Need a simple circuit analyzed for its troublesome behavior

I know the diode solution works. I've used it before. In the system I built in the late 80's early 90's, I used the diode method for a small motor. In the same thing, I used an LM317T as the motor power supply and I rigged it to put the motor into shutdown (1.2V output) via an SCR if the computer control failed. Power cycling was necessary to reset the protection. The motor was a small 6V plastic geared motor.
 

MisterBill2

Joined Jan 23, 2018
9,036
The initial complaint, as I understood it, was that operating either limit switch stops the motor no matter which way it is turning.
And what I see from the picture is that the red and black go right to the motor. I have at least one device that the leads go into an interface box where they connect with limit switches inside and not easily inspected. That had been my concern about this instance. Now I sww that is not the case.
 
I don't think it can be done with one SPDT limit switch. The key concepts are:
1) The switch has to interrupt power in one or both directions.
2) When the limit is engaged, only allow power in the other direction

It almost looks like the application is a cutting, so just maybe the TS would benefit more if the feed rate could be changed?

Here https://www.ebay.co.uk/itm/273932401292 is an example PWM generator.

They don't tell you if it's active high or active low.
 
Using this https://www.sparkfun.com/products/9056 is probably even easier to breadboard. CMOS inputs cannot float (be left unconnected)

I do agree, the diode method is the simplest. I'd use a 1N540x series diode.

With the IC-based logic, it's possible to add variable speed without sacrificing torque. The motor would have the same torque on a 12V supply running at 1/2 speed. It may or may not be a useful feature.
 

Thread Starter

jkbrand

Joined May 8, 2021
29
The reason that I asked about the motor is that there are motor ppackages that include switches to stop at the ends of rotation. And so there is a possibility, if it is one of those, that there could be some unintended interaction happening.
It would not be the first time that unseen switches have produced strange results.
Does the motor fail to run either way when you manually operate one of the limit switches when it is not at the end of motion position? That information will assist in making the analysis of the problem. I am thinking that there is more than the circuit shown involved here.
When the system fails to move in the opposite direction, off whichever switch is operated, how do you recover so that it can be run again? Does the power supply have an overload shutdown mode? Please answer these questions.
As far as I know there are no internal switches in the motor. And, yes, the motor fails to run when I manually trip either limit switch. I am able to recover simply by pulling back on the arm that tripped the switch to allow it to transition from NO to NC. I am not aware if the power supply has an overload shutdown protection. Recall the motor draws
only .35 amps as measured and indicated by the power supply.
 

Thread Starter

jkbrand

Joined May 8, 2021
29
Using this https://www.sparkfun.com/products/9056 is probably even easier to breadboard. CMOS inputs cannot float (be left unconnected)

I do agree, the diode method is the simplest. I'd use a 1N540x series diode.

With the IC-based logic, it's possible to add variable speed without sacrificing torque. The motor would have the same torque on a 12V supply running at 1/2 speed. It may or may not be a useful feature.
Any chance you could take my drawing and add where the diodes would go in the circuit??
 
I'll try. Due to get my 2nd Pfizer shot in the next 4 hours and I still have side effects from the first one. Muscle aches in thighs and very sleepy. Yesterday, I ran errand from about 5:00-7:00 and then got dinner for mom and fell asleep from 7:30 to midnight. I had no trouble falling asleep after doing her diaper change.

But anyway:
Put two NC microswitches (SL1, SL2) in series in the leads going to the motor. That's the C and NC contacts. So, that any microswitch stops the motor. You can put one in one lead and one in the other or both in one lead. It doesn't matter. Make sure the motor stops using either microswitch in both directions.

Make the motor go toward microswitch SL1. Stop the motor with the microswitch and hold it, i.e.. rubberband. REVERSE the motor. Since one microswitch is open, the motor will stop. Insert the diode, briefly, across the contacts of SL2 in the direction that causes the motor to move towards SL2. In one direction it won't move at all.

Do the similar thing for the other microswitch.

When you traverse the circuit to the motor and back using only the diodes, as though both switches are open, the diodes will have the opposite orientation.

You can put a diode in first and add the switch if you want too, one at at time. The diode alows the motor to run away from the limit only.
 

MisterBill2

Joined Jan 23, 2018
9,036
That is an interesting way to describe setting up the diode limit switches arrangement. And correct as well.

