Need a simple circuit analyzed for its troublesome behavior

jkbrand

Joined May 8, 2021
29
A newbie here, so if this isn't the proper forum, please let me know.

My project involves a DPDT switch, 2 limit switches and a 12Vdc motor.

The DPDT switch is wired as shown below. I copied a circuit on the net that I thought would meet my simple requirements: pressing the momentary
(ON-OFF-ON) switch one way and the motor drives an arm until it hits a limit switch. Pressing the switch the other way and the DC motor reverses
and travels until the arm hits its limit switch. MY PROBLEM: once the arm hits either switch, the NC switch opens, the arm immediately stops--as
expected--but pressing the switch to the other side does not energize the motor.

The limit switches in the illustration below are NC. I can alternate presses of the switch (left and right) and the motor will reverse directions with
no problem, UNTIL either of the limit switches is tripped. The arm stays stuck, as if both limit switches were open.

I can't understand this behavior and am hoping a simple explanation/solution will be obvious to those more analytically aware than I. TIA of
any help here.

AlbertHall

Joined Jun 4, 2014
12,397
The circuit as drawn should work as you expect so your problem suggests you have something wired incorrectly. Check your circuit carefully against the diagram.

jkbrand

Joined May 8, 2021
29
The circuit as drawn should work as you expect so your problem suggests you have something wired incorrectly. Check your circuit carefully against the diagram.
Thanks. It's good to know the wiring shown in the drawing is functional. I was having doubts on my ability to understand a
simple circuit. I'll have a friend who knows the issue I've been dealing with, and he suggested the same thing, ie, checking
the wiring. I will ask him to check the circuit against the diagram. A different set of eyes might discover something I've
overlooked.

MisterBill2

Joined Jan 23, 2018
19,474
Indeed. The circuit as drawn should work. So I am thinking that it is in how the supply connection is arranged at the limit switch terminals that is your problem. On switch is opening both supply connections.

jkbrand

Joined May 8, 2021
29
Indeed. The circuit as drawn should work. So I am thinking that it is in how the supply connection is arranged at the limit switch terminals that is your problem. On switch is opening both supply connections.
I am so befuddled. I have the common leads on both limit switches tied together. From there they connect to the DPDT switch at terminal 4 then to negative supply. The positive leads (NC) from the limit switches are tied together, connected to terminal 1, which is jumped to terminal 6 then to positive supply. The On switch operates in both directions, but when either limit switch
makes, the supply side appears to open. Is the tying together of the common and positive leads on the limit switch the cause of my problem???

AlbertHall

Joined Jun 4, 2014
12,397
I have the common leads on both limit switches tied together. From there they connect to the DPDT switch at terminal 4 then to negative supply. The positive leads (NC) from the limit switches are tied together,
This sounds as if the limit switches are connected in parallel which would be wrong but I don't see how that would cause your problem.

Can you take a picture (or pictures) which would show all the connections?

crutschow

Joined Mar 14, 2008
34,855
I have the common leads on both limit switches tied together.
The positive leads (NC) from the limit switches are tied together
That puts them in parallel, which is not what you want, and is not what your wiring diagram shows.

MisterBill2

Joined Jan 23, 2018
19,474
The positive supply does not connect to the limit switches in the circuit drawing and it should not connect in reality, either. Sothat could be a problem. One terminal of each limit witch should connect to the power source common negative, as shown, and the other terminal should connect to an end terminal of the direction switch, as shown.

jkbrand

Joined May 8, 2021
29
That puts them in parallel, which is not what you want, and is not what your wiring diagram shows.
Can the commons be tied in parallel if the positive sides of the limit switches wired in series?? Or
do both limit switches need to be wired in series?

crutschow

Joined Mar 14, 2008
34,855
Can the commons be tied in parallel if the positive sides of the limit switches wired in series?
They can have commons tied together as shown in your diagram.
Are you trying to deviate from the diagram you showed?

MisterBill2

Joined Jan 23, 2018
19,474
Can the commons be tied in parallel if the positive sides of the limit switches wired in series?? Or
do both limit switches need to be wired in series?
NO!!! Tying one side of each limit switch to the supply negative DOES NOT put anything in parallel! What exists in whatever connections are actually made, which is not the circuit shown, is that somehow the limit switches are now in series. If the circuit becomes wired as the drawing shows, it will function as desired.

jkbrand

Joined May 8, 2021
29
The positive supply does not connect to the limit switches in the circuit drawing and it should not connect in reality, either. Sothat could be a problem. One terminal of each limit witch should connect to the power source common negative, as shown, and the other terminal should connect to an en d terminal of the direction switch, as shown.
I chose to break the common side, which I assume you believe is OK. You say the positive supply does not connect to the limit switches (and "should not"). You then wrote "that could be a problem." Can you clarify what the "problem" could be? I will make
the change(s) recommended for the other limit switch terminals to their appropriate DPDT sides. Thanks!

jkbrand

Joined May 8, 2021
29
They can have commons tied together as shown in your diagram.
Are you trying to deviate from the diagram you showed?
Not intentionally, but I can see why you'd ask. I think I have a better understanding now.

