PCB Assembly Services - Screaming Circuits: Tip of the Day: Again with the Standard Not Standard


Tip of the Day: Again with the Standard Not Standard

Sometimes the standards-steering-you-wrong aren't just in house, but come from Industry. Case in point the three-terminal linear regulator.

Regulator non-standard pin out

Back when TO-220 packaged semiconductors were first hand chipped out of boiling lava, the LM7805 three-terminal voltage regulator established an informal standard pin-out for all such parts: Input, Ground, Output. Not every regulator uses this pin-out, but enough do that it's easy to just assume that pattern.

It is not completely universal though. Is saying "completely universal" repetitive and redundant? There are some regulators that divert from convention in thru-hole and in SMT form-factors. Despite the overwhelming prevalence of the 78XX pin-out pattern, you may find some parts that dispense with convention and can bite.

Take the LM1085, low drop out (LDO) regulator. It looks, for all intents and purposes, to be a standard TO-220 or TO-263 three-pin regulator. You'd look at it and assume that it's a direct replacement for any old 78XX series. But, rather than In-Ground-Out, it's pinned as Ground-Out-In. The LM1117T (Illustrated here) is the same.

You might think: "Of course, it's different, the part numbering doesn't follow the 78XX number scheme." That sounds logical until you look at the LM2940 LDO. It follows the 78XX pin convention, as does the MIC39100 LDO (also in the illustration). Most three-terminal V-regs follow the In-Ground-Out pattern, but not all do. You undoubtable know that, but a purchasing agent of yours trying to find a substitution in this messed up supply chain world may not.

Make sure you double check and approve all component substitutions, even if if may be obvious to you. It may not be obvious to everyone.

Duane Benson
Help stamp out and eliminate repetitive redundancy.

Now, quickly, jump over to screamingcircuits.com and see what on-demand electronics manufacturing can do for your project.


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