Debugging pwm motor control circuit

Discussion in 'The Projects Forum' started by IvarsOzolsDB, Nov 16, 2012.

  1. IvarsOzolsDB

    Thread Starter New Member

    Aug 24, 2012
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    I designed this schematic to control a small DC motor witch draws 250mA@4.5V. Made PCB, soldered all in place but it doesn't work. When I disconnect the motor, the LED lights up and reacts like it is supposed to. I'm electronics hobbyist so I'm asking you where is the error in design?
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,347
    Hello,

    What is value of the power supply of the circuit?
    For 4.5 Volts at the output of the regulator, you will need at least 6.5 Volts.

    Bertus
     
  3. IvarsOzolsDB

    Thread Starter New Member

    Aug 24, 2012
    27
    5
    Circuit is powered from 9V battery or adapter. If I hook up motor to regulator directly, motor runs as expected. Voltage measured in LM317T regulator output is 4.5V (adjusted by R4 1k trimpot), voltage measured connecting multimeter to motor outputs and with speed pot to max is also 4.5V
     
  4. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    You want the transistor to saturate. Then with a load of 250mA its base current should be 25mA.
    But your 1k series base resistor limits the base current to only 8.3mA which is not enough. Try 330 ohms.

    You also might have the collector and emitter pins of the transistor swapped.
     
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  5. tubeguy

    Well-Known Member

    Nov 3, 2012
    1,157
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    Another concern. If you are running a 250ma motor at 4.5v then you are running the 2n2222 at .25a x 4.5 volts = 1.125 watts. Too much wattage for a 2n2222. Is the transistor HOT?
    A TIP31 would be an option and is readily available.
     
  6. tubeguy

    Well-Known Member

    Nov 3, 2012
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    Re-thought my post. Now I'm not sure if my thought is correct....:confused:
     
  7. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
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    Yeah, if the transistor saturates as intended, Vce(sat) is typically less than 200mV. That times 250mA is only 50 milliwatts.
     
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  8. IvarsOzolsDB

    Thread Starter New Member

    Aug 24, 2012
    27
    5
    Thank you Audioguru, You were exactly right! I had collector and emitter pins of the transistor swapped. Did the 1k to 330R change too and circuit works like charm! :)
     
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