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Learning Unijuctions

Discussion in 'General Electronics Discussion' started by JTSharkAttack, Feb 26, 2017.

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  1. JTSharkAttack

    JTSharkAttack

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    Feb 26, 2017
    So I've been learning how to make electronics lately with a book called Make: Electronics. In "Experiment 11", there is a project teaching you how unijunction transistors work, as seen below. I assembled the circuit and plugged it in. The light quickly blinked once, then remained dark again. After some testing, I realized the capacitor wasn't discharging. I replaced all the parts and moved the circuit on a different location on the breadboard, but nothing changed. Is there a flaw in the design, or something I'm doing? If so, what is it? Thanks

    image2.jpg

    image1.jpg

    image (2).jpeg


    [mod edited: sorted out images]
     
    Last edited by a moderator: Feb 26, 2017
  2. BobK

    BobK

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    I can only say check all the connections and especially the pinout on the UJT.

    Bob
     
  3. davenn

    davenn Moderator

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    C1 isn't in the correct place ... it isn't doing anything where it is
    nor is R3

    compare your board layout with the one shown in the second image
     
  4. Bluejets

    Bluejets

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    C1 looks right to me but R3 definetely not correct.
     
  5. davenn

    davenn Moderator

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    maybe the angle of the photo fooled me ;)

    but I cannot tell if the + and - terminals of the cap are going to the right places ??
     
  6. Bluejets

    Bluejets

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    True.....I did notice that also.
     
  7. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Where did the schematic go? Anyway I didn't like the way it was drawn so I was going to redraw it. That's when I discovered I didn't have a 2N6027 in my library. Actually I found I didn't have any UJT's or PUT's at all. I didn't even have a symbol for them. So I downloaded a spice model for it and drew a symbol to go along with the spice code.

    Note that I couldn't get it to oscillate with R1=470K so I lowered it to 220K. There was too much A-G leakage for the 470K to overcome.

    Chris
    upload_2017-2-28_11-26-2.png
     
    Arouse1973 likes this.
  8. davenn

    davenn Moderator

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    it's still there ... as are the other images
     
  9. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Huh, I guess the page didn't load correctly when I posted that because, as you say, it's still there... now.

    BTW, if the spice model is accurate I don't see enough C1 discharge current through the PUT to light a LED. The datasheet includes an astable example but they drive the LED with an SCR gated by the PUT.

    Chris
     
  10. BobK

    BobK

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    Look at where you are measuring current. There should be a large pulse of current through the LED when the UJT fires seeing as the capacitor discharges very rapidly.

    Bob
     
  11. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Yes Bob, that was a very lame mistake.
    Chris
     
  12. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    I guess after a goof like that I should post a corrected schematic and plot. I added R4 to limit the LED current and stretch C1 discharge time a bit.

    Chris
    upload_2017-3-2_18-16-15.png
     
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  13. BobK

    BobK

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    Nice. I guess you would call that a spike.

    Bob
     
    Arouse1973 likes this.
  14. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Yes, it certainly is a spike. I guess with only 2uF one should expect a brief pulse. It could be a handy pulse source for someone playing with time domain reflectometry. I wonder where the OP went?

    Chris
     
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