Reminder for those new to electronics... and old that should know better ;-) - Page 1
Forum section: electronics / beginners

author: HalFoster
author_rank: Frequent Contributor
timestamp: December 06, 2023, 10:20:16 am
content: Always, always double check the footprint of transistors - don't assume that it is "EBC" or whatever, check it again! I just designed a fairly complex board and used quite a few 2N3904/2N3906s for jellybean transistors (in TO92 package.) Later in the design i changed them to BC546/BC556s - and never even thought about the footprint reversing. Sigh. At least in this case it is just a matter of installing them reversed but still... I should have double checked.HalPS: Relearning a lesson for the way-too-manyeth time.PSS: Yes, manyeth *is* a word. I used it in a sentence, didn't I.

author: magic
author_rank: Super Contributor
timestamp: December 06, 2023, 11:53:43 am
content: And P2N2222 is not PN2222

author: Terry Bites
author_rank: Super Contributor
timestamp: December 06, 2023, 01:04:09 pm
content: Most euro transistors with BCE pinout have a close CBE equivalent. Avoid owning the former!

author: SeanB
author_rank: Super Contributor
timestamp: December 06, 2023, 02:17:26 pm
content: Plus also remember there are a few where the suffix letter is important, as they have 2 variants, and that suffix also shows the gain group, plus the pinout. And also remember, datasheets can lie, especially ones from companies that got borged into others, where they took over lines, and did changes that were not noted in a datasheet other than in small print revision.

author: HalFoster
author_rank: Frequent Contributor
timestamp: December 06, 2023, 02:19:58 pm
content: The worst - or at least most embarrassing - part is that the parts drawer I got them out of had a little sticker with the pinout on it (I do that to all my transistors) but it never even registered for me.Hal

author: HalFoster
author_rank: Frequent Contributor
timestamp: December 06, 2023, 02:25:35 pm
content: And Lord save us from process changes. Most of what I do is restore old test equipment and it's for sure that yesterdays 2N3904 is **not** todays version. And even worse, many of the older parts were selected versions for the application so even matching an old datasheet doesn't necessarily keep it from being a roll of the dice. Kinda like replacing vacuum tubes - change 'em out until it works.

author: ArdWar
author_rank: Frequent Contributor
timestamp: December 06, 2023, 02:54:58 pm
content: Haha, 79xx have different "pinout" than 78xx. That bites me in the ass numerous times. I mean, if you think with relative voltage it kinda the same but that's not what most people expect.Also 78Lxx have "reversed" pinout, now that's just silly.

author: madires
author_rank: Super Contributor
timestamp: December 06, 2023, 03:01:57 pm
content: ... or get or build a Transistortester which will tell you the pinout.

author: HalFoster
author_rank: Frequent Contributor
timestamp: December 06, 2023, 03:21:46 pm
content: Quote from: madires on December 06, 2023, 03:01:57 pmThat's really not a bad idea - just a spot check before building could (would have) save a lot of effort. Mouser and Digikey certainly can make mistakes on the parts sent and, as was mentioned, a minor variation in the part number (or even just the manufacturer) can reflect a different pinout. I have several of those little testers rattling about - I might as well put them to use and potentially save myself some hassle.

author: magic
author_rank: Super Contributor
timestamp: December 06, 2023, 03:31:12 pm
content: Quote from: ArdWar on December 06, 2023, 02:54:58 pmIt sort of makes sense if you realize that the plastic blob which contains the die is on the front side of TO-220, but on the back side of TO-92.Of course they could have designed the 78L die to be mirrored with respect to 78, and they did just that with 79L. This is what really makes no sense.

author: shapirus
author_rank: Super Contributor
timestamp: December 06, 2023, 03:39:01 pm
content: Quote from: HalFoster on December 06, 2023, 03:21:46 pmThat idea is not only "not bad", it's actually an excellent idea and a very good advice. I can't even estimate how much time this little dirt cheap device saved me. It's not for precise measurements (even though it's actually pretty good within I'd say a percent or two), but it's invaluable for quickly checking if the part values are right, if the pinout is right (or to find what the pinout is), and if the part is dead. I do it for breadboard prototyping and for sanity checking before soldering. It's much faster and more convenient, in most cases, than using a multimeter. Less so for SMD parts, but still fine if you make some suitable spring-loaded hook probes.