Need help with motor control

[LCW]

Hi everyone,
I'm new to PLC and just got a T100MD888. I finished the tutorial on how to create the ladder logic and tbasic. I'm now trying to control a 5V motor using ON/OFF from the PLC. Using the online monitor and pause, I can click to turn digital output channel 1 on and off at will. I have connected the motor (with its own power source) to channel 1, and the ground to the PLC 0V.

Problem now is when I complete the circuit by connecting the motor to channel 1, the motor always turn on. Channel 1 and 2 LED will come on. No matter what I tried - turning channel 1 and 2 off, etc, the motor always runs. In short, motor always runs as long as PLC is on. What I'm trying to achieve is using a channel to turn the motor on and off.

Is is normal? I must be missing something   All help is appreciated.

[support]

T100MD888+ outputs are NPN type, which means it sinks current from the load to 0V (as oppose to sourcing current to external load). Ideally the motor should be using the same voltage as the PLC's power supply (12 to 24V DC), that way you can simply connect the + terminal of power to + terminal of DC motor and - terminal of DC motor to the PLC's output. Please refer to the T100MD888+ installation guide (white 8 page booklet that come in the box) for the wiring method.

If your motor is 5V, then you will need an external 5V power supply to connect to the motor "+", and the motor "-" will connect via a diode to the PLC output:

+5V ------(Motor)-------|>|-----o PLC output.

The diode is needed here to prevent the current from the internal pull up (approximately 22V for a 24V power supply) of the PLC output from flowing in reverse towards the +5V power source.  Without the diode although the motor can still be turned ON and OFF the output LED will be lit all the time.

[LCW]

Thank you! That did the trick. Is it possible to reverse the current to make motors turn the other way?

[support]

Reversing current to motor flow to control direction is not easily done. You need a H-bridge which can be formed by using two relays each with two independent contacts. The PLC must control the two relay coils and interlock them so that the two relays will never be on simultaneously (which can lead to direct short circuit of the power supply). For more details seach for H-bridge Google.

[Joel Moore]

I'm just using a single DPDT relay to reverse a motor.



Any reason why I should expect trouble?

[support]

This appear to be a clever way of using a single DPDT relay to achieve the H-bridge effect. In this case the output transistors served as the the lower half of the H-bridge and the relay serve as the upper half.

I think the two outputs shown on your diagram could be the same output. So only one additional output required to switch the relay.

[andrew_holt]

I would suggest a relay to isolate the power before switching direction.

Andrew