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Transistor switch suitable to run dc motor?

Discussion in 'General Electronics Discussion' started by Minu, Apr 10, 2018.

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  1. Minu

    Minu

    22
    1
    Apr 8, 2018
    Which kind of active switch can be used with a 12V 2000 rpm DC Motor. I used a BC547B transistor to act as a switch. Above dc motor is connected at the collector of this transistor. Once the motor turned on when i applied a 3V on transistor's BASE terminal. But transistor got damaged. How can i solve this problem?

    I got one more problem that the speed of dc motor is not as much as i get when i connect a 12V battery directly connected across the dc motor.

    I think i have to use a reverse connected diode across the dc motor and have to replace the BJT switch with a MOSFET switch. Is it so? 1523386716906-205684114.jpg
     
  2. BobK

    BobK

    7,682
    1,688
    Jan 5, 2010
    These are both good ideas. You probably destroyed the transistor because you exceeded its maximum current specification.

    Bob
     
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  3. Minu

    Minu

    22
    1
    Apr 8, 2018
    Thank you for your kind reply...can you suggest a MOSFET switch for this purpose?
     
  4. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    That's the first thing I would try to do. Always a good idea to place a diode across the motor armature connection unless you are going to reverse the polarity. You also need to use a "logic level" MOSFET that will turn on fully with just a few volts on its gate.

    You may need to precede the MOSFET gate connection with a transistor that will pull the gate toward +12 V DC through a resistor when the transistor is off. The resistor is connected to the NPN transistor collector to limit the collector current when the transistor turns on. A largeish value in the neighborhood of 50 kΩ to perhaps 1 Meg-ohm would work just fine. Note that this will invert the logic of your PIR device, turning on the MOSFET when the PIR output goes low.

    Depending on PIR actual output and MOSFET gate sensitivity, you might be able to use an emitter-follower configuration to drive the MOSFET gate, allowing the MOSFET to turn on when the PIR output goes high. Or add a second transistor to invert the output of the first transistor.

    Your BC547B is only rated for 100 mA of continuous collector current, and since the motor runs faster when directly connected to the 12 V DC supply, the transistor is either not being driven on hard enough for saturation of the collector-to-emitter voltage (typically less than 0.6 V for Ic = 100 mA) or you have exceed the 100 mA maximum continuous current specification. You may also have bricked the transistor with high-voltage motor transients since there was no reverse-connected diode across the motor. These transients are associated with turning the transistor off, and they can reach several hundred volts, destroying the collector-base junction almost instantly.

    I keep on hand a bunch of 2N7000 MOSFET transistors. Use Google to find equivalents in India.
     
    Last edited: Apr 10, 2018
    Minu likes this.
  5. BobK

    BobK

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    Jan 5, 2010
    If you can "suggest" the current required the motor, and what voltage you have to drive the gate. Note that the stall current is important, not just the no-load running current.

    Bob
     
  6. Minu

    Minu

    22
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    Apr 8, 2018
    Thank you for your detailed reply...i understood the concepts better. Thank you so much sir for your help
     
  7. Minu

    Minu

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    1
    Apr 8, 2018
    Actually the motor i got is a 12V 2000 rpm motor..dont know more detailes. To the base or to the gate of the switch i will get a 3V. Around 2.9V am getting practically.
    This is the motor i am using.
     

    Attached Files:

  8. Minu

    Minu

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    Apr 8, 2018
    I read your reply post again and again..i am trying to draw a circuit from your explanation..
     
  9. Minder

    Minder

    3,103
    664
    Apr 24, 2015
    For a general purpose Mosfet for small motor applications you can buy the IRF630 for about $1.00.
    M.
     
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  10. Minu

    Minu

    22
    1
    Apr 8, 2018
    Thank you..
     
  11. hevans1944

    hevans1944 Hop - AC8NS

    4,615
    2,154
    Jun 21, 2012
    The IRF630 has a lot more current capacity than the smaller 2N7000. The Rds(on) resistance is probably smaller too, meaning less power dissipation in the transistor for any given drain current. If you can find, and can afford to buy, the IRF630 please do that instead of using the 2N7000.

    In another thread you said you are using three PIR sensors to direct a fan at a person, using an Arduino for the control logic. If this is part of that same project, it would be better to drive the MOSFET gate from an Arduino output bit and sense the PIR state with an Arduino input bit. The program logic would then determine when to turn the MOSFET on or off. No extra transistor needed.
     
  12. Minu

    Minu

    22
    1
    Apr 8, 2018
    I wish to do 2 projects.
    1) i wish to make a fan which turn ON when human presence is there. Turn OFF when human presence is not there. The circuit have a single PIR sensor. No swing needed in this case. Also i dont want to program it in any sense. I need a pure CIRCUIT to do this work.
    Here In order to turn off the fan, i wish to set a delay for 10 minutes or so. During that time, if there is a pulse from PIR, again the fan will be ON for further 10 minutes.
    I wish to use a retriggerable Monostable multivibrator for this purpose.
    My aim is to make a low cost human sensing fan. This project also give me more awareness for my second Project
    2) make a human sensing and human locating fan with the help of programming using arduino uno. Here i use 3 pir sensors to find out the human position, one servo motor for the swing, one 12 V 2000 rpm dc motor for connecting with the blade and an arduino uno.
     
    hevans1944 likes this.
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