NPN Transistor question

Discussion in 'General Electronics Chat' started by odm4286, Nov 29, 2013.

  1. odm4286

    Thread Starter Active Member

    Sep 20, 2009
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    I ran into a little problem on a project I've been working on today and would like to learn more about what happened.

    Basically I am trying to control the speed of a small DC motor via PWM from a plc. My motor was connected to a floating 1.5vdc supply then connected to the collector of my NPN transistor, my transistor base was then connected to the plc output card that would handle the PWM, my transistor emitter was connected to my 1.5vdc supply ground.

    When I tried this circuit it didn't work, I scratched my head for a while and then it hit me. My PLC is powered by a GROUNDED power supply while my motor is powered by a floating 1.5vdc supply(batteries).

    So I'm guessing because of the difference references things didn't work. Sorry for the rambling explanation I'm still learning. Can anyone expand of this or did I pretty much wrap it up?
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    What is the voltage required to operate the motor?
    If the motor requires 1.5VDC then the voltage drop across the NPN transistor will be enough to make the motor not work.

    Yes, the emitter of the NPN transistor should be connected to the same GND as the PIC.
    You may have better success using a logic level MOSFET.
    You could also try increasing the supply voltage to the motor.
     
  3. MaxHeadRoom

    Expert

    Jul 18, 2013
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    What PLC are you using to control the motor? Is this PLC in the industrial sense, ladder logic controller?
    Max.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    You did not mention a current limiting resistor between the plc output pin and the base of the transistor. This will result in excessive I/O pin current, possibly destroying that pin or the plc.

    You need to provide more details about the motor, transistor and current limits for the plc I/O pins. You will also need a diode placed across the collector/emitter junction, or when the transistor turns off the motor current will cause a large reverse polarity voltage, frying the transistor and possibly taking the PLC with it.
     
  5. odm4286

    Thread Starter Active Member

    Sep 20, 2009
    155
    5
    I will post back with some more details soon. Including a schematic
     
  6. Lineout

    Member

    Nov 20, 2013
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    Can you place the diode across the + - of the motor as well or instead of the collector / emitter ?
    I thought I'd seen this done , but didn't know you could place the diode across emitter collector, maybe this explains how a lot of motors that I run across don't have the diode across the motor , must at the transistor junctions....:shrugs:

    If you have a reversing motor , this gets complicated ?
    How would you solve that, ?
    I have a reversing rc hoist/crane now that I don't have protection for reciever/relay, cause I'm not sure how to go about it, and just recently became aware of the back currents on 'STOP' .
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    Yes, you could simply place the diode across the motor terminals; the idea is to provide a current path for when the transistor turns off.

    Yes, it is a bit more complicated for a bi - directional motor; you need two diodes from each terminal; one to power and one to ground. These should be very fast diodes like Schottkys. Standard silicon diodes like 1n400x, 1n540x and the like are much too slow to turn off for PWM'ed motors.
     
  8. Lineout

    Member

    Nov 20, 2013
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    Cool , I just looked up shottkys on ebay and find some tiny orange ones and some black ones.

    I have some black and also some little orange ones that look like these but the source sent zero info on them.
    Are all shottkys 'fast' action ? Is that a brand name or style?

    Is there and easy way for me to identify whether mine are 'fast' acting.

    I guess whenever someone tells a newbie (like me and others) to slap a diode on 'it' , it would be helpful to include the justification or need for a 'fast acting' as oppossed to the slower silicon diode.

    So, I guess there is a little bit of damping with a slow diode, but not enough to do justice to the complete protection of the board components.
     
  9. Lineout

    Member

    Nov 20, 2013
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    Not sure if I understand what you're saying, but it sounds do able:

    Two diodes: one from pos of motor to + and - of (battery for instance) ? Cathodes in what direction for each ?

    Two diodes: one from neg of motor to + and - of (battery)?
    Cathodes in what direction for each?
     
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