long wire to transistor base

Discussion in 'The Projects Forum' started by zivbarlas, Apr 30, 2013.

  1. zivbarlas

    Thread Starter New Member

    Apr 30, 2013
    4
    0
    hello,
    im building an instrument for testing the circuit shown in the attached image. when pin x is open, the transistor is on and i see 0v in pin Y. But, when i connect a wire to pin x (6 feet wire), in order to test it, the transistor never turns on. when i disconnect the wire, the circuit works perfectly. the 6 feet wire is disconnected on its other side.

    what can be the problem? and how can i solve it without removing the cable?

    thanks

    circuit.png
     
  2. ScottWang

    Moderator

    Aug 23, 2012
    4,853
    767
    Try to increasing the 10K resistor.
    You can in series a pot (50K or 100K) with 10K, and then you could adjust the right current to let cable reduce the current to flow to the B of transistor.

    You can also in series the pot between the common pin of PinX and 10K.
     
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  3. zivbarlas

    Thread Starter New Member

    Apr 30, 2013
    4
    0
    hi scott,

    thanks alot for quick answer.

    the problem with what u suggested is that i cant change the card (card under test in the figure). i can only change things out of it.
     
  4. ScottWang

    Moderator

    Aug 23, 2012
    4,853
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    Then you can in parallel with a 5K pot between PinX and Ground, adjust the values of pot to a position that it called Threshold, you can test it what's value will make transistor turn on, but when you adding the cable will make the transistor trun off.

    I'm not sure what's value of pot that it should be, but I think you can get the right value through your testing.
     
  5. crutschow

    Expert

    Mar 14, 2008
    12,993
    3,229
    Are you sure the wire is completely open and not connected to anything?
     
  6. #12

    Expert

    Nov 30, 2010
    16,261
    6,768
    Must be a connection somewhere. A 6 foot antenna will not shut that circuit off.
     
  7. tubeguy

    Well-Known Member

    Nov 3, 2012
    1,157
    197
    Yes, doesn't make sense with 10k on the base that it would shut off with no connection on the wire.
    You could try a shielded conductor. Just ask if you don't know what that is.

    Edit: Or even a twisted pair.
     
  8. zivbarlas

    Thread Starter New Member

    Apr 30, 2013
    4
    0
    hi,

    im sure the cable in completly disconnected and not shorted. i even connected external cable instead of the original cable to check this issue... same problem.
    i tried also shielded cable.
     
  9. ScottWang

    Moderator

    Aug 23, 2012
    4,853
    767
    Normally there are two easy ways to turn off the transistor, the one is in parallel a resistor with PinX and Ground, the resistor may likes a shunt resistor to make b can't get enough current and the transistor will turn off, the other is adding a negtive power to Vbe(PinX and Ground), and it will also turn off the transistor.

    I tried to duplicated the circuit to do the test, connecting 10K to 15V, connecting 10K from c to Vcc(5V), and connecting a power wire to b about 9 feets, but it doesn't make any affected, even I used my fingers to touch on b(PinX), it's still nothing happening, the results were as respected. (the Oscilloscope was measuring on Collector)

    So does inside the card is so simple?

    Could you take some picture when you testing, including the cable and the meter and values?
     
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  10. zivbarlas

    Thread Starter New Member

    Apr 30, 2013
    4
    0
    hi scott,
    the card is not so simple - the circuit should remove Vcc from pin Y when Vcc gets too high. Vcc pin is connected to pin X through 30v zener and a resistor, so when Vcc is higher than 30V (+Vbe) the transistor should get on and remove Vcc from pin Y.
    when the long cable is disconnected, the circuit work fine, but when i connect the cable, its not.
    i measured pin x with oscilloscope and saw that if i raise Vcc above 30v i get a periodic wave (shapeless) with voltage peak to peak of 16v!
    in pin y i see that the voltage jump from vcc to 0v at high frequency.

    i think that the circuit gets out of stability in this case... but i dont understand why the long cable cause it. :confused:

    thanks again,
    ziv
     
  11. MrChips

    Moderator

    Oct 2, 2009
    12,429
    3,360
    Read carefully. Everyone is saying there is nothing wrong with the circuit as drawn.
    10 feet of cable will not turn off the transistor.
    There is something else going on with the actual circuit that you are not telling us about.
    Take photographs of your setup and post them.
     
  12. tubeguy

    Well-Known Member

    Nov 3, 2012
    1,157
    197
    Apparently the circuit does not match the schematic you originally posted which makes it difficult to offer help. It would help to post a schematic of the circuit you are describing.
     
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