What replaces a PC817, 4N35, 4N25, MOC3021 or TLP250?

The substitutes, and what each row actually is

Four pins against six pins

The PC817 is the optocoupler of an enormous range of consumer and industrial boards, and it is a four-pin part: light-emitting diode in, phototransistor out, nothing else bought with it. The 4N25 and 4N35 are six-pin parts of the same idea with the transistor base brought out to pin 6 — which is what makes them slower and more useful, because the base pin can be used to trim the response. Crossing between the two rows is a footprint change and, in the six-pin direction, a new circuit pin to decide about.

Within the six-pin row the pins are worth keeping at hand: 1 anode, 2 cathode, 3 no connection, 4 emitter, 5 collector, 6 base. The substituting hazard is orientation rather than order — some bodies carry variant markings, so the pin 1 dimple is the reliable reference.

The optotriac row, where the datasheet decides the circuit

The MOC3021 is a six-pin optotriac: an LED driving a triac output, used to switch mains loads from logic. Its family shares the DIP-6 outline and the pin order, and the members differ in a property the outline cannot express — whether the output turns on at the zero crossing of the mains cycle or at whatever point it is triggered. In a resistive load that is a detail; in an inductive load it changes what the triac sees and what the snubber has to handle. The package will accept the wrong member happily, so read the trigger type from the datasheet before substituting rather than diagnosing the result afterwards.

For a board that needs isolation it is worth remembering that a triac output optocoupler does not isolate the load the way a transistor-output part isolates a signal: the load side is still mains-referenced, and the isolation is between logic and load rather than around the load.

Gate-drive optocouplers are their own row

The TLP250 is the part that made it cheap to drive an IGBT or a power MOSFET from logic at mains potential, and its substitutes split into the ordinary eight-pin socket — the TLP350 and the FOD3120 — and the wide-body variant of the same part, where the isolation body is larger and the socket is not. The wide-body version is electrically the same device, which is exactly the substitution that fails a clearance review rather than a functional test.

What we will not claim

Every mapping above was checked against the manufacturer datasheet for package, pin order and output type, and where the datasheet did not support one it is not listed. One row carries do not insert because the pin count makes it impossible, and the optotriac rows are marked function only because the trigger type is the reader's to verify. Confirm against your own schematic and, on anything switching mains, against the load.