diff npn motor

Discussion in 'General Electronics Chat' started by duxbuz, May 20, 2014.

  1. duxbuz

    Thread Starter Member

    Feb 23, 2014
    When I look at this circuit it seems that both motor terminals are connected to 5v and when the transistor opens it then allows one of the motor terminals to ground.

    But when I look at other examples they seem to have a more common theme of one of the motor pins going only to the collector like this example:

    The first example seems weird to me.

    Is one more desirable than the other?

  2. k7elp60

    Distinguished Member

    Nov 4, 2008
    The first example is correct. When the transistor is open the motor is stopped. When the transistor is on (base +) then the motor is on as one lead is to +5V and the other lead is to ground.
  3. duxbuz

    Thread Starter Member

    Feb 23, 2014

    So the second example is wrong?

    I was wondering which is preferable. The second example seems more common.
  4. GopherT

    AAC Fanatic!

    Nov 23, 2012
    In the first example, a diode protects the circuit from reverse voltage. Reverse voltage occurs when the motor is turned off (transistor base is switched from 3 volts to 0 volts) and the motor slows down (motor acts like a generator).

    The first also includes a capacitor to remove electrical noise caused by the motor and prevent interference with the arduino board.

    The second is not wrong, it is just not as good or reliable.

    If you use the first circuit, make sure your diode is the correct direction.
  5. duxbuz

    Thread Starter Member

    Feb 23, 2014
    In regards to the first example here:

    He mentions using a BC547.

    I was looking at the datasheet and it mentions:
    that Ic is max 100mA

    He mentions prior to that it requires 340mA

    Then I follow the calculations and using the 1k resistor mentioned in article and 5v from arduino pin and using forced beta of 10

    I get:
    (5V-0.7V)/1000 = 4.3mA
    Ic = .0043 * 10(forced Beta) = 43mA

    How's that going to work?

    I am wondering if I am working this stuff out wrong?