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Example:

http://circuits.datasheetdir.com/142/PS2621-pinout.jpg

It is obvious that an optocoupler which provides a pin for the base of the internal transistor (e.g.; PS2621) cannot be in a 4-pin device. My question is not about this kind of optocouplers. But, an optocoupler which gives out only the collector and emitter pins (e.g.; PS2622) can be implemented in a 4-pin device. Yet in electronics, it is important to make the smallest possible design to save space and material cost. Why are 6-pin optocouplers so common in the market? Is there a reason for putting them in 6-pin packages?

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    \$\begingroup\$ Size isn't the only consideration - ease of manufacture and design. Drop-in replacements for other people's products are an important design consideration. Note that those two have the exact same pinout bar the base connection. Also if you're already making 6 pin ones it's cheaper to re-use the same rig than change for making a 4 pin one. \$\endgroup\$
    – Majenko
    Mar 23, 2015 at 12:11
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    \$\begingroup\$ In the example you gave its probably a matter of convenience (design and manufacturing). Its not uncommon that the same part series uses the same package with different pin assignments. \$\endgroup\$
    – Rev
    Mar 23, 2015 at 12:13
  • \$\begingroup\$ In this case you can even use the transistor in a dual-use purpose, once with the optocoupler, and once via external sources. \$\endgroup\$
    – arc_lupus
    Mar 23, 2015 at 12:32

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By adding a resistor from the base to the emitter you can trade off speed against sensitivity, so the base connection is not entirely useless. Of course you could use it in some other kind of fancy circuit with current fed to the base from an external source under some conditions, but I've not run into the need for that.

Since the very first DIP optocouplers were in 6-pin packages, the reasoning may have had to do with the space required internally to make the isolated lead frame connections using the methods they had available, but that's just speculation on my part. The 4-pin ones came later, and, if I recall correctly, primarily from Japanese suppliers such as Toshiba and NEC who were less committed to second-sourcing U.S. parts than to making parts that were attractive to their customers.

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