Beware the TO-220

-September 06, 2017

I needed some LM317T regulators for my new batch of powered-speakers, but got a bit less than I bargained for. It seems the TO-220 package isn’t necessarily what it used to be.

I’d ordered a bunch of Fairchild…uh, I mean, ON Semi, LM317s, and for starters, you don’t know what the hell you’re gonna get when a company is in transition like this and both companies involved make the same part! This was of some concern, as I wanted the higher-voltage version, and IIRC, Fairchild’s and Motorola’s…uh, I mean, ON Semi’s, have different specs.

But what I really noticed upon inspecting the parts was that the tab was much thinner than usual. Well…I never.

The thought of counterfeit parts even crossed my mind.


LM317 (top, U1) and LM337 (bottom, U2)


Heading to the Fairchild…uh, I mean, ON Semi, datasheet, reveals the cryptic terms Single Gauge and Dual Gauge in the Ordering Information block – for the LM317T at least. The high-voltage LM317AHVT only comes in single gauge.



Heading to the package drawing reveals the meaning: Dual Gauge is the standard TO-220 package, with a 50 mil (1.27 mm) tab thickness. The Single Gauge package’s tab is only 22 mils (0.56 mm) typical. Yikes. And to make matters worse, LM317T now refers to this thinner-tab package. If you want the classic tab, order a KA317TU. What the…‽


Has anyone notified JEDEC?

True, there won’t be many situations where tab thickness matters, and no electrical or thermal differences between the packages are listed in the datasheet. Still, this is a disconcerting development in component packaging. Caveat emptor.

And, gimme back my TO-220.



Michael Dunn is Editor in Chief at EDN with several decades of electronic design experience in various areas.

Follow Michael Dunn on and EDN on Instagram




Loading comments...

Write a Comment

To comment please Log In