Why when this Transistor appears to be reversed does this circuit work? - Page 1
Forum section: electronics / beginners

author: Nick0268
author_rank: Contributor
timestamp: May 22, 2025, 03:26:25 pm
content: In the attached diagram which is based on a Tinkercad simulation, why is the collector of T1 connected to the negative rail?I have read up on the operation of a 555 as a monostable and see that.A negative trigger at pin 2 (TR)Turns off an internal transistor to bring 7 (DIS) up from ground.The output goes highC3 then charges via R4 to 2/3 VccThen C3 discharges to groundThen the output goes low.I'm just not sure how the T1 produces the negative pulse needed.I though NPNs had the emitter on the more negative side so that base going positive would turn it on and current would flow from the collector to emitter (conventional current flow). Diagrams attached...

author: RoGeorge
author_rank: Super Contributor
timestamp: May 22, 2025, 03:41:41 pm
content: A transistor with CE reversed still works, though it will work as a transistor with a very small Beta than it really has when connected properly. There are a few other properties that will change when the CE are swapped, but I think none of those properties are relevant for the given schematic.Use the NPN transistor properly, with the emitter to GND.

author: Nick0268
author_rank: Contributor
timestamp: May 22, 2025, 03:56:10 pm
content: The link goes to the Tinkercad simulation. However, I have substituted a switch for the PIR. The switch brings the base of T1 high when pressed.When the simulation starts, the LED flashes for the time set by the monostable 555 U2, at the rate set by the astable 555 U1.T1 is again "The wrong way round", I think, but I'm prepared to be put right.https://www.tinkercad.com/things/eBikJv6AGPG-monostable-astable-555-flashers?sharecode=_nxWQ7XVVDar6qCGhem8BvatMChOqSwFR6Hx8J7eDiI

author: Nick0268
author_rank: Contributor
timestamp: May 22, 2025, 04:05:09 pm
content: Thanks for that.It was a design not of my own, but I have modified it as you suggest, so that the NPN is the correct way round, and it works fine, as a simulation in Tinkercad.I will ask the original author what their though process was.

author: PGPG
author_rank: Frequent Contributor
timestamp: May 22, 2025, 05:39:01 pm
content: Quote from: Nick0268 on May 22, 2025, 04:05:09 pmAs there is no pull-up resistor than may be cut-off current was taken into account.I don't know does a smaller cut-off current come with a smaller beta?

author: pcprogrammer
author_rank: Super Contributor
timestamp: May 22, 2025, 05:50:19 pm
content: I'm no expert, but does it not just work as a diode in this setup?To make it work with the transistor in it's "normal" orientation a pull up to VCC is needed on the collector, but it will then invert the signal.It does seem weird for sure and it depends on the input characteristics of the TR pin on the 555 if it will work. For this one should look at the datasheet.

author: Nick0268
author_rank: Contributor
timestamp: May 22, 2025, 08:18:38 pm
content: So I have inquired of the original author's thought process.Turns out he was in a dash, placed the Transistor components into the Tinkercad design, didn't notice that they were the wrong way round as the simulation worked!

author: David Hess
author_rank: Super Contributor
timestamp: May 22, 2025, 08:56:29 pm
content: This circuit is a good example of why Texas Instruments datasheets are horrible. (1)The original Signetics datasheet includes the equivalent circuit which shows that the trigger input at pin 2 drives the base of a Darlington PNP differential pair, resulting in a positive input bias current which will pull the input to Vcc with no load. So it works without a pull-up resistor, but it is not something I would normally rely on.The reversed NPN transistor will work fine in this case, albeit with low current gain, as long as the voltage at the reverse biased emitter is below its breakdown voltage of slightly more than 5 volts. If the supply voltage was 15 volts, then there would be problems. Bipolar transistors are sometimes deliberately used with their collector and emitter reversed for extra low saturation voltage, like when resetting the capacitor in a 555 circuit.(1) The Texas Instruments datasheet indicates that the input bias current is positive, however there is no consistency in their other datasheets about positive being from the positive supply. The only way to tell, at least for Texas Instruments, is to see the equivalent circuit.

