What replaces a BC547, 2N3904, 2N2222A, TIP122 or 2N3055 — and which ones need the leads bent?

The substitutes, and the lead order each one lands on

A drop-in here means the body, the mounting and the lead order all match — the part goes in with no lead forming. Anything marked otherwise needs the leads re-formed, the device rotated, or mounting planned, and that is the column worth reading first.

The trap is the lead order, not the gain

A transistor that fits and is the wrong way round behaves like a fault, not like a wrong part: the circuit may bias into conduction, draw current through the base, or simply do nothing, and the diagnosis goes off in the wrong direction. The three-rows-of-three-pins pattern above is worth memorising as a shape — European C-B-E, American E-B-C, Japanese E-C-B — because almost every substitute error is a crossing between two rows.

The trap compounds in complementary pairs. When the NPN half follows one order and the PNP half another, a substitution is two decisions rather than one. Read the drawing for the specific body you hold, including the tab: on a TO-126 the collector is the tab, so an uninsulated part against a grounded heatsink shorts the collector.

Grades, prefixes and the same part twice

Most of the letters in this family are grades rather than part numbers. In the BC547 row the trailing letter is the current-gain group, and BC547B is the same transistor as BC547 with a tighter hFE window. In the Darlington row the number is the current rating rather than the topology: TIP120, TIP121, TIP122, TIP125, TIP126, TIP127 are one design across 5 A and 8 A rungs and NPN and PNP halves, and moving between them is a rating decision.

The S8050 row hides the largest difference behind the smallest change: S8050 and SS8050 are the same three-pin body, and the SS prefix is the difference between roughly 0.7 A and 1.5 A. That two-letter spread is also the reason a "S8050" bought without a marking check may not carry the load the schematic expects.

Japanese repair parts need one extra check. 2SC945 and 2SA733 are the classic TO-92 pair of consumer gear, and their E-C-B order differs from both the European and the American row — so the substitutes that genuinely drop in are 2SC1815, 2SA1015 and KSC945C, not the BC or 2N parts that share the body.

What we will not claim

Every mapping above was checked against the manufacturer datasheet for the body, the lead order and the ratings that decide a swap, and where the datasheet did not support one it is not listed. A substitute is a starting point rather than a clearance: confirm against your own schematic, and treat the level as the scope of the risk — a part that needs its leads re-formed is a different job from one that drops in.