Problem with transistor and a motor

Discussion in 'The Projects Forum' started by terrik, Apr 5, 2013.

  1. terrik

    Thread Starter New Member

    Feb 23, 2013

    I'm trying to recycle/repair and old RC car and everything was going fine since last week when I got stuck trying to control the main motor.

    What I did is replace the old control board ( I lost the RF remote control ) with an arduino UNO board using bluetooth. I managed to build a battery pack, send commands to the arduino via bluetooth/serial and control the direction servo. To control the servo I build a H bridge circuit that is working fine.

    My problem is related with my inability to fully saturate the power transistor i recycled from the old board, it's a NPN transistor ( 2SC2654). First I tried to saturate it with the 5V 100% PWM from the arduino, the tried the 5V power pin (I read that can provide more current than pwd pins, not sure), no luck. Then I tried the 7-7,5V from the battery and the the transistor got more saturated allowing more current from collector to emitter, but not 100%. To fully saturate the transistor I have to connect the gate to 12V to get the motor running at almost 100%.

    My first hypothesis is that the arduino doesn't provide enouth current to saturate the power transistor. So, I tried to connect this transistor gate to the emitter of another (KN2222A) transistor, no luck either.

    The motor uses 0.8A at 12V when wired directly to the battery and runs perfectly.

    I have little electronics background so maybe I'm missing something obvious.

    The circuit below is the last attempt to make this work with the two transistors.


    Thank you in advance.
  2. tracecom

    AAC Fanatic!

    Apr 16, 2010
    The first thing I notice about the 2SC2654 is its pinout: from left to right, pin 1 is base, pin 2 is collector, pin 3 is emitter. I suppose that you have made certain of your wiring?
  3. terrik

    Thread Starter New Member

    Feb 23, 2013
    Thanks for the replay.

    Yes, I certainly did check many times :)
  4. patricktoday


    Feb 12, 2013
    Shouldn't there be a resistor between the arduino pin and the transistor base? Seems like it should work fine with a single transistor. If the motor needs 800mA, 10% of that should be ample for base current which would be 80mA so the resistor would be approximately (Vin - 0.7) / 0.080, say 33 ohms. I was just speculating the base current might be pulling down your voltage source (you could measure the base voltage to check for that).
  5. absf

    Senior Member

    Dec 29, 2010
    I simulated your motor circuit and and looks like it should work.

    I used a resistor to replace your motor (R=12V/0.8A~=15 ohm). I dont have 2sc2654 so I used 2sc1162 which is about half the spec of your transistor. On the Base of 2N2222A I put a 1K resistor or else the current on the motor would be too high.

    I suspect that your KN2222A is bad or low gain. You can try with any general purpose transistor like 2N3904 or BC547. Dont forget to put a diode across the motor to protect your 2sc2654.

  6. terrik

    Thread Starter New Member

    Feb 23, 2013

    I tried what you said and it worked. But, before that I bought some TIP120 (darlington) in a local store to replace the C2654, and it worked perfectly. Then I tried to compare the behaviour of the TIP120 VS the C2654 to confirm that the transistor wasn't a good choice for circuit. And then the C2654 was working also fine. WHAT!? Maybe my cheap multimeter was messing around, maybe the USB wasn't giving the 0.5A, I don't know. Connecting the gate at 12V immediately runs the motor, so transistor piout discarded. I checked so many times the wiring... :( Maybe I just needed a morning out with the bike and your help.

    Anyway, now, connecting the 5V from the Arduino to the base directly (just testing) fully saturates the power transistor, but doing so shutdown the board in less than a second. So, using the 30 ohms resistors, patricktoday suggested, works like charm.

    I'm going to put 8 diodes around the two H bridges thanks for the head up. Also, I saw somewhere a cap in parallel with the diode, is that a good idea?

    Thank you for all the replies.