Transistor swapping Collector and Emitter  - Page 1
Forum section: electronics / projects

author: Yamin
author_rank: Frequent Contributor
timestamp: January 27, 2025, 05:06:00 pm
content: Here is a little back story - Last year in college, we were working on simple transistor-switching circuits with an LED as the load on the collector. While testing, we encountered a transistor that initially tested fine with a multimeter but behaved oddly in the circuit. Specifically, the LED remained on even when the base was grounded. After close inspection, I discovered the issue: the collector and emitter leads were swapped inside the transistor case. (Photo attached showing the components used on the trainer board.)This year, armed with the knowledge that the B-C junction and B-E junction can be differentiated by their voltage drop using the diode mode on a multimeter (the B-C junction typically has a slightly lower voltage drop than the B-E junction), I decided to test two other transistors randomly taken from the trainer boards. To my surprise, both were wired incorrectly. I confirmed this by cross-referencing the datasheets of the S8060 and BC547 transistors.I decided to build a simple switching circuit with an LED on the collector to test further. It worked perfectly. Out of curiosity, I swapped the leads to the correct configuration, and it still worked fine. I even tested the transistor from last year, and it functioned correctly as well—I couldn’t recreate the problem we initially faced. In hindsight, the issue may have been due to a loose connection, leaving the base floating.It was quite surprising to realize that the transistors still worked even when the collector and emitter terminals were swapped. From the research I’ve done, this configuration puts the transistor in reverse active mode. As I understand it, reverse active mode is not efficient—the current gain is significantly lower, but the VCE(sat) is also relatively low.I conducted an experiment and noted the results. While writing this, I realized that I didn’t move the ammeter’s position during the test. So, when the leads were swapped, I was actually measuring the "emitter" current instead of the collector current—but I’m guessing the difference is negligible. Based on the results, I couldn’t determine if the current gain was lower as expected; the currents remained almost the same. Should I have used different resistor values? The only noticeable difference was that the VCE was lower in the correct orientation, which contradicts what I’ve read online.I was hoping that someone could explain/confirm the following.1. Why does transistor work when collector and emitter leads are swapped?2.Is it safe to assume that swapping the leads put the transistor in reverse active mode?3.From the experiment I did - can it be deduced that there is no significant loss in current gain? Beta = Ic/Ib4.Is there any advantage of reverse active mode?5.Would the B-C junction in a pnp transistor also show a lower voltage drop when measured in diode mode.6.Any more interesting experiments to do on this topic?Thanks in advance.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: January 27, 2025, 07:01:47 pm
content: Quote from: Yamin on January 27, 2025, 05:06:00 pmNo, you have tested in saturated mode, which does not test HFE. Check it in a meter that does read HFE.It is rare for HFE to be similar, forward and reverse, requiring transistors designed specifically. Look for Audio muting transistors.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 27, 2025, 07:17:30 pm
content: The reverse direction usually come with1) lower gain2) lower saturation voltage (with enough base current)3) lower breake down voltage (may be only in the 6 V range, like the negative limit for VBE)4) higher leakage (collector - emitter)How large the difference is depends on the transistor details. There may be special types with relatively little asymmetry between emitter and collector.About the only positive effect from the reverse direction is the lower saturation voltage.

author: mawyatt
author_rank: Super Contributor
timestamp: January 27, 2025, 07:37:09 pm
content: In the distant past (over 1/2 century) folks used certain bipolar transistors "Inverted" that is swapping the Collector and Emitter terminals. This "Inverted or Reversed" State was valuable because the Effective Collector to Emitter Saturation Voltage was much lower than the normal state and often employed as analog switches. Once JFETs and MOSFETs came around these "Inverted" Bipolar Transistor uses were forgotten.As mentioned the "HFE or Beta" in the Inverted state is much lower and the effective Collector to Emitter breakdown also is lower.If you can gain access to a Curve Tracer you can see the effects of Normal and Inverted states, very useful to understand how the DUT is behaving.See this link for more details (#5), also (#14) for how a reversed bias BE junction in breakdown can display "negative resistance" where over a small range thecurrent decreaseswithincreasing voltage!!https://www.eevblog.com/forum/testgear/fooln-around-with-dso-awg/msg4421512/#msg4421512Anyway, fun stuff indeedBest

author: garrettm
author_rank: Frequent Contributor
timestamp: January 28, 2025, 10:41:36 pm
content: Quote from: mawyatt on January 27, 2025, 07:37:09 pmThe common PN2222 show negative resistance when operated this way. I made a little square wave pulse generator and a sine wave oscillator using the effect. The square pulse mode requires two operating points that are discontinuous on the N curve, the sine wave mode requires the operating point to remain on the negative slope. There are a lot of old IEEE papers that go over these effects in intricate detail. Its really quite fascinating stuff. Possibly one of the simplest ways to make an oscillator that I know of is using negative differential resistance. I also believe JFETs can be used too. I had a bunch of photos and characteristic plots I made when playing around with this. Maybe I can find them...

author: Yamin
author_rank: Frequent Contributor
timestamp: January 29, 2025, 07:24:10 am
content: Thanks Guys, for the responses.I repeated the experiment with an hfe meter and it does show that beta is lower in the reverse mode.However if you look at the results from my previous experiment the VCEsat is lower not in the reverse active mode. Why is that?Also is there any reason that could have caused the problem last year because of the transistor being wired wrong mentioned in my first post.Thanks for the help and I'm sharing the setup I had today

author: Sacodepatatas
author_rank: Regular Contributor
timestamp: January 29, 2025, 12:17:09 pm
content: Quote from: Yamin on January 29, 2025, 07:24:10 amBecause while due to the high gain, the transistor in direct mode is hard saturated, once in reverse mode with the same base current, due to the lower gain, you are soft saturating or even though not saturating at all. If the gain in reverse mode decreases, let's say, 10 times, you need to increase the base current 10 fold (at least) to reach the same level of saturation.Quote from: Yamin on January 29, 2025, 07:24:10 amThe led might be on because the transistor connected backwards entered in avalanche. In reverse mode, the Vbr is much lower (about 7 to 12V). Look for information regarding the "Negistor" (that's actually a transistor connected in reverse mode and working also in avalanche mode for creating a negative slope impedance).

author: PGPG
author_rank: Frequent Contributor
timestamp: January 29, 2025, 03:45:07 pm
content: Quote from: Yamin on January 27, 2025, 05:06:00 pmUse Ib being higher than Ic. Check for example ib/ic = 2x, 5x, 10x, 20x and measure Vce in normal and reverse mode.