H-Bridge Using Remote Relay Board with Limit Switches - Works but missing something

Thread Starter

tandarco

Joined Mar 4, 2019
15
Hello,

I'm working on a project that uses a linear actuator, remote controlled relay board and 2 limit switches. There are two relays configured in an H-Bridge remotely controlled. When I hold the "UP" button (I have to keep it depressed) on the remote the linear actuator lifts a table upward until it hits the upper limit switch at which point it stops. When I hold the "Down" button on the remote the linear actuator lowers the table until it hits the lower limit switch at which point it stops. So it seem the wiring is working properly but I have a problem. When either limit switch is activated, the power to the motor gets cut, but the motor is coasting an extra 5-10mm and crushing the limit switch. Here is what is confusing me. Let’s say I am holding the "Up" button and the lift is moving upward, before it hits the limit switch if I let go the button the motor stops immediately(dead stop) and does not coast. I'm trying to figure out how to stop the coasting and make the motor stop when it hits the limit switches. Why does it stop immediately in one circumstance but not with the limit switches? Take a look at the wiring diagram attached. Any feedback is appreciated. Thanks. Frank
 

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MisterBill2

Joined Jan 23, 2018
11,948
Two options come to mind, the first is to adjust the limit switches so that the coasting does not crush them. Limit switches should never be in a position to be damaged by over travel. The second thought is to arrange for the H-bridge to short-circuit the motor when it is stopped, which should stop it much more rapidly with no coasting. Either approach should solve the problem.But dynamic braking does put a bit of stress on motors.
 

Thread Starter

tandarco

Joined Mar 4, 2019
15
Thanks for the information. How would I arrange the H-bridge to short-circuit the motor when it is stopped by hitting the limit switch?
 

BillB3857

Joined Feb 28, 2009
2,561
If you use the switches to control the relay coils instead of the power to the motor, with both relays dropped out, shorting the motor leads will result in dynamic breaking which will help to stop the actuator sooner.. The UP limit could be placed in series with the UP push button and the DOWN limit could be place in series with the DOWN switch.
 

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Thread Starter

tandarco

Joined Mar 4, 2019
15
If you use the switches to control the relay coils instead of the power to the motor, with both relays dropped out, shorting the motor leads will result in dynamic breaking which will help to stop the actuator sooner.. The UP limit could be placed in series with the UP push button and the DOWN limit could be place in series with the DOWN switch.
I understand what you are proposing and it makes sense. The current relay board I have has its own power supply which feeds both relays so I cannot individually control the power to each relay separately without hacking the pcb board they are on (maybe an option). I'm going to have to look at some other options. There weren't many that I found that had a remote interface. Thanks.
 

Thread Starter

tandarco

Joined Mar 4, 2019
15
I checked all connection for NO/C/NC. I just crimped some wire and made the connections you mentioned above. So the lower limit switch now stops on a dime perfectly so that corrected lowering the table. However the upper limit switch is still making the motor coast.
 

crutschow

Joined Mar 14, 2008
29,844
A better way, which would also have eliminated any arcing of the contacts due to motor inductance, might have been to add a diode bridge to carry the motor current when any switch opens, as shown below
upload_2019-3-4_21-46-54.png
 
Last edited:

MisterBill2

Joined Jan 23, 2018
11,948
Thanks for the information. How would I arrange the H-bridge to short-circuit the motor when it is stopped by hitting the limit switch?
If both lower sections of the H-bridge were switched on at the same time that would provide the short circuit. If the released sides of the relay switching circuit driving the motor were connected that could also provide the short-circuiting function. BUT you must be careful to avoid accidentally short-circuiting the supply voltage. So the circuit arrangement will need some careful thinking. And now, after seeing the most recent posts, it seems that there could also have been a connections in the wiring issue.
 

Thread Starter

tandarco

Joined Mar 4, 2019
15
A better way, which would also have eliminated any arcing of the contacts due to motor inductance, might have been to add a diode bridge to carry the motor current when any switch opens, as shown below
View attachment 171591
Thanks for the alternate concept. I will look into this as I read about using diodes but wasn't sure how to apply it to my specific setup. In the longer term I would want to prevent any arcing...
 
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