Bidirectional motor control

Posted on Feb 7, 2014

One of the simplest ways to get a motor to turn in both directions is by using a double-pole, double-throw (DPDT) relay. Along with the relay, this hookup requires two transistors and two Stamp pins, one for on/off control and the other for direction control. The diagram below contains the schematic for this setup. The DPDT relay is switching thedirection of current flow through the motor to get it to

Bidirectional motor control
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turn in either direction. The direction control transistor can be a 2N3904 because most relays require much less than 100 mA through the coil to trip the contacts. A TIP120 can also be used for the direction control. A TIP120 is required for on/off control because it must be able to handle the motor currents. For very high current motors, replace the TIP120 on/off control with a low on-resistance FET or with a SPST relay. In the diagram, the location of the relay pins matches a top view of the DPDT relay sold in the Robot Store (Jameco 174378) and the Radio Shack 275-249A relay. If your program hangs when you switch the motor off or change directions, you might be getting spurious voltage spikes that are causing your program to jump into never-never land. You`ll know this is happening when the motor keeps spinning when you expect it to stop or change direction. After confirming that this is indeed what is going on and that you don`t have any wiring errors, try adding an 0. 47 micro-farad, 100 V filter capacitor right across the motor terminals. If things are still bad, add a flyback diode between the two common pins on the relay (making sure that the plus or band end of the diode points toward the plus voltage of the battery). Consult the Controlling motors technote for additional info on noise suppression. If things are still not working, you may have to opto-isolate the circuit. With two SPDT relays and two 4N33...

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