dumb guy question-- through hole & smd

[plexi12000]

I was looking for diodes.  why does a "through hole" have two leads....but a similar "smd" has three tabs?

[Tony Forestiere]

So the "pick and place" machines can properly orient them.

[R.G.]

Good guess - but PnP machines commonly orient the two-lead diode packages correctly.

It's because the SOT-23 package is so very common, and two-diode combos in the same package exist. So in the same SOT-23 package, you can get one diode, two diodes with a common anode on the third lead, two diodes with a common cathode on the third lead, or two diodes in series with the middle tap on the third lead.

Placing components on a PCB is very close to following the old programming mantra - almost always is almost always as good as always. Commonality of package size and shape makes for very efficient layouts.

[Tony Forestiere]

Dang! I love to learn something new every day!
Thanks R.G.

[plexi12000]

awesome!  Thank you for sharing, R.G.!