DC motor direction control using relay circuit

A Relay based DC motor controller works with an H-bridge arrangement. With an H-bridge circuit, the polarity across a load can be altered in both directions.

H bridge circuit diagram

In Dual SPDT motor driver circuit, the DC motor terminals are connected between the common poles of the two relays. The normally closed terminal of both relays is connected to negative or ground. And the normally open terminals are connected to the positive terminal.

The coil terminals of the relay are connected to the supply with a push switch. The Switch S1 and S2 control the relay 1 and relay 2 respectively.motor direction control using relay

In the circuit diagram shown, the switch S1 is ON and switch S2 is OFF. So, the motor terminals will have a positive polarity on the left side and a negative polarity at the right side. Thus, the motor turns in a clockwise direction. Similarly, when the S2 is ON and S1 is OFF, the motor turns in an anticlockwise direction.

Motor modes of operation

S1 S2 Motor Movement
0 0 Motor brakes
1 0 Motor moves right
0 1 Motor moves left
1 1 Motor brakes

When two switches are open, both the relays are will be in a normally closed position. Which makes the ground or negative across the motor terminals, that is the same polarity on both sides. Similarly, when both switches are closed (here both terminals will have positive voltage) the same polarity will obtain at two sides. On both conditions, the motor terminals are shorted and the motor brakes.

In a relay H-bridge, the motor terminals have no any free floating terminal states. So a coast mode will not occur at any instant.

DC motor reversing circuit using DPDT relay

The direction of a DC motor can be controlled with the input polarity at the motor terminals. So here we are using a simple DPDT relay arrangement for polarity alternation.

In the circuit, the motor terminals are connected between the two common poles of the relay. The normally close terminal of the first pole is connected to the positive terminal and the normally open to the ground. Similarly, the normally open terminal of the second pole has connected to the ground and the normally closed to the positive terminal. That is, both normally open and normally closed terminals at a side are connected to opposite polarities.dc motor reversing relay
When the switch is OFF, the motor has positive on the left side and the ground on the right side. Hence the motor rotates in a clockwise direction or towards the right direction.
When the switch S1 is pressed, the common pole gets connected to normally open. Then the polarity of the motor get reversed and the motor rotates in a counter-clockwise direction.
Whenever the switch has pushed the polarity reverses, thereby the motor direction also changes with the same. In the remaining time when the switch has released the motor always turns in the clockwise direction.
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