Transistor pin identification: My first encounter with reverse active mode!? - Page 1
Forum section: electronics / beginners

author: ecclectrics
author_rank: Contributor
timestamp: December 14, 2024, 01:25:52 pm
content: I rigged up a supersimple circuit for go/nogo transistor (BJT) test (see image). Thought it could also be used to identify which pin is which for an unknown tranny without datasheet. Then something unexpected happened.Here is what I tried:1) Connecting any combination of two pins between 1 and 2 gives- for two combinations conduction with one polarity and no conduction with the other polarity;these are BC and BE- for one combination there is no conduction with both polarities;this must be CE2) Now knowing which is the base, I connected it to 3 and the others between 1 and 2.Expected to get conduction with one and only one polarity if C is at 2 and E at 1and no conduction with either polarity if E is at 2 and C at 1.But no: there is conduction with one and only one polarity for both ways to connect the other pins.Questions:1) Does the transistor operate in reverse active mode in the polarity that conducts with E at 2 and C at 1?2) Is there a way to distinguish forward and reverse active mode in this circuit or a simple modification of the circuit that would allow this distinction so I could decide which is C and which is E?3) Can the transistor suffer (degradation or destruction) by any connection in the circuit?Appreciate any help!Regards,Roland

author: magic
author_rank: Super Contributor
timestamp: December 14, 2024, 01:56:27 pm
content: Yep, you can swap C with E and it still works, but β is usually much lower. You should see it if you increase the base resistor - at some point the LED will become dimmer in reverse operation.

author: David Hess
author_rank: Super Contributor
timestamp: December 14, 2024, 02:48:02 pm
content: Quote from: ecclectrics on December 14, 2024, 01:25:52 pmYes, it does. Reverse operation has much lower current gain, but also has a much lower saturation voltage so it is sometimes used. Before JFETs and MOSFETs became available, they made "chopper" bipolar transistors which were symmetrical and had the same current gain in both directions.QuoteI know of at least 3 ways:1. Reverse current gain is much lower than forward current gain.2. The forward voltage drop of the collector junction is slightly lower than the forward voltage drop of the emitter junction.3. The reverse breakdown voltage of the collector junction is much higher than the reverse breakdown voltage of the emitter junction.QuoteReverse breakdown of the emitter junction will degrade current gain at low currents, especially for small signal devices. This will ruin circuits like low noise, high impedance, and precision amplifiers. This will not matter for power circuits where current gain is already falling.

author: ecclectrics
author_rank: Contributor
timestamp: December 15, 2024, 10:46:31 am
content: Thanks for the comments!Indeed if I look carefully, I can already see with the 10k resistor that the LEDs shine a bit less bright in the reverse mode configuration (tried with tranny of known pinout).So this is the way to identify pins using the proposed circuit;)Concerning base-emitter breakdown voltage: Is this a datasheet parameter? Or do I have to live with the general estimates of 5-7V? In any case, if I limit the driving voltage to 4.5Vas indicated in the figure, I should be on the safe side, shouldn't I?Best,Roland

author: rhb
author_rank: Super Contributor
timestamp: December 15, 2024, 12:33:52 pm
content: “The larger area of the collector results in a lower BC forward resistance than BE despite the higher doping of the emitter.”Until the cheap transistor testers appeared, that was how I verified leads with a DMM.Interest in the problem began with a package of 2N2222s from Radio Shack. I asked everyone how to tell the leads apart. No one knew. Nothing I built with them worked when I followed the package markings. Everyone knew how to identify the base, but that was all.As a break from looking through a petrographic microscope 8 hours a day, I was sitting in on an intro EE class taught by a semiconductor physicist. When he said that I ran home and tried it. All the things that didn’t work had C & E reversed and worked as expected when rewired.In the 40+ years since, I’ve asked that question many times. Only ONE person knew the answer.BTW. The physicist had built a computer from TTL logic just before the 8080 came out and made it instantly obsolete. Quite a guy.Have Fun!Reg

author: magic
author_rank: Super Contributor
timestamp: December 15, 2024, 12:36:12 pm
content: Yes, it's in datasheets.4.5V should be safe for practically all general purpose and power transistors, but close to the limit (many are 5V).May be unsafe for some RF transistors.

author: mawyatt
author_rank: Super Contributor
timestamp: December 15, 2024, 03:30:58 pm
content: Here's something the OP might find useful wrt to general transistor operation (curve tracing) and while in Vbe breakdown and inverted.https://www.eevblog.com/forum/testgear/fooln-around-with-dso-awg/Note reply #5. Also #14 and #15 where the reverse Vbe creates a negative resistance and can be utilized as a 2 terminal relaxation oscillator.Best