Simple Controls For DC Motors (MEC009E)

Simple dc Control

The simplest way to control a dc motor (or other load) is by adding an SPST switch in series as shown in Figure 1.

Figure 1 – The simplest control

This is an On/Off control using only three components.

When the switch is open, no current flows and the motor is off.

With the switch closed, the current can flow across the motor, and it is on.

Note that the switch can be wired in series with the circuit in either the positive or the negative line.

Reversing the Direction

The direction in which the shaft of a motor moves depends on the direction of the current.

We can reverse the current direction with a DPDT switch as shown by Figure 2.

Figure 2 – Reversing the direction

The power supply represented in this case is a battery, but you can use any dc voltage source.

Two-Way Control

The basic circuit shown in Figure 3 is very interesting for robotics and mechatronics applications.

Figure 3 – Two Way Control

With this block, we can control a motor from two different points or sensors.

Any of the switches (relays or sensors) can turn the motor on or off independently.

The circuit is the same one used to control lights from two switches in the wall.

Special two-way switches must be used in this application. or sensors with two contacts.

In a mobile robot, one sensor can be place in the front and another in the back.

These can be used to detect obstacles and feed information to a decision circuit.

Controlling Two Motors with One Switch (I)

This circuit can be used to control two motors from one switch or sensor. With the switch in position A, motor M1 is on, and with the switch in position B. motor M2 is powered.

Combining this circuit with the previous, we have a complete control for two motors-reversing directions, turning them on and off, and selecting which one is activated. The circuit uses a single pole, double throw switch as shown in Figure 4.

Figure 4 – Controlling two motors (I)

Controlling Two Motors with One Switch (II)

The difference between this circuit and circuit 4 is that we can use only a pair of wires to control two motors instead three as in the other configuration.

The circuit shown in Figure 5 uses diodes, selecting the motors to be powered according the direction of the current flow.

Figure 5 – Controlling two motors (II)

When the switch S1 is in position 1, D1 is forward biased, and motor M1 is powered on.

When S1 is placed in position 2, D2 is forward biased and motor M2 is on.

The diodes are mounted near the motors and their positions must be chosen according the desired direction that the motor must turn.

Diodes 1N4002, or others of the same series, are suitable for motors up to 1 A.