Bjt PNP transistor disguised as a NPN? - Page 1
Forum section: electronics / general

author: ManlishPotato
author_rank: Regular Contributor
timestamp: September 09, 2020, 01:14:59 pm
content: I was troubleshooting a perfboard circuit today and i was measuring a bjt transtor to see if it was broken. As i understand it when you measure bjt transistors they are basically like two diodes in series with a common anode or cathode.w2aew explains it here:https://youtu.be/qM7_h8my8eAAnyway the transistor was a BC550 which is supposed to be a NPN transistor, but when i measured it, it was like a PNP transistor, meaning that current flows from collector and emitter to base, not the other way around. This was of course what was causing the error in my circuit. Did i miss something in the datasheet? Is there some kind of error with the transistor? I tested other not ever used BC550 transistors and they gave the same result.

author: madires
author_rank: Super Contributor
timestamp: September 09, 2020, 01:32:47 pm
content: If your check is correct that BJT is apparently a PNP. Where did you get the BC550s from?

author: ManlishPotato
author_rank: Regular Contributor
timestamp: September 09, 2020, 01:38:08 pm
content: I'm sure i measured it correctly. The transistors are from Kjell och co here in Sweden. But i believe they were distributed to them, by luxorparts.I included i picture of it.

author: bdunham7
author_rank: Super Contributor
timestamp: September 09, 2020, 01:44:42 pm
content: Quote from: ManlishPotato on September 09, 2020, 01:38:08 pmExactly how--and with what---did you test this device?

author: ManlishPotato
author_rank: Regular Contributor
timestamp: September 09, 2020, 03:11:50 pm
content: I used an dmm on diode test mode and measured the forward voltage of the transistor. I first measured positive on base and negative on collector, which gave OL. I then measured positive on base and negative on emitter which also gave OL. I then did the same but with negative to base, and that both gave reading of around 0.5v. I tested this several times on different BC550s.

author: bdunham7
author_rank: Super Contributor
timestamp: September 09, 2020, 04:16:00 pm
content: Make and model of the DMM?Whenever you run into these crazy conflicts, you have to stop and reexamine everything, including your own assumptions. It seems that you are testing everything correctly, but check these:1. DMM polarity. Some DMMs have reverse polarity for resistance, but I've not heard of one that is reversed for diode checks--but tell us what you have. And in case gremlins have rewired it, do you have another DMM that you can use to absolutely verify that the diode check polarity is correct?2. Another component. Do you have any other semiconductors you can quickly check to verify that your measuring system performs as expected?3. The datasheet. Make sure you have the correct pinout. The base is the center terminal from what I can see, so this is probably a non-issue.I have one of those cheap-o multi-component testers that I often use just for a final check before I install parts in repair projects, just to avoid silly problems like you are having. Absent some pretty unlikely scenarios like the ones I just listed, it looks like your transistors are mismarked. Or, they might not be BJT transistors at all.https://www.ebay.com/itm/Green-12864-Mega328-ESR-Transistor-Resistor-Diode-Capacitor-Mosfet-Tester-w-hook/322498217999?hash=item4b1664480f:g:c8EAAOSwCQZZA8gO

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 09, 2020, 04:23:09 pm
content: Measure anything C-E or E-C? (Try resistance too.)I don't know how mainstream that outlet is; I guess from the sound of it, they're like a Swedish RadioShack / Tandy? It could still happen that mislabeled or counterfeit parts slip into mainstream distribution channels; uncommon, but not at all unheard of.Is it just the one, or did you get others in the pack that measure wrong too, or have you purchased the same part in the past that functioned normally?Tim

author: ManlishPotato
author_rank: Regular Contributor
timestamp: September 09, 2020, 04:30:12 pm
content: Will try what you guys suggested later tonight or worst case tomorrow morning. Anyway I bet it's a mislabeled PNP doesn't sound like its impossible.

author: LA7SJA
author_rank: Regular Contributor
timestamp: September 09, 2020, 07:00:23 pm
content: What article number from Kjell & Co is this?

author: exe
author_rank: Unknown
timestamp: September 09, 2020, 08:16:20 pm
content: May be I'm spoiled by transistors from "big brands", but the one you show looks a bit dodgy. There is no manufacturing logo.I'm a bit reserved buying jelly-bean components from lesser known brands. For my use-cases saving is insignificant. It doesn't mean I don't buy dirt-cheap components from unknown sources (including used parts from aliexpress), but I double-check them

author: ManlishPotato
author_rank: Regular Contributor
timestamp: September 09, 2020, 08:36:43 pm
content: Quote from: T3sl4co1l on September 09, 2020, 04:23:09 pmI measured resistance on everything as well and it gave the same result, very high resistance on one side, and OL on the other. I Measured C-E and E-C and that gave OL on both side like it's supposed to.I suppose you could say that Kjell och Company is kinda like Radio Shack, but they focus more on mainstream products, like hdmi cables and Iphone chargers. As far as i know we actually don't have a chain of physical stores that go a bit more deep into electronics in Sweden. I found a good webbshop for that, but it would be way more awesome if you could go into a physical store to pick up stuff for your next project!I measured other BC550s that i had and they gave the same result.