But right now there is still the very interesting puzzle of why a correct circuit fails to work as it should. I am still thinking that it is related to the power connection that is common to both limit switches.
 

Thread Starter

jkbrand

Joined May 8, 2021
29
I'll try. Due to get my 2nd Pfizer shot in the next 4 hours and I still have side effects from the first one. Muscle aches in thighs and very sleepy. Yesterday, I ran errand from about 5:00-7:00 and then got dinner for mom and fell asleep from 7:30 to midnight. I had no trouble falling asleep after doing her diaper change.

But anyway:
Put two NC microswitches (SL1, SL2) in series in the leads going to the motor. That's the C and NC contacts. So, that any microswitch stops the motor. You can put one in one lead and one in the other or both in one lead. It doesn't matter. Make sure the motor stops using either microswitch in both directions.

Make the motor go toward microswitch SL1. Stop the motor with the microswitch and hold it, i.e.. rubberband. REVERSE the motor. Since one microswitch is open, the motor will stop. Insert the diode, briefly, across the contacts of SL2 in the direction that causes the motor to move towards SL2. In one direction it won't move at all.

Do the similar thing for the other microswitch.

When you traverse the circuit to the motor and back using only the diodes, as though both switches are open, the diodes will have the opposite orientation.

You can put a diode in first and add the switch if you want too, one at at time. The diode alows the motor to run away from the limit only.
Take care of yourself with the 2nd vaccine. My wife and 52-yr-old son were fine after our second shot. We awoke the following morning not feeling too bad. But after just a couple more hours we were down and out. Chills, fever, headaches, and all other signs of flu-like symptoms (muscle aches, fatigue and general malaise) for the rest of the day and into the evening. Next morning, all was well. I've read for Moderna, at least, 50% of those who received the vaccine were similarly afflicted. Not too sure, but Pfizer might have similar odds....

No hurry on the annotation of the circuit drawing I provided. I can live with the issue of non-reversing for a while yet.
Get and stay well!!
 

LesJones

Joined Jan 8, 2017
3,303
Re post #45. You must have the two limit switches wired in series for it to behave the way you describe. As you have not yet provided pictures of how you have wired it we can't point out your error. Without the pictures the only solution I can suggest is to directly connect a diode in parallel (Between the common and NC contact.) with each limit switch. To get the correct polarity for the diodes do the following. Let the motor reach one of the limit switches. Move the reversing switch to the other direction. (You may have to get someone to hold it in that position is it is a biased center off switch.) Connect the diode across the limit switch that has been actuated. I the diode is connected with the correct polarity the motor will move away from the limit switch. If not reverse the polarity of the diode. Repeat this at the other end of the travel.

Les.
 

Thread Starter

jkbrand

Joined May 8, 2021
29
Re post #45. You must have the two limit switches wired in series for it to behave the way you describe. As you have not yet provided pictures of how you have wired it we can't point out your error. Without the pictures the only solution I can suggest is to directly connect a diode in parallel (Between the common and NC contact.) with each limit switch. To get the correct polarity for the diodes do the following. Let the motor reach one of the limit switches. Move the reversing switch to the other direction. (You may have to get someone to hold it in that position is it is a biased center off switch.) Connect the diode across the limit switch that has been actuated. I the diode is connected with the correct polarity the motor will move away from the limit switch. If not reverse the polarity of the diode. Repeat this at the other end of the travel.

Les.
Les, thanks for the post. I took a few pictures of the wiring, but honestly, even I have problems tracing the wires. I imagined that other folks would, too. Not being lazy, just realistic. It's a maze of wires and challenging to trace from a picture. Nonetheless, if you still think it'll be useful, I'll upload a couple of perspectives with the caveat "don't expect too much."
 

MisterBill2

Joined Jan 23, 2018
9,036
Les, thanks for the post. I took a few pictures of the wiring, but honestly, even I have problems tracing the wires. I imagined that other folks would, too. Not being lazy, just realistic. It's a maze of wires and challenging to trace from a picture. Nonetheless, if you still think it'll be useful, I'll upload a couple of perspectives with the caveat "don't expect too much."
Once again, the diodes scheme is very different from the switching scheme in this discussion. Both circuits work as shown, and so the fact is that some part of the circuit does not match the circuit presented. AND the circuit that we see is the minimum parts count and the most reliable form. The diodes circuit is most useful when both limit switches are close to the motor, such as antenna rotators and ball valve actuators.
 

