PNP "Wire Break" Circuit Help

Discussion in 'Homework Help' started by nullsys, May 20, 2012.

  1. nullsys

    Thread Starter New Member

    May 20, 2012
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    Hello,
    I'm hoping you can help.
    As far as I can tell, the circuit works using Yenka simulation software, but I'm loathed to subject my poor PNP transistor to testing, just in case ;)

    Here's a lil info:
    I wanted to make a simple - "If the wire breaks, the led comes on" circuit. This is my first attempt, as I say, it appears to work.

    Details:
    S1 closed, no LED.
    S1 open, LED.

    S1 is of course simulating a wire break.

    [​IMG]

    What I'm concerned about is that if I remove the 47 Ohm resistor, the circuit fails, and the simulation "blows up" several components.
    But, does this really mean the circuit is safe with it there?

    Any help would be greatly appreciated, thank you!
     
  2. absf

    Senior Member

    Dec 29, 2010
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    Removing the 47 ohm is the same as pushing the other button on the right. It affectively disconnect the battery from your circuit and I dont see how any other components can be affected (get blown).:D

    May be it's time for you to get your hands dirty and construct the circuit on a breadboard. even something get spoiled, it wouldnt cost you more then 50 cents to replace it except the battery. :p

    Allen
     
  3. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    There is two situations tht arise from what u ask ?

    Remove the 47Ω R.
    Now what I am thinking is tht once u remove the resistor u will leave the connections as it is. In this case nothing will happen .
    When u take the 47Ω R u will be breaking a connection in the power line. Without power, LED won't light and the circuit simply won't work. So there is nothing to blow.

    The real question is, Are u connecting the two points where the resistor is connected after u remove it. If this is the case, U circuit will definitely blow up.
    It will blow up because without that resistor u are simply short circuiting the 9V battery.

    I don't think the circuit will blow but the battery will infidelity have an episode on not trying to blow.
     
  4. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    The 47Ω is a power waster,giving the battery[6 AAAs] about an hours life, short it out, along with the top 100Ω, & the 1kΩ. Increase bottom 100Ω to 450Ω. Move 100K from emitter to LED-k & drop value to 5k; cut wire from LED-k to 1k.
     
  5. strantor

    AAC Fanatic!

    Oct 3, 2010
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    if you remove the 47ohm resistor (and replace it with a wire) you short out the battery directly
     
  6. nullsys

    Thread Starter New Member

    May 20, 2012
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    Thanks all for your reply.
    Yes, I see it now. It does cause a huge problem by removing the 47ohm resistor and just leaving wire there.

    What I'm worried about is: If I leave the circuit as is... is it safe? I have a feeling the simulation software is kind of botching it. Where as in real life, I think it might mess up the battery or other components.

    Bernard: I tried what you said, but if I close S1, the simulation fails and causes several blown components.

    I think in general, having this setup where there us a connection along S1 is the problem. I don't think it really works for what I want it to do.
     
  7. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    Origional circuit is faulted, with S1 closed, the 47 Ω will be continusly disipating about 1.7W, discharging battery rapidly, LED off. S1 open, LED on with about 25mA. As modified, I do not see why simulation failed, but ckt will work; drawing about 2mA S1 closed, LED off; S1 open LED on, drawing about 20mA.
     
    nullsys likes this.
  8. nullsys

    Thread Starter New Member

    May 20, 2012
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    Hi Bernard!
    Wow, I just tried that out. It worked.

    I'm stunned. It's a complete design change from mine.
    I'm going to try and work out "why" this is easier and actually works, but right now, it's over my head!

    Thank you so much buddy!

    PS: What is the purpose of the 5k resistor? I've tried altering it to 10k for example, and it seems to draw less current across itself and the PNP, perhaps a higher value is preferred?
     
    Last edited: May 22, 2012
  9. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    Depends on transistor gain, could possibly go to 10k or better.
     
  10. nullsys

    Thread Starter New Member

    May 20, 2012
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    Thanks Bernard.
    I have included another version, with the 10k and in NPN format.

    At 5k, it draws 25.13mA and at 10k 24.25mA. So I think you're right, it's the transistor gain. Silly me.

    Here's the final NPN version too, thanks so much!
    [​IMG]
     
  11. absf

    Senior Member

    Dec 29, 2010
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    Glad that you've solved your problem...

    BTW what program did you used to draw your schematics? Symbols Kinda cute!
     
  12. nullsys

    Thread Starter New Member

    May 20, 2012
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    Thanks!
    The program is called Yenka.
    It's free as long as it's used at "home" and after 3pm.
     
  13. absf

    Senior Member

    Dec 29, 2010
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    Thanks, I'll download it and take a look.
    Allen
     
  14. strantor

    AAC Fanatic!

    Oct 3, 2010
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    i looked at yenka for about 30min and then uninstalled it. I can't remember why.
     
  15. absf

    Senior Member

    Dec 29, 2010
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    I cant even download it successfully. It was V 3.4.0 and 63 MB. The link was slow and I have downloaded 5 MB and it stopped.

    Maybe next time...

    Allen
     
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