MisterBill2

Joined Jan 23, 2018
19,474
Looking back at post #1, which if that is thge way it actually is wired, it will work as you hope. Now I hear about commons going to the directional switch, and I am finding it a bit confusing. Are you meaning the "C" terminals on the limit switch? Or the terminals of the limit switch connected to the power source negative, also referenced as "common." Most limit switches have 3 terminals, N.O (normally open)., Common, And N.C. ( normally closed to common.

Post#9 is so very confused that it tends to muddle the discussion quite a lot. I am guessing that some confusion goes with "common" on the limit switches versus "common referencing the power supply negative terminal.
But once again, if it is in fact wired as the drawing in post#1 shows, it will work.

Last edited:

jkbrand

Joined May 8, 2021
29
Looking back at post #1, which if that is thge way it actually is wired, it will work as you hope. Now I hear about commons going to the directional switch, and I am finding it a bit confusing. Are you meaning the "C" terminals on the limit switch? Or the terminals of the limit switch connected to the power source negative, also referenced as "common." Most limit switches have 3 terminals, N.O (normally open)., Common, And N.C. ( normally closed to common.

Post#9 is so very confused that it tends to muddle the discussion quite a lot. I am guessing that some confusion goes with "common" on the limit switches versus "common referencing the power supply negative terminal.
But once again, if it is in fact wired as the drawing in post#1 shows, it will work.
I agree that my post #9 is confusing. When referring to 'common' in that post I was referring to the limit switches. When it was
pointed out that I had wired incorrectly the other side of the limit switches directly to one another and then directly to the + side of the supply (and NOT through the switch), I was hopeful. The other side of the limit switches now both go through the DPDT switch, albeit on opposite sides. Strangely, (to me at least), the DPDT switch drives the motor in both directions as it should, but it still gets stuck when either limit switch is allowed to trip from NC to NO. As frustrating as this is, please know I do appreciate the support I'm receiving. I'm thankful knowing I'm much closer now than I had been before posting.

MisterBill2

Joined Jan 23, 2018
19,474
That description tells me that the problem is that the limit switch wiring is not correct. Are both limit switches the same type? Is the DPDT , center off, switch the standard configuration with the middle terminals connecting to one side or the other side? Do you have a continuity checker of some kind, either a meter or a beeper, or even a light will do. If so, I will be able to suggest some checks to be done.
Is anything else connected, like diodes or capacitors, that do not show on the drawing?
And is it possible to re-post that same drawing, but with all of the terminals identified by numbers on the DPDT switch, and by the switch markings on the limit switches? That would make future discussions much simpler.
And one final suggestion is to use a separate wire from each limit switch to the power source terminal.

jkbrand

Joined May 8, 2021
29
Yes, the limit switches are the same (ordered a pack of 6) manufacturer. The DPDT is momentary ON-OFF-ON. The middle terminals go directly to the motor. I'm not sure what you mean by "middle terminals connecting to one side or the other." Yes,
I have a continuity checker and meter. No other electrical components are in the circuit. I will add the terminal identification on the DPDT switch and will repost. I will also take a picture of the limit switch and will annotate the three contacts. Finally, you suggest separate wires from the limit switches to the power source terminal. Does that mean no tying together of the "common" terminals on the limit switches, or does your suggestion apply only to the other (+) sides of the limit switches? One final clarification: when I press either side of the DPDT switch, the motor energizes properly. If I press the other side before the arm hits the limit switch, the motor reverses fine and continues to allow me to alternately change directions. It's only when the arm hits the limit switch that I'm not able to reverse direction. I hope that clarifies things somewhat. . .

MisterBill2

Joined Jan 23, 2018
19,474
My point was, and still is, that since the motor can be operated as intended with reversing by the switch, and that, it seems, operating either limit switch prevents motion in both directions, somehow the limit switch connections are not the same as the circuit drawing.
So here is one more experiment:: With the mechanism not at either limit switch, manually operate the limit switches and verify that either the motor will, or will not, run in the opposite direction with one limit switch activated. And do this for both limit switches in both directions.
AND now I am wondering if the motor/mechanism has some built-in limit switches somehow. I have seen that arrangement at least once.

KeepItSimpleStupid

Joined Mar 4, 2014
5,088
There is a more complex design, I posted in this https://forum.allaboutcircuits.com/...house-roof-window-control.134958/post-1131056 thread.

A 12V automotive relay is a good choice for the relays.

1. You get dynamic braking (motor stops instantly)
2. If FWD and REV relays are on or off at the same time, the motor is off.
3. The limit switches only have to handle the relay current. It's still around 150 mA or so.
4. There are signals for @ limit A, @ limitB and moving

You should stop the motor briefly before changing direction.