author: golden_labels
author_rank: Super Contributor
timestamp: May 23, 2025, 08:13:31 am
content: RoGeorge and David Hess confirmed it may work and explained the limitations. Let me amend this with the “why.”If you look at a simplifieddiagram of a bipolar junction transistor, this is nothing more than P-type semiconductor sandwiched between two N-type semiconductors. This ideal diagram is perfectly symmetrical. Same forMOSFET diagram. This is not how they’re actually manufactured, but conceptually this explains how they operate. Obviously, the diagram can be flipped and the story stays the same. And there really is a plain P-N junction, which — as pcprogrammer mentioned — is sometimes used in place of an actual diode. This is also why in almost all discrete MOSFETs we see a diode across gate-source. Since the body can’t be left floating, it’s connected to one end. Which inevitably turns one of the P-N junctions into a diode.A transistor built as drawn in that simplified schematic would have pretty poor characteristics. The manufacturing process available also limits what we can do, and what we can do is removing or depositing ultra-thin layers, making such nice sandwiched construction impossible. To improve characteristics and to accommodate manufacturing limitations, we make things assymetrical. And this is why in practice we use transistors as polarised devices. They’re designed to excel, but only in one direction.In a sense you can think of it ashandednessin humans. In principle both hands are equal and there is no physical limitation, that would prevent using either. But the brain optimizes resources, making humans directional.

author: PGPG
author_rank: Frequent Contributor
timestamp: May 23, 2025, 10:07:59 am
content: Quote from: pcprogrammer on May 22, 2025, 05:50:19 pmNo.npn reversed is still npn.Just both junctions are used opposite way. B-C works like B-E and B-E works like B-C.If I am not mistaken, the oldest transistors behaved very similarly in both situations. Due to technological differences in the implementation of both junctions, modern transistors used in reverse have a much lower beta, but still work like a transistor.The only purposeful use of a transistor connected in reverse that I know of is to obtain a C-E (E-C) voltage close to 0 (single mV) in saturation (with a base current greater than the output current). This cannot be achieved in a normal connection.Before the era of MOSFETs, the reverse connection of bipolar transistors was used to obtain a minimum voltage drop on a transistor acting as an analog switch.

author: David Hess
author_rank: Super Contributor
timestamp: May 23, 2025, 10:35:26 am
content: Quote from: golden_labels on May 23, 2025, 08:13:31 amThey used to make "chopper" transistors which were symmetrical, and they are still available:https://www.mouser.com/ProductDetail/Central-Semiconductor/CMPT404A-TR-PBFREE?qs=u16ybLDytRYUXlomUEKF%2FQ%3D%3Dhttps://www.mouser.com/ProductDetail/Toshiba/2SC3326-ALF?qs=EPmvyOv1YlNngpLn%252Ba3SPw%3D%3DQuoteChopper transistors were actually pretty good with reasonable gain in both directions. Their low saturation voltage and high base-emitter breakdown voltage make them useful in specific applications like differential pairs where high differential voltage must be tolerated.

author: RoGeorge
author_rank: Super Contributor
timestamp: May 23, 2025, 10:37:27 am
content: Another fun trick to do with a swapped CE BJT (unrelated with the 555 schematic) is to exploit the negative resistance to be seen in (some?) reversed BJT, like this:https://www.eevblog.com/forum/testgear/fooln-around-with-dso-awg/msg4422982/#msg4422982and to turn that into an oscillator by adding a parallel capacitor:(pics not mine, stolen frommawyatt's topichttps://www.eevblog.com/forum/testgear/fooln-around-with-dso-awg/)

author: David Hess
author_rank: Super Contributor
timestamp: May 23, 2025, 10:42:48 am
content: Forward gain transistors can display the same thing, but at higher voltages, which makes then suitable for avalanche operation.https://ibm-1401.info/LowAvalancheBreakdownHypothesis.html

author: Doctorandus_P
author_rank: Super Contributor
timestamp: May 23, 2025, 03:40:18 pm
content: Quote from: Nick0268 on May 22, 2025, 08:18:38 pmngSpice is integrated in KiCad, and it works. I'm stil a noob with spice, but I have done some nice simulations with it, and see it's benefits. However, ngSpice in KiCad does not come with built in libraries which connect schematic symbols to spice models. In KiCad it's quite mandatory for pin numbers to match with the footprints on the PCB. For spice however, there is often a pin number mismatch with the real world. KiCad does have a pin renumbering scheme built in, so you can both keep the real pin numbers in the schematic, and map those to the pin numbering in the spice models. But because the libraries are not integrated, you have to do this mapping yourself. So overall, it's quite possible that the Collector and the Emitter are reversed in the spice model. (I.e. KiCad also has BJT's with all 6 possible pin mappings built in the "Device" library)