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long wire to transistor base

Discussion in 'General Electronics Discussion' started by zivbarlas, Apr 30, 2013.

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  1. zivbarlas

    zivbarlas

    2
    0
    Apr 30, 2013
    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. Harald Kapp

    Harald Kapp Moderator Moderator

    11,532
    2,656
    Nov 17, 2011
    The wire alone, if really not connected to anything else, should not be able to turn off the transistor.
    Have you measured the voltage from base to emitter of the transistor both with and without the wire attached? It should be approx. 0.6V in both cases.
     
  3. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

    4,960
    652
    May 8, 2012
    Do you have a connector on the far end of the cable? If you do it's probably shorted and that's easy for a nube to do if it's a 1/8" mini-plug.

    Yes, these are assumptions because there aren't many suspects here.

    Chris
     
  4. zivbarlas

    zivbarlas

    2
    0
    Apr 30, 2013
    hi,

    thanks for replies.

    Harald, i measured the voltage from base to emitter of the transistor both cases. without the cable connected, i see 0.7v, but when i connect the cable i see small negative voltage (-50mv~).

    CDRIVE, 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.
     
  5. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

    4,960
    652
    May 8, 2012
    Grabbing at straws here because what you're describing is mind boggling. Please re-post the schematic with the collector resistor value and the transistor part number included. Also, is the collector circuit being sourced by that same 15V rail as shown for the base bias?

    Chris
     
    Last edited: May 1, 2013
  6. Raven Luni

    Raven Luni

    798
    8
    Oct 15, 2011
    My money's on the collector and base resistor values. I've been caught out by this before. If you want to use a transistor as a switch, drive it well into saturation otherwise any current on the collector might not be enough to switch future stages.
     
  7. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

    4,960
    652
    May 8, 2012
    Understood but I can't conceive how 6 feet of cable could manifest this. Maybe he's sitting in the field of a transmitter?? :confused:

    Chris
     
  8. BobK

    BobK

    7,682
    1,688
    Jan 5, 2010
    Actually I would expect to see a nice 60Hz signal at the output when the 6 foot wire is connected, but certainly not turning the transistor off.

    Bob
     
  9. Raven Luni

    Raven Luni

    798
    8
    Oct 15, 2011
    Just reread the post - hadnt noticed the part about it being disconnected. When you say disconnected did mean like actually not touching anything or did you actually mean grounded??
     
  10. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

    4,960
    652
    May 8, 2012
    Unfortunately the transistor and collector load is as yet unknown . I know It's assuming too much but if it's a small signal transistor I would expect it to be saturated or near it. With a 15V bias supply and 10K base resistor Ib= 1.4mA. It would take quite a bit of 60Hz hum to overcome this forward bias and show up on the collector.

    I certainly hope that he doesn't vanish. I need to know!

    Chris
     
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