The Unijunction Transistor

[@frasermderry]

Another well-done video. Thanks, Bill. As an Electrical Engineer trained in the 1970s these videos recall those times. 👍

[@markfiddament9383]

I remember using these to build simple speed controllers for mains powered AC motors - using triad for power switching… I built 3 under the supervision of my father, at the age of about 15…

[@edgeeffect]

I think it must have been about 1982-1983 that I first asked myself "I wonder what unijunction transistors are actually for"... today I find out. :) ;)

[@LockdownElectronics]

Yes, I found out whilst making the video!

[@sebastian19745]

I got two UJT transistors when I was a kid and made my first ever oscillators. Later I found them used in various schematics as oscillators but since they were scarce, hard to find and quite expensive, I often replaced them with a equivalent schematic (BC107+BC177) or simply used another oscillator. I remeber they were used in contolls for thyristors, audio oscillators and even as clock genetrators in some TTL IC circuits.

[@SeanBZA]

Yes used them as a dimmer, and also worked on equipment that used them for dimming lamps, where the common failures would always result in a blown pair of lamps, because the natural thing on lamp failure was to switch in the spare lamp, which would promptly, because they were run on a much higher voltage than rated, and used PWM to dim them, burn out. If you were lucky the first lamp on going out it would blow the fuses, saving the second lamp. Lots of changing of unijunctions that had shifted parameter, and also the LM111 comparator used to do the PWM, and of course the darlington drivers that ran hot enough to unsolder the emitter or base lead.

[@mohinderkaur6671]

Great design of using a spare lamp in case of failure. Like you lose 1 tire, another tire drops down and is also lost right away! Great redundancy!

[@uni-byte]

Thanks for re-introducing me to the UJT!

[@99icd]

Bill, well done, good video, very clear. Never had a UJT in my box of bits - so something new for me. Thanks !

[@TRONMAGNUM2099]

Thanks for the great lesson!

[@JxH]

The RC charging curve is presumably one reason that the famous 555 timer chip constrains the sawtooth wave to oscillate between 1/3 and 2/3 of the Vcc. This avoids the most curved sections near the extremes, giving a much straighter waveform. Cheers.

[@analoghardwaretops3976]

No ..simple rc charging on 555 is also from Vcc & has similar chg.slope... For linearity there , it requires similar const. curr. chg. thro. a similar transistor fixed bias ckt.

[@TerryClarkAccordioncrazy]

This was in a Hobby Electronics magazine circuit for driving a strobe lamp, one of the first circuits I ever built. Never used one since, guess the range of applications was quite narrow. The electric fence driver I fixed around the same time used a diac to get the same function, I presume that was a cheaper solution.

[@Sctronic209]

Very good, thanks for sharing Bill.

[@justicelut]

Thanks Bill, found your presentation excellent!

[@JxH]

Minor nitpick - perhaps mixed up the two signals on the Bases (using Bases and Test Points assignments as shown at 3:03, Bottom is TP3 = Base 1, and Top TP2 = is Base 2). 5:10 "...now going to switch on Base 2 [=TP2] ..." But the pink trace is an upward spike from near Gnd, therefore it must be on the top of the 47R resistor on the bottom (TP3), thus Base 1. 5:21 "...switch on Base 1 [=TP3]..." Greenish trace shows a downward spike from near VCC, therefore it must be on the bottom of the 470R resistor on the top (TP2), thus Base 2. Apologies for the nitpick, but these sorts of things jump out at me almost instantly. 🙂

[@simonyoungglostog]

I'd never even heard of this component! I hope you are well.

[@LockdownElectronics]

Hello Si, am still going, hope you are too....

[@fredmitchel1236]

Right on Bill- I think these are still used...like you say to drive and trigger an SCR....or at least those two components are found in CD ignition in small engines and dirt bikes.....probably a scheme used in early MSD ignitions... I remember using the 2N4871 and the 2N2646....playing with them in school... Intrinsic standoff ratio..... They interest me..cause I have a tired KTM magneto.... Maybe it is time to tinker... Good job Kind regards Fred

[@campbellmorrison8540]

Very clearly explained video, excellent stuff thank you

[@0dbm]

I like your point of view Nice video

[@terryblackman6217]

Thank you Bill.

[@t1d100]

Thank you.

[@tecnisdaimondm.g9321]

Excelente 👌

[@jimhenderson2308]

There must be a huge discharge current to arrest that capacitor

[@guyfradasyl88650]

Hi, could you speak more about the transistor buffer.

[@alexanderstrelets8234]

Many thanks for this video! Still, I don't understand what was the reason to develop such a component having so narrow (so unique) region of application. Moreover it can be easily replaced with a pair of pnp and npn bi-polar transistor.

[@kennmossman8701]

I think the DIAC replaced the UJT in mains circuits

[@martijnklasing7815]

Dear sir, could you help me with finding a LTSpice model for a Uni Junction Transistor (UJT). It would be nice if I could find a model for a 2N6026 UJT or a 2N2646 UJT. The models I found up until now didn't work like they should. Thanks in advance for your help.

[@grahambambrook313]

😗

[@simontay4851]

Why is it the emitter when it is not emitting. The arrow points inwards. Its an input.

[@zedcarr6128]

The arrow indicates the direction of the flow of ''Conventional Current'' i.e. positive to negative. This is the same with Bipolar Transistors also. In an NPN transistor, the Collector is positive with respect to the Emitter. In a PNP Transistor, the opposite is true. In the case of the Unijunction Transistor it does ''Emit''. It emits positive charge carriers (holes) which are the absence of an electron in the semiconductor. Electrons (Negatively Charged Carriers) and holes (Positively Charged Carriers) travel in opposite directions to each other. The collector of a transistor ''Collects'' charge carriers the emitter ''Emits'' charge carriers, depending on the PN construction of the device, the charge carrier is either negative (electrons) or positive (holes). Remember that a ''hole'' is the absence of an electron which behaves as positive charge in the silicon or other semiconductor material. I hope I've explained this well. If any other electronic engineers / hobbyists / buffs want to chip in and correct anything I've got wrong or missed out, I'm all ears (eyes). 🙂

[@floranape913]

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