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"Switching Transistor" OFF Impedance

  1. Oct 15, 2014 #1
    Hey all,

    When a Bipolar Junction Transistor is "OFF" (i.e., Vb = 0), what are typical Emitter-Collector impedances? We are using a switching BJT as a switch for a stepper motor, however, we have just realized that the impedance is rather low when the transistor is off (470 Ohms). I would have expected this to be much larger for a transistor that is built for switching.
     
    Last edited: Oct 15, 2014
  2. jcsd
  3. Oct 15, 2014 #2
    The datasheet should list some of this - however how are you switching it "off" - do you have a pull down resistor?
     
  4. Oct 15, 2014 #3
    We are sending it a signal from a microcontroller. Attached you will see a portion of the circuit. The problem is that when the bases are "low", or even floating, the circuit is drawing current.
     

    Attached Files:

  5. Oct 15, 2014 #4
    I would use a 1K (?) resistor between the Base of Q1 and Ground. When the uC is Low, it is probably more like "floating" and the Q1 becomes a Voltage divider - allowing some leakage current - This also seems to be reverse logic ( when the uC is Low(Q1 off) the Motor is ON because the Gate of M1 gets Pulled up ) - is this what you want?
     
  6. Oct 15, 2014 #5
    Wouldn't a positive signal at the input of Q1 cause a current to flow into the base and thus bias Q1 "ON"? This is merely a common-emitter amplifier, no?
     

    Attached Files:

  7. Oct 16, 2014 #6

    davenn

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    Science Advisor
    Gold Member

    yes and that turns M1 ( the FET) off

    As windact said its kinda reverse logic
     
  8. Oct 16, 2014 #7
    In a switching circuit - think of ideals -- perfect switch.. If Q1 is "on" - ideally - what is the V at M1s Gate ... and conversely - if Q1 is "off" -- ideally -- what is the V at G-M1?

    Your number 2 diagram --- is not the same as your #1 diagram...redraw #2 - in the arrangement of #1 and then add the parts that are missing.

    As a matter of design best practices - the MOSFET is VERY FAST - what happens if the motor is running ( current flowing) any you "try" to turn it OFF VERY FAST??
     
  9. Oct 17, 2014 #8
    Which particular NMOS transistor are you using?
     
  10. Oct 17, 2014 #9

    NascentOxygen

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    Staff: Mentor

    You believe it has always behaved like this, or are you investigating now only because something has recently stopped working properly?

    Or is this a new circuit that isn't yet working how you want it to? Certainly, an OFF transistor should present more than a few kΩ to low voltage DC.
     
    Last edited: Oct 17, 2014
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