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Problems with electrical circuit to control 2 motors

  1. Nov 28, 2012 #1


    The above picture shows the electrical circuit I'm using for controlling a DC-motor and a servomotor.
    The power source is a 12V AC to DC adaptor.

    At the right side of the drawing, you see (from left to right) a potentiometer and two switches, which are connected to 3 pins of the Arduino, the ground and a 12V->3.3V transformator. These enable the user to control the DC- and servomotor.

    After processing these input signals, the arduino sends out the necessary signals to two FET's with a threshold voltage of maximam 2.1V (or something very close). One is controlling the relay so the motor can be made to turn left or right, and the other one acts as a 'power switch' for the DC-motor: when it's opened, the motor doesn't draw any current and vice versa.

    At first, I used 2 NPN FET's. The sources of both FET's are connected to ground, the two gates are connected to two digital arduino pins. The drain of the FET acting as a power switch for the motor is connected to ground and the output of the relay. The drain of the other FET is connected to ground and to the control input of the relay.

    To control the FET's, the pins on the arduino send out a sigal of either 0V or 5V.

    Alas, I keep encountering a major problem with the DC-motor - the input switches and the servomotor are not yet tested. The DC-motor drains current, but doesn't react to changes of the FET's. Also, when measuring the voltages on the FET's pins (compared to ground), the voltage on the drain is 0V when the FET should be closed and about 0.7V when it should be open (or high resistance). The pins on the arduino do their job just fine - although when LOW, the pins give a voltage of about 0.12V, so not exactly 0V.

    Do any of you guys know what causes this problem? Is the network in the drawing fine, or are there some mistakes? Also, when replacing the 'power switch' FET with a PNP FET, the motor does stop and start. Why so?

    Thanks in advance !
  2. jcsd
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