What replaces an IRF540N, IRF9540, IRF840, IRFP460 or 2N7002?

The substitutes, and the class each one runs in

A drop-in here means the outline and the pin order both match and the voltage class is the same. Everything else has a note attached, and in this family the note is usually about voltage rather than about fit.

The trap: every pin matches and the dielectric does not

Family numbers in the IRF rows are not a consistent code. The middle of the part number looks like a specification and is not one: the IRFP250 and the IRFP460 sit in the same TO-247 body with the same pin order, and differ by 300 V of breakdown. That card exists because the cross gets made from memory — "IRFP, TO-247, close enough" — and it is not close at all. In a mains-side bus the low-voltage part fails on the first transient rather than on the first overload, so the diagnosis afterwards points at the driver or the snubber instead of at the part that was fitted.

The same arithmetic applies one rung down, less dramatically. Moving from an IRFP460 to an IRFP450 keeps the 500 V class but drops from 20 A and 0.27 Ω to 14 A and about 0.4 Ω, and conduction loss goes as current squared times resistance — so in welder duty the extra heat lands exactly where the heatsink was already marginal. Grading up is free margin; grading down is a measurement you owe the board.

The logic-level question, which is a gate-drive question

The IRLZ44N against the IRF540N is the most common substitution request in hobby and light industrial repair, and the reason is in the gate rather than in the channel. A standard-gate part driven from 5 V operates half-enhanced, in the linear region, and dissipates heavily — which is what "the IRF540N gets hot on the Arduino" actually means. The logic-level part is fully enhanced at a 5 V gate and runs cool at a couple of amps, at the cost of a 55 V ceiling against the IRF540N's 100 V. Either fit the logic-level device, or keep the standard part and give it a real 10–12 V gate drive.

The small-signal end hides the same trap in miniature. SOT-23 MOSFETs do not share one pin map: the 2N7002, the BSS138 and the AO3400 are all three-pin SOT-23 parts whose gate, source and drain move between makers and families. Read the manufacturer drawing for the specific orderable part, and remember that the through-hole 2N7000 is a different pin order again.

What we will not claim

Every mapping above was checked against the manufacturer datasheet for outline, pin order and voltage class, and where the datasheet did not support one it is not listed. The IRFP250 row is here as a warning rather than as a substitute. Confirm against your own schematic and your measured load before a redesign, and treat the level as the scope of the risk.