Motor forward and reverse is a very common seen application in our daily life, for example, we can use DC motor to control garage door open and close, gate extract and retract, curtain up and down etc, these all have one thing in common, the DC motor movement in two directions.
While some motors have this feature built-in, most of them we seen today doesn’t, so we may need a little trick to achieve the goal.
The trick involves a DC motor and two SPDT relays – the most common seen one which has three pins marked N/C COM N/O, and the matching circuit to control the two SPDT relays, or you can buy our wireless remote relay SR-RCS-202U, which has integrated two SPDT relays and control mechanism.
As you can see from above schematic diagram, the connection is very simple, and here is a short description on how to achieve motor forward and reverse using above connection.
The short principle is only one relay activated at one time, no more, no less.
When, for example, relay 1 activated and relay 2 de-activated, NO1 connected to COM1 and NC2 connected to COM2, battery current flows from positive contact -> NC2 -> COM2 -> DC motor’s lower contact -> DC motor’s upper contact -> COM1 -> NO1 -> battery negative contact
And when relay 1 is de-activated and relay 2 is activated, NC1 connected to COM1 and NO2 connected to COM2, battery current flows from positive contact -> NC1 -> COM1 -> DC motor’s upper contact -> DC motor’s lower contact -> COM2 -> NO2 -> battery negative contact
As you can see, we have switched the polarity connection to DC motor by switching the on-off status of SPDT relays, thus simply can control the garage open or close etc.
However, simple switch between relay status, such as Relay 1 on Relay 2 off -> Relay 2 on Relay 1 off may cause damage to DC motor, especially when the load is high, since the relay switch is very quick compared to motor mechanical move forward to stop and then move backwards.
We have solution by controlling the relay switching by software programming, to add middle transition status – both relays off in between, so the new working logic has become Relay 1 on Relay 2 off -> Both Relays off -> Relay 2 on Relay 1 off, and we can further add a timer to better protect our DC motor.
Another factor to notice is since current to motor goes through both relays, it is very important to make sure both relay rating is enough for DC motor, and make sure the wires or PCB connection is enough too, or sometimes too high motor load current would cause damage to our control board, especially to the PCB track on board.