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Half Wave Rectifier Full Wave Rectifier in a DC Motor Control (MEC117E)

This article is part of my book Mechatronics, Robotics and Artificial Intgelligence (exhausted). Here I provide several suggestions on the use of diodes in motion controls, also based on articles I have published in other media, such as magazines in many countries.

When powering a dc motor from an ac power source (e.g., the secondary of a low voltage transformer), it is necessary to add a rectifier to the circuit. The complete block diagram of a half-wave rectifier with filter is shown in Figure 1.




Figure 1 – Half wave motor control
Figure 1 – Half wave motor control



The diode conducts only half of the semi cycles of the ac power line voltage. The capacitor filters (smooths out) the voltage pulses, keeping the voltage level applied in the motor as constant as possible. The value of the capacitor depends on the current drained by the motor in these applications. A simple rule for motors powered from 3 to 15 V is adopt 1,000 ?F of capacitance for each ampere of current. For instance, use a 470 ?F (500 ?F) for a 500 mA motor. Using this circuit, you can powered your application from such ac supplies as small transformers.



Full-Wave Control


The circuit shown in Block 6 uses only half of the semi cycles of the ac voltage. If you don’t intend to use the other semi cycles in another part of the circuit (to control another motor for instance), it would be better use the configuration shown in Figure 2.



Figure 2 – full have control
Figure 2 – full have control



The filter capacitor is chosen in the same manner as described in the half wave control. This circuit can also be used in dc circuits to keep the direction of a motor independent of the polarity of the voltage source, as shown in Figure 1.


In this figure, we show a circuit in which a single switch is used to reverse the dc power supply, reversing the direction of motor M1 but not M2. M2 is wired to the same line, but it runs only in one direction, thanks to the design of this block.