In regard to the use of external diodes, people often wonder when power supplies are used to energize DC motors. Most people know that a diode has to be used, but are unclear where to place them or why to use them.
Concerning power supply, it is intended to introduce two types of DC motors: brushed DC motor and brushless DC motor.
Brushed DC motors
In a brushed DC motor, the magnets are stationary and the coil rotates. Electricity can pass through the rotating coil by the use of “brushes”.
The advantages of brushed DC motor are low initial cost and easy speed control.
When at outage, the motor coil will act like an inductor and continues to produce current, effectively becoming an inverted voltage source. This will apply a reverse polarity to the power supply and can cause damage.
By using a diode, as shown in the following diagram, the diode provides a current path for the reverse motor current and will clamp the reverse voltage to a level no greater than the forward voltage drop of the diode, protecting the power supply’s output capacitors and other components from being stressed by the reverse voltage.
Brushless DC motors
Brushless DC motors have permanent magnets that rotate and the armature is stationary. Though they are more expensive, they have many advantages compared with brushed DC motors. For example, they have no brushes so that there is no brush or commutator wear problem. And they enable the position control to be more precise.
When the motor is turned off or reversed, it will act as a generator and produce a high voltage spike, which may cause the power supply’s overvoltage protection to trip, shutting down the unit.
Fortunately, by using a diode in series with the output, as shown below, the spike will be blocked from interfering with the power supply.
In general, a general purpose diode can be used in both cases providing that the voltage and current ratings for the diode are correctly calculated.