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Need help with oscillating PUT

Discussion in 'General Electronics Discussion' started by flypadre, Feb 9, 2013.

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  1. flypadre

    flypadre

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    Jan 30, 2013
    I am working through this book Make:Electronics by Charles Platt and have come to a circuit that I am unable to duplicate the results of.

    Here is a link to the circuit.
    http://books.google.com/books?id=PQ...nepage&q=Make:Electronics figure 2-98&f=false

    I have built the circuit as is but my LED doesn't blink it simply remains lit. My understanding is that the 2.2uF capacitor is supposed to cause the oscillation. However, I can remove the capacitor and the LED remains on.

    I am beginning to wonder if there is an error in the book (however I am expecting that I am the one that is in error :eek:) which is why I am posting this here to get all of your opinions.

    Thanks
    -Sean
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Check your component values again really carefully. It is quite possible that you have an incorrect value resistor.

    As an alternative, check that you have places the PUT in the circuit correctly.

    Finally, try another PUT in case you've damaged this one.
     
  3. flypadre

    flypadre

    14
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    Jan 30, 2013
    Well I didn't have the exact resistors. I replaced the 27k with a 33k. However, I also tried the circuit by replacing the 27k with a 22k.
    My understanding though is that those resistors in series set the voltage at the gate necessary for the anode to reach before electricity will flow from anode the cathode.

    What I am not getting is why my LED is not blinking. I've tried different capacitors thinking that maybe one was defective, however my results don't change. In fact I can remove the capacitor and the circuit continues to operate in the same way. I've included a couple of pictures of my breadboard to show you what I have done.

    [​IMG]

    [​IMG]
     
    Last edited: Feb 10, 2013
  4. flypadre

    flypadre

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    Jan 30, 2013
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,496
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    Jan 21, 2010
    If it remains lit, I would be looking seriously at the 470k resistor.

    The only way current can get continuously to the LED is via that resistor (or via the 15k resistor -- but that would imply the PUT is in circuit incorrectly.

    With a 470k series resistor, I'd be surprised if you could see any glow from the LED.

    Try connecting the LED to the battery via the various resistors you're using and tell me which one gives the same light intensity from the LED as you're seeing when the LED is on continuously in this circuit.

    I would also suggest placing a 220 ohm resistor in series with the LED to protect it and the PUT from the sudden discharge of the capacitor though the LED.
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Wow. That's way too bright. I don't think you have the configuration of the leads correct.

    Google for a datasheet for the PUT you're using and confirm the correct pinout.
     
  7. flypadre

    flypadre

    14
    0
    Jan 30, 2013
    Steve, you called it right in the beginning when you told me to check my resistors. I was reading your post and something caught my eye. You drew my attention to the 470 resistor. I noticed that you kept writing 470k. I was using a 470 ohm. I put in the 470k and it worked perfectly. The LED blinks and is much much less bright.

    I think that I understand now what I was seeing. The 470 ohm resistor was allowing the capacitor to charge so quickly that I couldn't see the oscillation in the LED, which is why when I pulled the cap I didn't notice anything different. However, I think that if I had an oscilloscope I would have seen a difference. Do I have that right?

    I feel like this is a newbie mistake, but I don't feel to bad because I am a newbie!

    Thanks for taking the time to help me out. You helped me take a frustrating situation and learn from it.

    -Sean
     
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,496
    2,837
    Jan 21, 2010
    The 470 ohm resistor was possibly charging the capacitor faster than the PUT could discharge it, so it may have been permanently alight (however the fact that you did not have a series resistor for the LED means that your explanation could also be true).

    Either way, it always pays to check the multiplier band on the resistors :)

    These days I check resistors with a multimeter because I can see the multimeter, but the bands are too tiny without changing glasses :(
     
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