UJT curve - mallet to the head needed - Page 1
Forum section: electronics / beginners

author: Quantumplate
author_rank: Regular Contributor
timestamp: June 02, 2022, 03:26:45 pm
content: Going insane once again, hope you can help. It's the emitter voltage/emitter current curve, the hill and valley one.I don't get the fact that as one increases the input emitter voltage, it should keep going up and up surely? How can the graph show it plunging into a valley when you are increasing the input voltage?

author: rfeecs
author_rank: Frequent Contributor
timestamp: June 02, 2022, 05:55:53 pm
content: What curve are you referring to?The usual curve is Voltage vs current, not current vs voltage.At the peak, the emitter-B1 junction is forward biased. This causes extra carriers to flow into the region between emitter and B1 contacts. The extra carriers will lower the resistance, so the voltage will drop, hence the negative slope of the curve.

author: Quantumplate
author_rank: Regular Contributor
timestamp: June 02, 2022, 06:40:21 pm
content: Quote from: rfeecs on June 02, 2022, 05:55:53 pm

author: free_electron
author_rank: Super Contributor
timestamp: June 02, 2022, 06:56:07 pm
content: well . it is a UJT . unijunction transistor. that is the specialty of such thing. when you go over a certain threshold they shoot into conduction. this collapses the required voltage to maintain conduction. these things are typically used in oscillators.

author: Quantumplate
author_rank: Regular Contributor
timestamp: June 02, 2022, 07:16:30 pm
content: Quote from: free_electron on June 02, 2022, 06:56:07 pmAh, so once you get into conduction you can lower the input voltage and still conduct, this time with more current.But for a given Ve-b1 (above the valley point minimum), how do you know which side of the valley point you are on?

author: TimFox
author_rank: Super Contributor
timestamp: June 02, 2022, 07:19:00 pm
content: The curve graphed above,VEBas a function ofIE, is not monotonic, and therefore its inverseI(V) is multi-valued, and not a "well-behaved function".

author: free_electron
author_rank: Super Contributor
timestamp: June 02, 2022, 08:11:26 pm
content: Quote from: Quantumplate on June 02, 2022, 07:16:30 pmcorrect. that's whey they are often use in oscillators. use r-c at the base. the voltage rises until the trip voltage. at that point the transistor shoots into conduction. it stays in conduction until the base voltage goes below the hold value. this thing is essentially atransistor that has input hysteresis. you need to pierce the upper threshold for it to go into conduction , and pierce the lower threshold to take it out of conduction.

author: TimFox
author_rank: Super Contributor
timestamp: June 02, 2022, 08:15:02 pm
content: The UJT can do many things (not all) normally done by a 555 IC.

author: rfeecs
author_rank: Frequent Contributor
timestamp: June 02, 2022, 08:43:00 pm
content: They're not really used anymore. I think there are only a few models still made.You might take a look at this GE Transistor Manual from 1964:https://worldradiohistory.com/BOOKSHELF-ARH/GE-Books/GE-Transistor-Manual-1964.pdfIt has a whole chapter on unijunction transistors and applications, describing all the parameters, how they are measured, etc.It also has chapters on other near extinct devices like tunnel diodes and silicon controlled switches.These old manuals are great in that they explain semiconductors in a clear understandable way to engineers and hobbyists that were completely new to solid state at the time.

author: Quantumplate
author_rank: Regular Contributor
timestamp: June 02, 2022, 10:39:22 pm
content: Quote from: rfeecs on June 02, 2022, 08:43:00 pmThanks for that. I like the old books too...well some of them. My UJT info I found in a course book from 1974 - ICS Electronics course distance learning. Quality paper in those days.

author: free_electron
author_rank: Super Contributor
timestamp: June 03, 2022, 03:00:13 pm
content: wow. that's an interesting book ! saved. Thanks !lots and lot of diagrams and pictures to explain it as opposed to a bunch of dry equations where you have no clue where they come from or what they actually do.

author: Benta
author_rank: Super Contributor
timestamp: June 03, 2022, 06:31:03 pm
content: For similar behaviour of a semiconductor device, check out Diacs, widely used in dimmers.Example:https://www.st.com/resource/en/datasheet/db3.pdf

author: Quantumplate
author_rank: Regular Contributor
timestamp: June 07, 2022, 11:20:52 am
content: Quote from: rfeecs on June 02, 2022, 08:43:00 pmBought the book! Nice bedtime reading. Still a nagging question as to what that graph would look like if you were just raising the input voltage and carried on raising it beyond Vp.