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let's see if i'm getting this right.

Discussion in 'General Electronics Chat' started by mybuickskill6979, Jan 26, 2007.

  1. audioguru

    audioguru Well-Known Member Most Helpful Member

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    RadioShack are gone from Canada because their prices for cheap Chinese parts were way too high and their "specifications" were meaningless and written by morons. Most of their salesmen were morons who could hardly speak Engrish.

    I bought an expensive video extension cable at RadioShack. My wifey took the label to The Dollar Store and bought exactly the same thing and brand for nearly nothing. I took back the overpriced RadioShack one, then they went bankrupt. Good riddence RadioShack!

    For years I joked with their salesmen about their spec's for LEDs that didn't say a word about their brightness.
    The spec's mentioned length and width, and weight. Sometimes their spec's said what colour were their LEDs. Typical forward voltage? Max current? You need to guess!

    My son's first cell phone was purchased at RadioShack. It was free.

    My daughter gave me a cell phone with a built-in camera for my birthday last year. She got one the same. Both are free. We don't use them on the internet and we don't talk much when they charge a fortune. Evenings and weekends are free. Calls between us at anytime are free.
     
  2. mybuickskill6979

    mybuickskill6979 New Member

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    dude i tried adjusting the voltage. adjusting the amperage. flipping and flopping parts and pieces. but nothing i do seems to kill the LED. it always shines at least a little bit!

    gonna check the schematic again!!
     
  3. mybuickskill6979

    mybuickskill6979 New Member

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    2n3904

    i'm messing with this thing and i can't for the life of me get the led to shut off or brighten without adjusting the pot (thats in place of the resistor before the LED) with or without charge to the base the led shines. just as bright as before. conductivity seems not to change in the trans no matter what i do resistance in the trans doesn't change either!! could this be one of those RS mess ups? \

    And i just melted another pot.

    i think i'm a have to put off the experimenting a little while till i can afford more stuff. all my resistors are 1/8w and not high ohmage enough to even work.
     
    Last edited: Feb 4, 2007
  4. dave

    Dave New Member

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  5. sam2

    sam2 New Member

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  6. ljcox

    ljcox Well-Known Member

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    You need to post a drawing that we can read!! Then we'll be able to help.
     
  7. AllVol

    AllVol New Member

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    Hi, Buickskill

    I hope you are still around.

    I dropped by the local Radio Shack this morning and picked up one of the cheap Chinese second PNP transistors like you have been using. You can see the results in the accompanying attachments.

    The first shows the basic breadboard arrangement: The green LED is attached from negative (ground) through its current-limiting resistor to the collector of the transistor (center pin). The black wire connects the emitter (right most pin in the pic) to positive. The base (left-most pin) is connected through a resistor and switch to negative.

    The second shows the switch closed, putting base to ground, since its a PNP transistor. Voila, the LED glows.

    Maybe this will help you. If you need further information, ljcox will be glad to assist, I'm sure. >)

    Regards,
    AllVol
     

    Attached Files:

    Last edited: Feb 5, 2007
  8. mybuickskill6979

    mybuickskill6979 New Member

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    yeah the TIP42 pnp working perfectly and found that if i have at least a little resistance on the base that it will conduct, with out res it won't and at pos to base it won't. i changed the pot a little and measured resistances and they seemsd to drop as the resistance on the base dropped.

    i got to working with out a problem but now the npn i found some 0 continuity in it so im gonna have to test it and play a little more and hope not to smell burning plastic (from the pot) again lol. i need to attain some better stuffs lol!!
     
    Last edited: Feb 5, 2007
  9. mybuickskill6979

    mybuickskill6979 New Member

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    okay i got it working i need a little more resistance( i'll try my next higher ohmage on resistors) and see if i can' get broader control but none the less. i can switch the npn on and off and the pnp on and off and in between yay!! lol
     
  10. mybuickskill6979

    mybuickskill6979 New Member

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    well i tried two 470k resistors in series to the base and it still lights but dimmer then full flow. didn't test the I or V on it yet but whats the biggest resistor in existence? and is it possible to have a conductivity of .00001 MHO? just wondering. i need to get some bigger pots the big ones i have 10k and 100k are all goners lol!!
     
  11. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Pots burn out when your circuit allows too much current to flow in them.

    Add a small transistor to drive the big one like a darlington transistor then its base current is extremely low and a high value pot across the supply can be adjusted to turn it fully on or all the way down to off and anywhere in between.
     
  12. mybuickskill6979

    mybuickskill6979 New Member

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    heres the schematic of what i have now its blurry but none the less its my PNP
    theres five 470k resistors and a 10k pot. to the base
    one 330 ohm resistor to the led to the to the collector

    the emitter to ground

    oh and the values to the base are this. 0.0 mA and .645V and the LED is still shining, but pretty dim lol!!

    [​IMG]
     
    Last edited: Feb 5, 2007
  13. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Your PNP transistor is connected upside down. Its emitter-base junction is having over-voltage breakdown at about 7V like a zener diode. Its collector-base junction is forward biased at 0.7V so the LED has a low current and is dim all the time.

    The emitter of a PNP common-emitter transistor is supposed to go to the positive supply. The emitter of a common emitter NPN transistor is supposed to go to ground like Len showed.

    Your picture is too fuzzy to see why you have so many resistors.
     

    Attached Files:

  14. AllVol

    AllVol New Member

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    buickskill

    Did you not even look at my posting with the photos showing how to set up your PNP transistor?

    AllVol
     
  15. mybuickskill6979

    mybuickskill6979 New Member

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    the circ posted was from the toying with 2 npn transnistors

    i first hooked up my 2n3904 like this. then my RS cat number 276-1617.

    okay lets see! okay pnp's the base must be grounded witha little resistance. NPN must have a little voltage/current to the base. is that right? i'll try to get a less fuzzy pic!!
     
  16. audioguru

    audioguru Well-Known Member Most Helpful Member

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    You can't use "a little resistance". You must calculate the value of each resistor so you don't burn out the resistor or the transistor.
     
  17. ljcox

    ljcox Well-Known Member

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    I suggest you draw it on a large sheet of paper using a wide felt tipped pen.

    Then you should be able to hold the camera further away and, hopefully, it will be in focus.

    Even if its not in focus, we should be able to read it. Make the letters large and mark the C, B & E of the transistor.
     
  18. ljcox

    ljcox Well-Known Member

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    yes, we showed you how to do it and how to do the calculations.

    Fooling around with pots is unnecessary.
     
  19. AllVol

    AllVol New Member

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    Well, I admit I'm totally confused. First its an NPN, then its a PNP, and it works, but it doesn't work. If the latest schematic (the envelope on a keyboard) is of an NPN, it certainly will not work with 2.35 megs of resistance on the base, which implies a current of .000005 amps with 12 volts. What next?

    As Ron H would say in this situation "If its not one thing, its a mother".
     
  20. mybuickskill6979

    mybuickskill6979 New Member

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    nope sorry to the base was .645v and no measurable amperage what i was saying is i can't get it YET to shut off completely i found bigger resistors today so it will happen today lol!! sorry for the confusion :D
     
  21. audioguru

    audioguru Well-Known Member Most Helpful Member

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    A transistor turns off if you or a pot or a resistor shorts its base to its emitter.
     

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