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Circuits / Transistors

DC motor speed with a transistor

Started, the motor draws nearly its stall current and speeds up with a time constant of its inertia; turning, its back-EMF rises with the speed until it nearly balances the supply, and the current falls

DC motor speed with a transistor, simulated on a breadboard

Tap a part to see its voltages and currents. It is a model, not a measurement.

What do you need?

How does a transistor control a DC motor's speed?

A TIP120 is a Darlington: two transistors in one case, so a tiny base current controls a much larger one. Here it works as an emitter follower: the motor gets the voltage at its base less about 1.3 V. With the knob at 80 %, the base sags to 3.63 V, so that is 2.35 V and about 3750 rpm, with only 0.14 mA into the base. The current is then about 39 mA, low because a spinning motor is also a generator: its back-EMF, 2.05 V here, opposes the supply, and only the difference pushes current through the winding. At the start there is no back-EMF yet, so the current jumps to about 270 mA and falls as the motor speeds up, in about 100 ms. A load does the same: at 1.5 mN·m the motor stalls and takes about 276 mA. The voltage the motor does not use falls across the transistor: 0.1 W of heat. D1 (1N4007) takes the winding's current if the transistor cuts off suddenly.

Values from the Protoboard ISO simulation: a model, not a bench measurement.