further enhanced by 12 years of teaching (interwoven). With an MSEE degree from Minnesota State University, Mankato, Dahlen has taught in an ABET accredited EE program, served as the program coordinator for an EET program, and taught component-level repair to military electronics technicians. Dahlen has returned to his Northern Minnesota home and thoroughly enjoys researching and writing articles such as this.