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Electric components to toggle DC motor polarity

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by dcwatson84, Jul 21, 2016.

  1. dcwatson84

    dcwatson84 Member

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    First off - this project is all about automation, so im looking to have it run with no manual intervention except for initial setup, and im pretty new to working with electrical relays.

    I have a DC motor that triggers it's own mechanical switch to reverse it's direction. I have an electronic timer that feeds 12v to the DC motor. The motor has mechanical components which trigger the polarity reversal at upper/lower limits of operation, but these components were very low quality and are starting to malfunction. I have a project in mind to fix this but have a few questions which I haven't been able to answer on my own....
    1. Due to the consistent nature of the operation, I think I can scrap the mechanical upper/lower triggers and simply use the timer to control how often & how long the motor operates, given that the speed of the motor is consistent. Reasonable?
    2. If so, I'll need a device between the timer and the motor which swaps the polarity somehow, correct?
    3. I've seen these devices, but they are manual switches. Is there a latch/relay that I can use to swap the polarity, the only input to which is a single current path? Note - the difficult part of this seems to be that the *same* current, the next time around, also needs to swap the polarity again. A polarity toggle.
    So basically I have one timer that delivers timed periods of current always at the same polarity. Can I build a device that each time the current is shut off / on , swaps the polarity? Such that the motor always alternates between forward & reverse, without any manual intervention?

    Feel free to tell me if this isnt possible with simple components (relays, latches, etc).

    Thanks!
     
  2. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi 84,
    One problem in using a timed period to drive the motor forward and reverse, is that that any slight differnce in the forward and reverse physical travel rate will be accumulative.
    It will eventually run into an end stop.
    E
     
  3. Reloadron

    Reloadron Well-Known Member Most Helpful Member

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    I would heed what Eric mentions. Using a timer in motor reversal applications is not a good idea. There are plenty of circuits which will do as you wish. Some data about the motor would be useful like how much current the motor draws? The horsepower rating if possible?

    Ron
     
  4. dave

    Dave New Member

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  5. dcwatson84

    dcwatson84 Member

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    The timer can be set to run different amounts of time for each cycle. If the *up* takes 10 seconds and the *down* takes 5 seconds, then it can be setup to account for that. The only thing that I can imagine would affect it would be that if over time the motors performance degraded and it couldnt perform the same work in the same time, then the timer method couldnt account for that. Im hoping that that sort of degradation would only require manual intervention on the scale of every few months, which is acceptable for this project.

    As far as motor specs, I'll get some details. It's a 12v motor that currently runs off a 12v car battery. It lifts a 2-3lb load vertically on a reel. I'll get the amperage today.
     
  6. Reloadron

    Reloadron Well-Known Member Most Helpful Member

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    Can the motor go from Fwd to Rev without a pause? Again, using a timer, if that is what you want, can be done.

    Ron
     
  7. dcwatson84

    dcwatson84 Member

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    The motor *itself* doesn't need a pause, but the specific implementation does. I need it to go up at a very specific time and down at a specific time, specified on the timing device. I've considered simply buying two timers and reversing the polarity on one, but the timers were ~$40 and I was hoping these components would be cheaper.
     
  8. dcwatson84

    dcwatson84 Member

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    I checked the motor - the draw is .8 A at 12-14 V.

    So It sounds like this is possible. This "polarity toggle" (as I called it), does it have some other name? Or is it rare enough that it needs to be assembled from simpler components?

    Other than the motor speed degrading, are there any other issues I should consider before using the timed approach? I'm defaulting to that method right now because those are the components that I have and because it could be installed right next to the timer and the battery. Aside from the motor itself, the mechanical switching mechanisms (gears with tabs that click a physical switch) are all plastic, which is why they are failing, and the load is not shielded from the weather. So Im somewhat limited in how I can control the polarity reversal.
     
  9. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    What mechanism and load is the fwd/rev motor action performing/driving.?

    EDIT:
    For weather sealing you could consider either a Hall effect device or slotted opto to detect the end travel limits, driving a solid state fwd/rev drive.

    OR a pair of these

    https://www.amazon.co.uk/Water-Proof-Micro-Switch-Attached/dp/B00MFB81CQ
     
    Last edited: Jul 21, 2016
  10. Reloadron

    Reloadron Well-Known Member Most Helpful Member

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    OK, things considered to start off you may want to look at some relay configurations which will reverse the current to your DC motor. Generally a DPDT (Double Pole Double Throw) relay will take care of Fwd and Rev. The relay can be controlled using as Eric mentioned hall effect sensors or switches, any of several sensors will work. The motor travels to a limit sensor in one direction and then reverses traveling in the opposite direction to the other sensor. I like using weather proof micro switches but you use what works best for your application.

    Ron
     
  11. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    These posts are confusing. You did specify current and voltage - good.
    Up and down - good; so probably different speeds when not controlled - OK

    The movement and the frequency wasn't really specified.
    Is this like the "chicken coop door"? Up at t1 ; down at t2;

    Or something entirely different?

    If it's constantly going up and down, then it would be good to pause briefly at the endpoints. e.g. UP-Pause-DOWN-Pause-UP....

    There's the standard 2 automotive style relay method that incorporates limit switches. One coil is effectively UP, the other DOWN. If no or both coils are activated the motor brakes, so no over-runs at the limits.

    The limit switches don't have to carry the full motor current.

    So, it's easy to convert this to an UP/DOWN mode of operation with a another relay and timer. e.g. ON = DOWN; OFF= UP

    These are comments because I don't fully understand the problem. The "chicken coop door" is a common problem.
     
  12. cowboybob

    cowboybob Well-Known Member Most Helpful Member

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    We need to know this.

    If I may, dc, you seem to discuss two, different, timing events, i.e.,:
    (My emphasis).
    1. the motor's "on" time is variable, or:
    (My emphasis)
    2. the motor "action" occurs at specific times.

    Which is it? Or are both needed?

    It would help if you could describe for us, in detail, what the Motor/Drive system is meant to doing, when it is doing it and for how long.

    <EDIT> KISS, you beat me by a few seconds, but we both seem to have the same issues.
     
  13. dcwatson84

    dcwatson84 Member

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    I was being intentionally vague hoping to simplify the conversation, but that was my mistake :) .

    Y'all hit the nail on the head - Yes this is a chicken coop door. It is a .8 A motor attached to a reel, which winds up a door attached to string. The reel has plastic gears and movable tabs. Those tabs can be adjusted such that at a certain point in the cycle they flip (mechanically) a switch. This switch turns the motor off and also swaps the direction, such that the next time current is applied, it turns in the opposite direction until the switch is flipped the other way. The tabs and gears are very cheap and are malfunctioning, leading to over/under runs. The motor and timer work fine though.

    So I'm looking to use what I have as much as is possible - so this is where my design came from. The timer allows me to set multiple times and durations. So each morning and evening I can run power to the motor for a specific duration. I was hoping to have some device in the middle which automatically toggled the polarity each time current was applied.

    I see that the ideal and most reliable method might be to add contacts/switches to the door itself, such that when it hit its limit it automatically flipped a switch, so that the door can never override its boundaries. This becomes increasingly expensive because the door is exposed to lots of elements, including wildlife. Im willing to trade that reliability for the ease of simply installing this one device that swaps the polarity (if it's that simple) because I think (hope) the timer will offer the same reliability on a scale of weeks/months. I dont mind slightly recalibrating it every 2-3 months if necessary.

    Let me know if I've left any important parameters out. Does my polarity-toggle component seem to fit the bill? I'm trying to make the system as simple as possible, and maintain month-scale reliability.
     
  14. spec

    spec Well-Known Member Most Helpful Member

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    Hy dcwatson84,

    To answer what I think is the essence of your question, yes what you require can be achieved quit simply using standard and simple electronic techniques and components.

    The thing is, do do have the skills and tools to construct a simple circuit.

    I see that you are from the US, which is good because it means you will have unlimited access to components and tools. Care to tell us which state you are at?

    spec
     
  15. Les Jones

    Les Jones Well-Known Member

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    I use a circuit to open and close curtains. It gets round the problem of not having limt switches by sensing the current drawn by the motor when it stalls at the limits of its travel. It is controlled by a plug in timer which applies power for 2 minutes in the morning an 2 minutes in the evening. It remembers which way it last moved so it always moves in the opposite direction when power is first applied. If this would suit your needs then I will provide the details.

    Les.
     
    • Informative Informative x 1
  16. dcwatson84

    dcwatson84 Member

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    Spec,
    Yes I'm fine connecting/soldering the circuitry, but no one has, of yet, described the basic components that would needed. I'm a little new, but Im very familiar with electronic hardware in general so if someone could describe the circuit components I could take it from there.

    Les,
    You say yours"remembers" which way it last went. What does that mean in terms of specific electrical components? Thats the problem I'm trying to solve.
     
  17. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    All you had to say was "chicken coop door".

    Anywy, these http://www.texasindustrialelectric.com/relays_0332209150.asp or equlivelent relays are available cheaply. They need about 200 mA for the coil and that's all you need for a limit switch (>~200 mA and 12 V DC).

    By cheap: http://www.parts-express.com/12-vdc-automotive-5-pin-relay-spdt-30-40a-bosch-type--330-073#l

    You can also get a dual socket for them with wires attached.

    On the above page there's a link for wiring diagrams and common uses. What's at the bottom of page PDF/Real page 15 and top of 16 is the way (pick one) you need to wire them.

    OK!

    You need some sort of contact that is OPEN at each limit. Environmentally, your right it may cost you, but the contacts won;t see the full motor current just about 160 mA at 12 V.

    Using Amazon as my unfavorite etailer, I find something like this: https://www.amazon.com/Apex-Curtain..._SR160,160_&psc=1&refRID=SYK43727961WG8YN8CT7

    Costly - Yes. With some flex conduit it's water and animal proof. There has to be other options. We have no idea what you have to work with.

    You might be able to put a pipe on the door and enclose a non environmentally rated switch behind a plexiglass panel covering the studs. Fix the switches and use step collars for adjustments. It's just an idea.

    The limit relays will not draw power at the limits.

    One's elegant. One's quick and dirty.

    You then need a relay that is connected to your timer. The timer is not fully explained.If your timer has SPDT contacts your totally set. Again, they will only have to handle about 200 mA. You get to define what on does (Open or close).

    If we have to cut the power consumption, we can do that. Need more timer info.

    So, maybe little more info about the timer and picture of the door so we have more to work with.

    Aside:
    There is a missing piece of the puzzle. Diodes should go reverse biased across the relay coils. These would protect your 12 V timer and there's a preferred way (circuit) to where you wire 12 V. Supposedly, the relays can be bought with internal diodes, The easiest, would probably be soldering them direct to the relay pins.
     
  18. dcwatson84

    dcwatson84 Member

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

    The timer is a digital timer with a 2-wire 12v output, which can be configured to pass current through at a specific time(s) and specific duration(s). And it is designed to handle the amperage directly, no relay needed.
     
  19. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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  20. dcwatson84

    dcwatson84 Member

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    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?
     
  21. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    I don;t think it's a good idea at all. If it were an AC synchronous motor, then probably OK.

    The relay method with brake is much like the power windows/door locks in your car but with limit switches. For the two relay OPEN/CLOSE to work, you need to apply (<200 mA) power to one or the other relay, so SPDT is needed from the timer. Power or ground to the common contact, your choice).

    Another relay can create that SPDT contact. There are Solid state relays that should work that would draw less current A few extra parts required.
     

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