Dual Motor Speed Controller
Circuit : George Quirk, Albuquerque, NM
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A higher power dual motor control circuit capable of driving motors up to 24V DC and 5 Amp current.

Based on the original constant motor circuit George has modified this circuit for dual motors and changed the components to drive higher power motors. The LM158 or NTE928M can both work up to 32Volts DC and will swing output voltage to within a couple of volts of the supply voltage. The output current is boosted up to 5 Amps by the TIP120 Darlington transistor, wired in emitter follower. The base-emitter voltage drop varies but is typically 2.5Volts at 3A load so with the maximum supply voltage of 32V DC about 27.5V would be delivered to the load. The collector-emitter saturation voltage is also higher in a Darlington and varies with collector current, typically 2V at 3A and 4V at 5A. This should be kept in mind when choosing your motor.

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Maximum Ratings
It is important that the maximum power dissipation of the darlington transistor is not exceeded. For the TIP120 this is a maximum collector-emitter voltage, Vce up to 60V, a collector current, Ic of 5A and maximum dissipation of 65W. If in doubt measure the collector current and then voltage across the collector and emitter terminals. At all positions of the speed control, the product of Ic x Vce must not exceed 65W.

TIP120 Datasheet

As with anything that can create a spark, a running motor is a source of interference. To minimize the high frequency noise a 22uF capacitor is connected across the motor armature. The 1N4002 prevents any back EMF from damaging the darlington transistor.

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