EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: schmitt trigger on January 26, 2019, 10:34:08 pm
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A follow up of a previous thread. https://www.eevblog.com/forum/projects/having-fun-with-discrete-transistors/msg2108917/#msg2108917 (https://www.eevblog.com/forum/projects/having-fun-with-discrete-transistors/msg2108917/#msg2108917)
I have continued to play with discrete semiconductors, ....I am just sharing the fun.
Most everyone, at least if you are nearing retirement age, has seen the bipolar transistor astable multivibrator.
This is a classic circuit: http://www.davidbridgen.com/astable.htm (http://www.davidbridgen.com/astable.htm)
A little rarer is the programmable unijunction transistor astable multivibrator. I remember seeing a description in an old GE SCR manual, 5th edition. See the attached image and description.
Assembling the circuit, and probing the cathode for each device, one can see the alternating pulses.
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Nice that it only uses one capacitor -- reminds me of the CD4066 oscillator and such.
Tim
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An acquaintance of mine would joke that the PUT is the triple-lie semiconductor:
-It is not programmable, it’s parameters are adjustable.
-It is not an UNI-junction, it actually has three junctions like a SCR.
-It is not a transistor, as it cannot operate linearly. Again and similar to the SCR, which is a thyristor, it operates either off or triggered.
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You used to be able to get one with both anode and cathode gates, the BRY39 (http://www.datasheetcatalog.com/datasheets_pdf/B/R/Y/3/BRY39.shtml), in a four lead TO-72 metal can package. It could operate as a normal (low power) thyristor, or as a PUT, which obviously gave you extra triggering options. Although its still available N.O.S. from various specialist suppliers, prices have unfortunately gone crazy.
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This is really a unique component.
Thanks for sharing, I have never seen this device applied in a circuit.