author: ManlishPotato
author_rank: Regular Contributor
timestamp: September 09, 2020, 08:38:45 pm
content: Quote from: LA7SJA on September 09, 2020, 07:00:23 pmI can't seem to find it, but it was a set of like 6 different bjt transistor types with like 50 of each.

author: ManlishPotato
author_rank: Regular Contributor
timestamp: September 09, 2020, 08:51:30 pm
content: Quote from: exe on September 09, 2020, 08:16:20 pmManufacturing logo?? I have never seen a company logo on a bjt transistor, is that a thing? Isn't kind of iffy who owns these old transistors anyway, now it's ON semi i think, but it used to be something else. Like is these old transistors even manufactured even more? I always thought they were old stock. To clarify what i'm talking about are old bjt transistor like the one in the picture.

author: ManlishPotato
author_rank: Regular Contributor
timestamp: September 09, 2020, 08:53:01 pm
content: I will answer bdunham7's question tomorrow, gettin' late gotta get some sleep!

author: Sredni
author_rank: Frequent Contributor
timestamp: September 10, 2020, 01:04:58 am
content: I remember reading a review on Amazon about those boxes of mixed transistors, and one of the customers said that he had found all BJT of a certain type to be PNP instead of NPN. And if I am not mistaken it was a BC550 or similar.EDIT: checked and yes, it's the BC550 in those 200 transistor kits.

author: ManlishPotato
author_rank: Regular Contributor
timestamp: September 10, 2020, 08:50:31 am
content: Quote from: bdunham7 on September 09, 2020, 04:16:00 pmIt's a UNI-T UT33A auto-ranging multimeter.I know that the polarity is right because i have run the same diode test on other trusted transistors and that gave the right results.I have tested other transistors like i said, they gave the result you'd expect. It has only been the BC550s that have been giving the opposite results.Yes like you said the pinout is no doubt correct, although i have never seen a bjt transistor that isn't center base. Here is the datasheet that i used:https://www.mouser.com/datasheet/2/149/BC550-888526.pdfI have a component checker that works other than it's screen is broken, so you can't se it's lower half. But i tried it anyway and the BC550 came out as a PNP actually.With that i am 99% sure that it's a mislabeled PNP transistor, pretty crazy actually. Like how do they even mess that up? This isn't lika a deformed potato chip or something, this is a precision sandwich of silicon made with a lot of quality testing. The transistor is of course not manufactured by Kjell och company or Luxorparts or anything, this thing has probably been manufactured in some far of land, and been in storage for a long time.

author: madires
author_rank: Super Contributor
timestamp: September 10, 2020, 11:31:42 am
content: Quote from: ManlishPotato on September 10, 2020, 08:50:31 amThat should be a ST7565 based display.

author: ManlishPotato
author_rank: Regular Contributor
timestamp: September 10, 2020, 01:30:26 pm
content: Oh i guess if i could find a good replacement part i might be able to fix it then huh? Haven't had any good luck importing stuff from china to Sweden though, took like 1-2 months last time, and i had to pay a lot in customs too.

author: bdunham7
author_rank: Super Contributor
timestamp: September 10, 2020, 01:33:31 pm
content: Quote from: ManlishPotato on September 10, 2020, 08:50:31 amThey're out there. Always check!QuoteThat's the exact tester I use. It's 100% accurate for common low-voltage parts, at least in my experience.QuoteNo, you are 100% sure. There's not much left to doubt! It really does make you question the supply chain in this case. If they can't get the polarity right, how do you know that any of the other parameters are correct on the other units and that the whole kit is not just mismarked miscellaneous garbage? If one of my suppliers (not eBay) sent me that product I'd be demanding some explanations.

author: ManlishPotato
author_rank: Regular Contributor
timestamp: September 10, 2020, 02:55:08 pm
content: Well i mean in this case there's no doubt that it was miss marked from the factory, meaning that they were delivered faulty as BC550s. Kjell och Company probably being the like 20 hand in the supply chain, and normally doing mainstream stuff like i said, it's hard for them to know that a BC560 was mislabeled as a BC550. But hey, that's what those component tester are for ey?