LesJones

Joined Jan 8, 2017
3,303
Try doing the following resistance tests with no power to the circuit. Find a way to actuate both limit switches. Set the DPDT switch to the middle off position.
Test 1 Measure the resistance between the power supply negative terminal and the common terminal on LS1.
Test 2 Measure the resistance between the power supply negative terminal and the common terminal on LS2.
Test 3 .Measure the resistance between the LS1 NC terminal and the top right hand terminal on the DPDT switch.
Test 4 .Measure the resistance between the LS2 NC terminal and the bottom left hand terminal on the DPDT switch.

Report the results of these tests. (I think this is the most likely place that you have made an error.)

Les.
 

Thread Starter

jkbrand

Joined May 8, 2021
29
Once again, the diodes scheme is very different from the switching scheme in this discussion. Both circuits work as shown, and so the fact is that some part of the circuit does not match the circuit presented. AND the circuit that we see is the minimum parts count and the most reliable form. The diodes circuit is most useful when both limit switches are close to the motor, such as antenna rotators and ball valve actuators.
In my case the limit switches are about 8" away from the motor. I will shoot some pix of the wiring and post them up.
 

Thread Starter

jkbrand

Joined May 8, 2021
29
Try doing the following resistance tests with no power to the circuit. Find a way to actuate both limit switches. Set the DPDT switch to the middle off position.
Test 1 Measure the resistance between the power supply negative terminal and the common terminal on LS1.
Test 2 Measure the resistance between the power supply negative terminal and the common terminal on LS2.
Test 3 .Measure the resistance between the LS1 NC terminal and the top right hand terminal on the DPDT switch.
Test 4 .Measure the resistance between the LS2 NC terminal and the bottom left hand terminal on the DPDT switch.

Report the results of these tests. (I think this is the most likely place that you have made an error.)

Les.
Will do. I'll post results this afternoon.
 

Thread Starter

jkbrand

Joined May 8, 2021
29
OK, meter set to 200K ohms. Once probes were attached, I pressed the appropriate LS switch. The results are:

Test 1 -- PS neg to COM on LS1 reads .6

Test 2 -- PS neg to COM on LS2 reads .6 and .7

Test 3 -- LS1 NC to top right DPDT reads 0

Test 4 -- LS2 NC to bottom left DPDT reads 0

Photos (of how things are wired up) to follow. . .
 

Thread Starter

jkbrand

Joined May 8, 2021
29
Here are some photos. I don't know the sequence in which they will display, but one is a close-up of the DPDT switch and its wiring. Another shows the wiring overall. Note the red and black wires going to the motor that are coming from the center posts on the DPDT switch. Another wiring picture shows a different perspective and finally, there's a shot of LS2 with its wires. In the foreground is the wiring going to LS1. Sorry for the change in POV for the pix, but I wanted to separate wires as much as possible to make it easier to trace.
 

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Thread Starter

jkbrand

Joined May 8, 2021
29
Using this https://www.sparkfun.com/products/9056 is probably even easier to breadboard. CMOS inputs cannot float (be left unconnected)

I do agree, the diode method is the simplest. I'd use a 1N540x series diode.

With the IC-based logic, it's possible to add variable speed without sacrificing torque. The motor would have the same torque on a 12V supply running at 1/2 speed. It may or may not be a useful feature.
Does the value for "x" in the diode you recommend matter? If so, based on the motor's draw of 350 milliamps can you suggest what value should have? I'm assuming that too high or too low might be problematic?
 
The x is related to the inverse voltage rating. A 1n5400 is 50V which is fine. Datasheet: https://www.vishay.com/docs/88516/1n5400.pdf If it were a car, the PIV should be 200 for a different reason. The diode is only "in play" to get the motor off the limit switch. "in play"means, that the diode will drop some voltage. The rule of thumb is 0.6V. That number is dependent on current and temperature and type of diode.
 

MisterBill2

Joined Jan 23, 2018
9,036
When I look at the wiring t does not seem like it matches the circuit shown. What it looks like is that terminals 4 and 4 are tied, as are terminals 1 and 6, which are supposed to be tied. If that is the case, then somehow the two switches are in series, and that would cause exactly the problem as described. The black wires from 3 and 4 go to a junction block but they should go each to a limit switch only. And there is another wire leaving the connections to 1 and 6, while in the circuit that is not the case. So it is not wired like the diagram shows.
The construction has way too many connectors to be easily traced.
 
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