By David Norton, TDK-Lambda Americas
Often, there is confusion regarding the use of external diodes when power supplies are used to power dc motors. Most people know that a diode has to be used, but are unsure where to place them or what their purpose is.
From a power supply concern, diodes may be used to prevent reverse motor current and blocking spikes that interfere with the power supply source. To illustrate, here are examples for brushed dc motor and a brushless dc motor.
Brushed dc motors
With this type of motor, permanent magnets are stationary (stator) and the armature is comprised of coils that rotate. Electricity is connected to the spinning coils by the use of “brushes” that alternately reverse each coil’s polarity, causing the armature to rotate. The advantages of this type of motor are low initial cost and easy speed control.
When the power is interrupted, the armature coils will act as inductors and will try to continue to produce current, effectively becoming an inverted voltage source. This will apply a reverse polarity to the power supply and can cause damage (back EMF, electro-magnetic flux).
By using a diode, as shown below, 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. This protects the power supply’s output capacitors and other components from being stressed by the reverse voltage.
Brushless dc motors
Brushless dc motors, often referred to as BLDC motors, have permanent magnets on the rotor and the stator coils are fixed. Although more expensive because it requires an integral controller to sequentially activate the stator coils that causes the rotor to move, it is more reliable in the long term as there are no brush or commutator wear, position control is more accurate and it is more efficient.
When the motor is turned off or reversed, it will act as a generator and produce a positive high voltage spike. This spike can cause the power supply’s overvoltage protection to trip, shutting down the unit.
By using a diode in series with the power supply’s output, as shown below, the spike will be blocked from interfering with the power supply.
With either brushed or brushless dc motors, a general-purpose diode can be used to protect the power supply providing it is sized to handle the maximum voltage and current for the application.
Filed Under: Design World articles, Electronics • electrical, Motors • dc, Power supplies
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