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transistor pain

Discussion in 'General Electronics Discussion' started by Britannicus, Feb 27, 2012.

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  1. Britannicus

    Britannicus

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    Feb 26, 2012
    Capture.JPG

    Hi All - I'm a real newbie here, so I'm going to have to air my ignorance big time.

    I've been plaing with STAMP microcontrollers for a bit - mostly driving stuff from their Board of education, but I'm trying something new and failing totally.

    The STAMP doesn't deliver enough power to run a motor really - I'm therefore trying to use a pin out to control a PNP transistor. This in turn I hope to switch on/off a 9V supply which has enough power to turn the motor.

    I actually want to pulse the motor so came up with the circuit attached. The LED flashes so I know the base on my transistor is getting a regular 1 second on / off 10Ma 5V signal. I'd hope this would permit the motor to then pulse 1 second at a time. I'm getting nothing through the emittor to the battery and am confused as to why -

    Am I ingorant of transistors ???
    Is my circuit wrong
    have I trashed my transistor and I don' tknow it

    Hi probability is all 3 !

    I've tried swapping the 3906 PNP with a 3904 NPN on the basis that this would work bu tas though the pin lo/hi was swapped around

    I'd really value some help for a dummy !
     
  2. jackorocko

    jackorocko

    1,284
    1
    Apr 4, 2010
    Sry, disregard what I said earlier, it was totally wrong.
     
  3. jackorocko

    jackorocko

    1,284
    1
    Apr 4, 2010
    Let me start again.

    A PNP transistor turns on when the base is more negative in respect to the emitter by about -0.7V. You have 9V on the emitter and 0 - 5V on the base at any given time. This should keep the transistor in the ON state continuously.

    What is the current rating of the motor?
     
    Last edited: Feb 27, 2012
  4. Britannicus

    Britannicus

    9
    0
    Feb 26, 2012
    Hi jackorocko,

    Thanks for the tip - I'm really green with this stuff. I fed the base with straight 10Ma from the STAMP output.

    I'd expected the Hfe gain being between 100 and 300 if I'm reading my datasheet right. So this should give me an output of 1 to 3 amps providing the battery can supply that yes ?

    not sure how I'd calculate the resistor you mention but I'd thought I didn't need one.

    as why a PNP - well,.. I had one handy for the experiment - I tried a NPN for good measure, still no result.

    I was wondering if I need a flyback Diode in parallel with the motor ??
     
  5. Britannicus

    Britannicus

    9
    0
    Feb 26, 2012
    Hi Jackorocko -

    rating of the motor - I'm not sure - all I know is if I connect the 9V battery to the thing direct then it will run - I'm figuring about 1 amp, but there's nothing on the motor to indicate
     
  6. Britannicus

    Britannicus

    9
    0
    Feb 26, 2012
    just read your secod post - so my 5 V is not high enough to switch the transistor off so in theory my motor should run all the time
     
  7. jackorocko

    jackorocko

    1,284
    1
    Apr 4, 2010
    I believe so. The 3906 is only rated at a max 200mA continuous collector current. You also need to remember that the transistor has a voltage drop across it as well, so the voltage the motor sees is less then 9V.

    So either you have to low voltage or not enough current to drive the motor. It would help if you knew the specs of the motor.

    Can you measure the voltage across the transistor C-E junction and across the motor?

    You should always have one when where an inductor is being switched on and off. But this is to protect the transistor from high voltages when switching off the motor. It won't help get the motor running.
     
    Last edited: Feb 27, 2012
  8. Britannicus

    Britannicus

    9
    0
    Feb 26, 2012
    Cheers Jacko,

    Much obliged for the help - sounds like multiple issues to sort out here...

    I'll have to go back and look at the motor to see if I can de-bug this bit by bit.

    In short, sounds like my circuit is OK but that my transistor is the wrong one, at least I seem to have understood the principle, if not the application - thanks for being so helpful
     
  9. jackorocko

    jackorocko

    1,284
    1
    Apr 4, 2010
    seeing as how you have two different voltages, I would use an NPN. That way you have a common voltage to turn the transistor off.

    to select the base resistor is quite easy. We know the max C-E current of the 2N3904 is 200mA and if the gain of the transistor is 100 min. then 200mA /100 = 2mA base current. 5V/0.002A = 2500Ohm resistor.
     
    Last edited: Feb 27, 2012
  10. BobK

    BobK

    7,682
    1,688
    Jan 5, 2010
    Agree with jackorocko about using an NPN, but you need to connect it differently.

    The emitter of the NPN should be connected to ground, the collector to one motor lead and the other motor lead to +9V.

    Bob
     
  11. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,497
    2,839
    Jan 21, 2010
    Drawing the circuit nbsiqəqomu doesn't help.

    It is conventional to place the most +ve rail at the top and the least positive (or most negative) rail at the bottom..
     
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