Confused by DMM Diode Test for Darlington Transistor BC517 - Page 1
Forum section: electronics / beginners

author: RonFred2
author_rank: Contributor
timestamp: March 07, 2021, 08:13:56 am
content: I am testing different transistors with my DMM in Diode mode with good results until I came to the BC517 NPN Darlington silicon transistor.My test using DMM in diode mode:I test Vbe (positive lead to base, negative lead to emitter) and get a reading of .7 VDC.I test Vcb (positive lead to base, negative lead to collector) and get a reading of 1.3 VDC.Different BC517's from the same purchased batch supplied by Hilitchi (Amazon) and different DMM's show similar results.My DCA Pro Transistor Tester shows the Vbe to be 1.45 VDC so I am suspecting DMM Diode Test behavior is creating this issue, but don't fully understand why if this is indeed the problem.I was expecting the Vbe voltage reading to be similar to Vcb at 1.3 VDC. Any idea what I am doing or interpreting wrong?

author: Simon
author_rank: Unknown
timestamp: March 07, 2021, 09:13:48 am
content: Are you sure you are following the correct pinout? Datasheets I think usually show the pinout from the bottom and different manufacturers will use different pinouts so make sure you know who made them and get their datasheet up.

author: David Hess
author_rank: Super Contributor
timestamp: March 07, 2021, 10:21:42 am
content: Integrated Darlington transistors almost always have a low value of resistance in parallel with the base-emitter junction of the output transistor to remove charge for faster turn-off. So a low current diode test will will see essentially zero volts across the base-emitter junction of the output transistor.

author: Simon
author_rank: Unknown
timestamp: March 07, 2021, 10:50:32 am
content: I don't think this rather small one does:https://docs.rs-online.com/9d4e/0900766b816f7472.pdfhttps://www.mouser.com/datasheet/2/149/BC517-888613.pdfOn semi and Fairchild seem to be the main manufacturers and at least they both use the same pinout.

author: RonFred2
author_rank: Contributor
timestamp: March 08, 2021, 03:22:42 am
content: Simon you are right.I was relying on the pin-outs for my BC517 transistor as I learn details about my DMM Diode test function. Even though all datasheets I could find show the same pin-out as the two links from Simon above - never the less - the pin-out for my Fairchild manufactured BC517 transistor has the E and C pins reversed! As Simon pointed out, some manufactures make different pin-outs for the same part number. So I should not rely solely on an Internet datasheet (or even a bunch of datasheets).I see the "F" logo on the transistor case for Fairchild. But the Fairchild datasheet shows the same pin-out as every other BC517 datasheet I can find. As background, I bought 10 different transistor types @25 each in an assortment kit bundled by Hilitchi with each part labeled:https://www.amazon.com/gp/product/B06Y4M2N29Apparently even a known manufacturer can make reverse pinouts perhaps purchased in bulk on some grey market at a discount by Hiltchi.But using my DCA Pro Transistor Tester by Peak and with Simon's encouragement, I did get the true pin-outs and they are indeed reversed to the datasheets I have seen. So my DMM readings make sense now, the BE junction does have the higher voltage drop because of the two emitter junctions in series as expected, compared to the BC junction. As my confidence builds, I could have known the E and C pins were reversed just by my DMM readings alone. To help others I am putting a blog post together with my DMM trials and tribulations in testing transistors using DMM (with confirmations from by DCA Pro curve tester).See attached photo - the DCA Pro test results show the C and E pins in reverse order compared to all the BC517 datasheets I have seen.

author: Simon
author_rank: Unknown
timestamp: March 08, 2021, 06:28:30 am
content: Are you sure you didn't just get confused with which way round they go. Stuff like that drives me mad, between the possibility of getting up/down and front/back mixed up there are more wrong ways to look at it that the one right one.

