Question: What’s the easiest way to operate (stop/rotate) DC brush motors?
Answer: Simply inserting a switch between the DC brush motor and power supply will enable the ON/OFF operation, allowing the motor to stop and rotate in one direction. The same operation can be achieved by replacing the switch with a semiconductor. However, the moment the switch is turned OFF the coil will attempt to continue to supply current, generating a substantial amount of voltage. Therefore, in order to minimize voltage and prevent damage to the semiconductor a diode must be connected in parallel to the motor.
Figure 1. DC Brush Motor Switch Circuit Examples
A switch will make it possible to control the ON/OFF operation by connecting the motor to a DC power supply or disconnecting the power supply by opening the circuit. When the switch is turned ON the motor will rotate in one direction. Conversely, turning the switch OFF cuts power to the motor, putting it into idle mode. The switch can be inserted at either the ground or power supply side, and when using a power transistor as the switch it becomes a part of the actual electronic circuit. Figure 2 shows circuit examples, including an Nch MOS connected to the ground side ② and a Pch MOS at the power supply side ④.
However, in an actual circuit due to the inductance of the motor current will to continue to flow immediately after the motor turns OFF, causing the voltage to swing to below the ground level at the negative (–) terminal and above the supply voltage level at the positive (+) terminal. This can result in the generated voltage of the motor exceeding the supply voltage. To prevent this, when using a power transistor as the switch it is necessary to connect a power diode in parallel in order to suppress (clamp) the generated voltage using the forward voltage of the diode.
Figure 2. DC brush Motor Switch Circuit Examples
The advantage of this type of circuit is that configuration is simple, requiring just one power transistor. There are some drawbacks, however, including the fact that the motor can only rotate in one direction. Plus, it takes some time to stop, and a power diode is needed for clamping EMF.
However, by configuring the MOS Gate for pulse drive PWM operation becomes possible. Equivalently, varying the voltage supplied to the motor (supply voltage x on duty ratio) allows control of the motor speed. With PWM drive, when the power transistor is turned OFF the regenerative current will flow through the diode connected in parallel to the motor. Therefore, it is important to take into consideration the diode power consumption so that the package power is never exceeded.
