Methods for Easily Driving Brushed DC Motors
In this column, we will explain, starting from the basics, methods for driving brushed DC motors. It is likely that nearly all readers have connected a battery to a motor and run the motor, perhaps in a grade-school science experiment or in shop class, for example. And not a few people have probably built a circuit that is turned on and off with a switch, or have build a model, toy, or the like that can be turned on and off. I'll begin with these kinds of circuits and devices.
When a Motor is Connected to a Power Supply, It Runs; When Disconnected, It Stops
First, we consider a case in which a brushed DC motor in only rotated one direction and then stopped. When the brushed DC motor is connected to a power supply, it rotates, and when they are disconnected, after idle rotation, the motor stops. But, this of course will not suffice, so instead please imagine that a mechanical switch is inserted in the power supply line connected to the motor to manually turn the motor on and off. Such a circuit appears on the left in Fig. 1.
In order to upgrade this electronically, the switch (SW1) can be replaced with a transistor. An example of this appears on the right in Fig. 1, where the transistor is an N-channel MOSFET. By applying a voltage to the gate of the MOSFET Q1, to turn the MOSFET on and off, the brushed DC motor can similarly be rotated and stopped.
A Simple but Important Point
Here, a word of caution. In both of these circuits, at a moment when the switch is turned off, a high voltage (a back emf) occurs so as to cause current to continue to flow in the motor coils. In particular, when a transistor is used, if a voltage exceeding the maximum rating for the transistor is applied, transistor degradation or destruction occurs. Hence such voltages must be suppressed. To do so, a diode must be connected in parallel with the motor, as shown in the circuit diagram. This gives us something that looks a bit more like an electronic circuit.
The Switch May be Inserted on Either the Positive (+) or the Negative (-) Side
In this way, by using a single switch we can achieve our initial goal of operation of only "rotating the motor in one direction and stopping the motor." In this case, the switch may be on either the positive (+) or the negative (-) side of the power supply. Examples of these appear in Fig. 2. When using a MOSFET, to facilitate transistor driving (turning on/off), a P-channel MOSFET is used on the (+) side, and an N-channel MOSFET is used on the (-) side.
As previously explained, regardless of the side of the power supply on which the switch is inserted, the inductance (coils) of the motor causes a current to continue flowing immediately after turn-off; when the switch is on the (-) side, the (-) terminal of the motor falls below the (-) side voltage (in general, below GND potential), and when the switch is on the (+) side, the (+) terminal of the motor rises above the (+) side voltage (the power supply voltage). In other words, the voltage occurring at turn-off is higher than the power supply voltage. For this reason, in both cases a diode must be connected in parallel with the motor when using a MOSFET, as shown in the circuit diagrams, to suppress (clamp) the back emf that occurs in the diode forward direction.
These circuit configurations are easy and simple, using only a single transistor, but on the other hand rotation is only possible in one direction, and when turned off, some time passes during idle rotation before the motor stops. And there is the further matter that a diode is needed for back emf clamping.
In contrast, there are driving methods using H bridges that can provide further functions such as the ability to select the rotation direction and the ability to brake when stopping the motor. There are also PWM driving methods that enable control of the rotation rate (these methods can be used in both single-switch driving and in H-bridge driving). Explanations of these are provided in the Tech Web Motor "Basic Knowledge" section, available here.