Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

"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


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    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


    User Avatar

    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
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook