Electric components to toggle DC motor polarity

dcwatson84 said:

I definitely read about these. They seem to cut current on/off so I couldn't figure out how I could use them to reverse the polarity of the current?

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In the details, there is FWD/REV/BRAKE/COAST

Simplifying there is FWD/REV; Default = brake

There's also DIRECTION and NOT ENABLE methods.

The classic reversing switch is the DPDT relay with the corners crossed. DC say enters the common terminals and exists on the diagonal. So, yo can get FWD - OFF - REV with a single DPDT Center-Off switch.

There is burst of high current when a motor first turns on, so that burst has to be ignored. e.g. I>x for some length of time to software detect a limit.

This https://cache.freescale.com/files/microcontrollers/doc/ref_manual/DRM160.pdf gives you an idea of how it's done in a car. Note,they detect stall, not a slam.

will a pair of reed switches work?

Yes. Anything that's open when it's at an extreme and is DC rated to carry about 200 mA.

well a picture of your setup posted here certainly would be helpful but if you could mount two reed switches, coupled with a pic chip and a h-bridge would be able to do both travel limit and direction as it would have the required inputs.
but without knowing what the physical setup looks like makes it hard to give you futher ideas.

Hy dcwatson84,

Another question: does your timer have two channels, ie two outputs that can be individually programed, or just a singe output? Two outputs would be ideal because it means that an absolute system can be designed rather than relative. The danger with relative systems is that they have no absolute reference and can get out of step.

This may sound complex but what it means in your case is that if the control gets out of step for any reason, the coup door could be closing when it should be opening and vice versa. This would not be good for the chickens I suspect.

spec

spec said:

Two outputs would be ideal because it means that an absolute system can be designed rather than relative. The danger with relative systems is that they have no absolute reference and can get out of step.

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Two outputs would be ideal, but not essential.

This https://www.ixysic.com/home/pdfs.nsf/www/LBA716.pdf/$file/LBA716.pdf part coupled with a few more parts only requires 2 mA to operate. It's effectively a solid state SPDT relay, so if you need a way to convert a steady out into an SPDT contact low power, this is a way to do it.

Any SPDT relay would also work. The automotive relays are power grubby.

ON can become open and OFF closed or vice versa.

I've often mentioned Astronomic timers with the coop door instead of a basic timer. Astronomic times need latitude and long as inputs as well and will calculate sunrise and sunset. I have a timer at home on a lamp that turns on at 1 hour after sunset and off at 11:30.

Hi dcwatson84,
My curtain controller remembers which way it last moved by the PIC microcontroller setting or reseting a bit in the EEPROM memory. It could also be done using a latching relay. If you used limit switches (Reed switches would be the easiest.) then the limit switch that was actuated a power on could be used to decide which way the motor should move. Should you not also have some type of sensor on the door to prevent it closing if it was obstructed.

Les.

PTC thermisters mounted on the motor are cheap protection, but they have to be sized right.
Brass or Nylon tipped set screws can also help. I used this method recently.
Or you design a sacrificial part.
Adjustable clutch (expensive).

Stall detection is even better. It basically detects "your supposed to be moving but your not."

dcwatson84 said:

So it sounds like you're saying my original idea (just using the timer to control the operating limit) is not practical?

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The timer on its own isn't practical. The motor would stall at the end of its travel and possibly burn out. Why? Because although it is rated 0.8A, the stall current would be many times that.

KeepItSimpleStupid said:

Two outputs would be ideal, but not essential.

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You have completely missed the point in your eagerness to contradict.

I do no contradict or comment negatively on your posts- would you like me to start?

spec

KeepItSimpleStupid said:

PTC thermisters mounted on the motor are cheap protection, but they have to be sized right.
Brass or Nylon tipped set screws can also help. I used this method recently.
Or you design a sacrificial part.
Adjustable clutch (expensive).

Stall detection is even better. It basically detects "your supposed to be moving but your not."

Click to expand...

No, a high current is caused by the door being closed or open and at that point the power should be switched off. High current arises because the motor has stopped for any reason: door closed, door open, chicken in the way, door jammed due to dirt, heat, cold, etc.

That is one of the reasons why I recommend an absolute system to keep the door opening when it should in absolute time and that errors in the door opening and closing do not accumulate.

spec

I'm always going to get misunderstood. It always happens.
When i told my boss that the system we are installing is going to be obsolete in 3 months he took it as:

1) We have to install something else
2) I "thought" I was giving him the "heads up" that parts were going to become unavailable

I didn't appreciate being yelled at.

My so called "contradicting" isn't supposed to be a "tug-of-war".

I totally agree with you that not knowing what direction the motor is to go is bad.

I agree that asking about two outputs is a good thing. One can be used for up and the other down. That reduces the parts count. Good. A relay can do the same with ON and OFF representing the open and close commands. Sort of good. What if, the system is essentially battery powered all the time? then the relay isn't a good idea.

Reminder:
That it's the OP or Thread Starter that has to make the choice and we don't have all of the details. He/She is the ultimate boss so to speak. It's up to us to present choices, but not dictate the choice. alec_t just contradicted a choice in his post #29. So what? I also made the same contradiction.

I guessed a "chicken coop" door.

Two timer outputs, if available, as suggested by spec could make the design less complex!

KeepItSimpleStupid said:

I'm always going to get misunderstood. It always happens.
When i told my boss that the system we are installing is going to be obsolete in 3 months he took it as:

1) We have to install something else
2) I "thought" I was giving him the "heads up" that parts were going to become unavailable

I didn't appreciate being yelled at.

My so called "contradicting" isn't supposed to be a "tug-of-war".

I totally agree with you that not knowing what direction the motor is to go is bad.

I agree that asking about two outputs is a good thing. One can be used for up and the other down. That reduces the parts count. Good. A relay can do the same with ON and OFF representing the open and close commands. Sort of good. What if, the system is essentially battery powered all the time? then the relay isn't a good idea.

Reminder:
That it's the OP or Thread Starter that has to make the choice and we don't have all of the details. He/She is the ultimate boss so to speak. It's up to us to present choices, but not dictate the choice. alec_t just contradicted a choice in his post #29. So what? I also made the same contradiction.

I guessed a "chicken coop" door.

Two timer outputs, if available, as suggested by spec could make the design less complex!

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It have been dodging a lot of flack on ETO in the last couple of days Keep.

spec

spec said:

No, a high current is caused by the door being closed or open and at that point the power should be switched off.

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So, if that were the case the motor would never start because the high starting current would instantly shut the motor off, right?

No problem, spec.

That is one of the reasons why I recommend an absolute system to keep the door opening when it should in absolute time and that errors in the door opening and closing do not accumulate.

spec

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We all (alec. spec, KISS, etc) agree with that.

Aside:
That piece of equipment that was becoming obsolete had a $40,000 USD replacement cost, no naturally he was upset. He did assign someone to look at a possible replacement and it was eventually replaced. The used market became a viable option. I had only learned about the end of life date because I had to order parts for the current machine because of the upgrade I was doing to increase the number of sampled channels. Lesson learned? Keep your mouth shut? If I was still working there, I would have changed my MO if I was ever in the same sort of situation again.

I kinda tried that, but got in trouble again, by not doing anything, A guy who was leaving in the next few weeks locked his keys in his office. I let him kick down the door instead of making a phone call that would have obtained keys easily.

Same boss. I noticed that the contractors were framing the opening such that the doors would be installed "wrong". The specs said they needed to open out, but not into someone's face in the hallway when someone exits the lab. I noticed this and I was asked, "What was I doing in there?", not the expected "Thank you". We did eventually, grab some people and did an inspection to uncover issues before completion. The one I remember most was a lever for a duct shutoff being embedded into the wall and spackled so the lever could not move. So, I guess, it was a good idea to catch stuff early, right?

You can't win.

dc, plug "chicken coop door" into the "search" window (top right of this page).

Scroll down to the older posts. Lots of ideas.

I'll try to reply to most everyone!

More details about the door - It's a 2lb piece of plywood raised by a line on a reel, and lowered via the same + gravity.

Spec,

• No the timer only has one channel, which has been a crux of the issue, because there is no way to differentiate between one signal and the other, it must be kept track of down stream.

KISS,

• Dont SPST relays still require polarity reversal to deactivate? That's the original problem.

Les,

• As for obstructions - None. the door is lowered by gravity, so any obstruction would simply prop the door and the motor would continue to unwind just fine. The same is not true for the lifting - but that is easily solved by a cotter pin which slips when a higher force is applied. It doesnt reset the system, but it saves the motor. Simple is my game .
  
• If my understanding is correct - all those latch relays still need *reverse* polarity to un-latch them. And that results in the exact same problem - I only have access to *one* polarity of power from the timer. The only relay that seems to be able to truly swap is an "alternating" relay. Its usually used to swap the load - but I could hook it up in reverse so that the input comes from a different polarity each time, and the "load" is really a single? But those relays are very expensive ($80+).

I should probably point out that Im not trying to build this from scratch . I was trying to see if I could do it using the timer I already have as the power source. But it seems like that wont work, as I can't control relays with that single polarity of power (unless the alternating relay does that - but im still uncertain). So with that said - the two timer idea is starting to sound more reasonable. I'm still leaning in favor of just using the timers themselves to control the door. So If I simply reversed the polarity of one timer, then one becomes the "up" signal, the other becomes the "down" signal. Im still a little disappointed that I couldn't find a single cheap relay that handled that for me (see alternating relay above).

If you can provide up and down limit switches whose contacts OPEN at each of the extremes, it's cake from there.

Two SPDT relays (The automotive ones are really cheap) can very simply provide a means to go UP or DOWN.

The relays are essentially connected such that the common terminal goes to the motor and the Normally open (NO) terminal goes to +12 Always. The Normally Closed (NC) terminal of both relays go to ground.

Thus, the motor is shorted if neither relay is activated, so the motor acts as a generator into a short and stops instantly.

At this point there is no limit switches.

You can chose to make the coils common to +12 or common to GND. Let's use Ground as a common to make things a bit easier to understand.

So, we now have two wires left over. If we apply +12 to one, the motor goes up. If we apply +12 to the other, the motor goes down. If we apply +12 to both, the motor also stops.

So, we take the UP limit switch and put it in series with the UP relay coil, so it stops the motor when the door is fully up.

We do the same for the DOWN signal.

Now we have a circuit where when +12 is applied through the UP limit switch to the UP relay, the door goes up and vice versa,

Your timer provides +12, so have that run to an SPDT relay to have potential free contacts. Take the common of this relay and attach it to +12. Take NC and attach to UP and NO and attach to the down relay. Now, when the timer is ON, the door goes up. When the timer is OFF, the door goes down.

That's basically it in words.

I can see it working in my head. I'll have to see if I have some reasonable places for contacts around the door track.

That's good.

This https://www.alliedelec.com/m/d/9642d43e44cef566355cba3524b7342a.pdf sort of part is only a few dollars <$2.00 each. It's just not a sealed switch. These could be edge mounted.