author: RonFred2
author_rank: Contributor
timestamp: March 10, 2021, 04:00:37 am
content: Occam's Razor would definitely suggest that I simply see the wrong pins for my BC517 part.I decided to do my best to confirm that my first BC517 part really does have its E and C pins in reverse of every datasheet I can find online. For this effort I ordered another transistor assortment kit from a different vendor on Amazon that included a BC517. I then put both parts onto my Atlas DCA Pro tester using the same color probes for each of the two parts.Now I have confirmed that my original BC517 part does have its E and C pins reversed - a "Goofy Footed" surfer in the industry.I am attaching two photos: 1 for each of the parts showing the reversal of pins. Both parts have the BC517 and an "F" logo for Fairchild but the other manufacturer markings are different. (Click the thumbnails to expand the images)

author: retiredfeline
author_rank: Frequent Contributor
timestamp: March 10, 2021, 04:24:31 am
content: Maybe it's worth money like a flawed stamp?

author: Simon
author_rank: Unknown
timestamp: March 10, 2021, 07:57:15 am
content: where did you get the original part? Places like amazon and ebay will be full of sellers selling stuff that may not necessarily be genuine. For example it was common to brand any TO3 package parts 2N3055 (I think) given their popularity but often they were just swept up off the floor.

author: RonFred2
author_rank: Contributor
timestamp: March 10, 2021, 10:26:48 am
content: Goofy Footed BC517 Part:https://www.amazon.com/gp/product/B06Y4M2N29Normal testing BC517 Part:https://www.amazon.com/gp/product/B08KR961SWFor a transistor testing article I suppose its cool to find some fake or "one-off" parts that require just the kind of testing I discuss before being used in a circuit.

author: richard.cs
author_rank: Super Contributor
timestamp: March 10, 2021, 12:09:40 pm
content: Quote from: Simon on March 10, 2021, 07:57:15 amGiven the ridiculously wide specs of the 2N3055, this is mostly legitimate. Almost any silicon NPN power transistor in that package will meet them.

author: Simon
author_rank: Unknown
timestamp: March 10, 2021, 12:41:41 pm
content: Quote from: richard.cs on March 10, 2021, 12:09:40 pmNot quite, there were reports and warnings that less than parts were being branded.

author: rsjsouza
author_rank: Super Contributor
timestamp: March 10, 2021, 01:29:27 pm
content: Quote from: richard.cs on March 10, 2021, 12:09:40 pmThis is a problem since the 1980s - same with TIP31 and BD139...I have seen "2N3055" fakes blow up when they were subjected to currents marginally higher than 2A - this was back in the day, where lower power Silicon or even some Germanium TO-3 transistors were still widely common in the back of the warehouses and being reprinted.

author: richard.cs
author_rank: Super Contributor
timestamp: March 10, 2021, 01:53:43 pm
content: Also common is for modern, genuine 2N3055s to oscillate or otherwise misbehave in a circuit designed around the 1970s version because the specification only sets lower limits on gain and Ft.

author: David Hess
author_rank: Super Contributor
timestamp: March 10, 2021, 05:37:52 pm
content: Quote from: richard.cs on March 10, 2021, 01:53:43 pmEarly in its life, the 2N3055 was caught up in some major process changes. If you want a better specified part, then you use a different part like the 2N3772 or 2N3773 which are good substitutes for old 2N3055s or a more specific variant like the 2N3055AG.Tektronix qualified 2N3055s from different manufacturers and had at least 4 versions, with some instruments using 3 in different places. They can be roughly divided into 0.3 MHz (mesa process?), 0.8 MHz, and "modern" (triple diffused?) 2N3055s which are at least 2 MHz.

author: 33to78
author_rank: Newbie
timestamp: March 16, 2023, 05:31:34 pm
content: Back to the original topic: Even if it is 2 years old, I stumbled on the same problem. I purchased a virtually identical transistor set from the 'DollaTek' brand via Amazon. Since it is from China I considered it sensible to check all the transistors. Interestingly, all of them seem ok apart from the BC517s that also have the E and C terminals swapped. So this is another confirmation that goofy things can happen. These BC517s also have, as RonFred2 states, a small 'F.' next to the label - which might mean something different from Fairchild.BTW when checking the BC517 data sheets I found online, the only one showing an abnormal pinout (that matches the one of my BC517s) is the one by Siemens dated 1991. 'German Gruendlichkeit' anyone?CheersRobert

author: Simon
author_rank: Unknown
timestamp: March 16, 2023, 08:10:09 pm
content: make sure you find the correct manufacturers datasheet. I have seen 2n7000 parts with different pinouts according to which manufacturer.