Reversing DC motor using relay and limit switches.

Thread Starter

rosiemk

Joined Jan 31, 2018
2
Hello,

I am new to circuit design and very much a novice and am trying to get my head around limit switches.

I'd like to make a belt drive system that is powered by a DC motor. The motor will turn in one direction until the belt reaches a certain point (hits a limit switch), and then the motor will turn in the other direction (until that limit), and so on. I was planning on fixing some sort of plate onto the belt itself to act as the contact with the two limit switches. There would also be a basic on/off switch for the whole thing.

I don't really want to use a microcontroller as I want the system to be as reliable as possible since it will need to be running for a number of weeks.

I think I've just about got my head around DPDT latching relays but I'm unclear how to reverse the polarity to the latching relay using the limit switches, to energise/de-energise the relay and reverse the polarity to the motor.

I'm aware there are a lot of threads relating to similar topics, but I'm unclear about the variations of each one and what's the best approach to take in this instance.

Any help or advice or just general chat would be greatly appreciated.

Thank you in advance.
 

crutschow

Joined Mar 14, 2008
27,231
Below is a simple circuit that should do what you want.
It consists of a DPDT relay, two SPDT limit switches, and 5 diodes.

  • The motor runs to the right with the relay de-energized as shown.
  • When the Right Limit Sw is activated, the relay coil is energized, reversing the motor direction to the left. This also latches the relay through the Left Limit Sw NC contacts and diode D1 to keep the motor running to the left after the Right Limit Sw is deactivated.
  • When the Left Limit Sw is activiated, the voltage to the relay coil is removed, de-energizing the relay and reversing the motor back to the right.
D1 keeps the Right Limit Sw voltage from being shorted through the Left Limit Sw and the relay contacts to ground.
D2 through D5 suppress the motor voltage transients to protect the relay contacts.

Note that the limit switches carry only the relay coil current, not the motor current.

What is the voltage and current rating of the motor?

upload_2018-1-31_23-58-55.png
 
Last edited:

Alec_t

Joined Sep 17, 2013
12,080
Ideally the motor should be allowed to come to a stop before being reversed. It may not matter for a small motor but would be necessary for a large one.
 

Thread Starter

rosiemk

Joined Jan 31, 2018
2
This is so helpful @crutschow, thank you.
Sorry to ask an obvious question...
Note that the limit switches carry only the relay coil current, not the motor current.
When the relay is de-energised and Left limit SW closed, the current doesn't run through the relay coil, because it's 'easier' ( ie. less resistance) for it to go to ground through the relay switches? Is this correct?

I'm not certain of the spec of my motor yet, as I wanted to get my head around the physics of it before trying it out. I do have something like this, but I think it might be overkill.

Ideally the motor should be allowed to come to a stop before being reversed
Can you achieve a delay with a capacitor and resistor, to allow the motor to stop?
 

Bernard

Joined Aug 7, 2008
5,788
Rotation specified as CCW which may mean it is not advisable to operate CW. The brushes might be mounted at an angle, reversible motors usually have brushes perpendicular to armature.
If not using external speed reduction, might consider a gear head motor.
 
Last edited:

crutschow

Joined Mar 14, 2008
27,231
When the relay is de-energised and Left limit SW closed, the current doesn't run through the relay coil, because it's 'easier' ( ie. less resistance) for it to go to ground through the relay switches? Is this correct?
Yes.
The resistance through the relay contact to ground is very low, so the voltage at that point is likely no more than a few tens of millivolts.
 
Last edited:

MisterBill2

Joined Jan 23, 2018
8,792
See the posting that I did for "change the direction of a 9V motor.

The function required can be provided by using two 3Pole Double Throw relays and 4 reed switches with 4 magnets. 2 of the reed switches must be normally closed and the other 2 normally open. Also a start button, NO, and a stop button NC are needed. I will call the two directions north and south. Each relay has a self-sealing circuit engaged by an NO reed switch and released by an NC reed switch. The slowdown at each end is by coasting after the NC reed switch is triggered by it's magnet.
The drive starts with the pushbutton latching the drive north relay, running the motor forward, until it passes magnet #1, releasing the drive north relay, and then the dolly coasts until it reaches magnet#2, which seals in the drive south relay. The dolly then rolls south until it reaches magnet #3, releasing the drive south relay, and then the dolly coasts until magnet #4 seals in the drive north relay, and the cycle repeats. The stop button opens the power feed to the latching contacts of both relays. I would send a circuit but I am not at a computer with the cad software to draw it today.
 
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