Search for "Towers International Transistor Selector PDF" or "ECG Replacement Guide PDF" – these are scanned versions of the classic books, ideal for workshops without internet.
MOSFET substitution is less forgiving than BJTs. Pay attention to Vgs(th) (Gate threshold voltage) and Rds(on).
| Original MOSFET | Type / Voltage | Good Equivalent | Critical Check | | :--- | :--- | :--- | :--- | | IRF540N | N-Chan, 100V, 33A | IRF540Z, STP55NF06, FQP50N06 | Rds(on) is lower on subs (good). Check gate charge. | | IRF9540 | P-Chan, -100V, -19A | IRF9540N, FQP27P06 | P-channel substitutes are rare. Stay in family. | | 2N7000 | N-Chan, 60V, 200mA | BS170 (higher current), 2N7002 (SMD) | Logic level (Vgs=5V). | | BS170 | N-Chan, 60V, 500mA | 2N7000 (lower current), VN2222 | Pinout: BS170 is D-S-G (center drain). | | IRFZ44N | N-Chan, 55V, 49A | IRF3205, IRLZ44N (logic level) | IRLZ44N requires only 5V gate drive; check original drive voltage. | | IRFP460 | N-Chan, 500V, 20A | IRFP460A, FQA24N50F | High voltage. Sub must have same or lower Coss (output capacitance). |
Avoid these substitutions unless you enjoy burning components: all transistor equivalent book
| If you have... | Do NOT substitute with... | Why | | :--- | :--- | :--- | | A Darlington (TIP120) | A single BJT (2N3904) | The base current will be 100x too low. | | A JFET (2N5457) | Any MOSFET or BJT | JFETs are depletion-mode; MOSFETs are enhancement-mode. Not interchangeable. | | A Matched pair (LM394) | Two random individual transistors | Vbe mismatch will cause DC offset. | | A High-voltage CRT driver (BU508A) | A standard power transistor (TIP31) | The standard part will instantly breakdown at 1500V. | | A RF transistor (2SC1971) | A general purpose transistor | Low ft and high Cob will kill RF power output. |
As surface-mount (SMD) devices dominate, part numbers become cryptic (e.g., "1P" for a MMBT3904). The "all transistor equivalent book" is evolving into SMD code books. These decode the 2-3 letter marking on a tiny transistor back to a standard part number, which you then cross-reference.
Furthermore, AI tools (like ChatGPT or specialized electronics LLMs) are beginning to serve as dynamic equivalent books. You can ask: "Find a modern equivalent for a 2SA970 in a TO-92 package for a phono preamp." The AI returns KSA992 (fair) or ZTX795A (better). Search for "Towers International Transistor Selector PDF" or
Warning: Always verify AI suggestions against a real datasheet. Equivalent books were written by human engineers; AI is still learning.
The biggest failure when using an equivalent book is ignoring pinout differences.
If you plug a 2N3904 into a socket designed for a BC548 without crossing leads, you will destroy the transistor instantly. A good equivalent book provides pinout diagrams or footnotes. MOSFET substitution is less forgiving than BJTs
The last printed "all transistor equivalent book" was published around 2005. Today, the equivalent concept lives online. Here are the modern successors:
While online resources are handy, there are books that can serve as a good reference for transistor equivalents and more: