reverse polarity of 12dc dpdt relay

Discussion in 'The Projects Forum' started by bergybob, Dec 28, 2011.

  1. bergybob

    Thread Starter New Member

    Dec 28, 2011
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    I read this thread with interest as it is exactly what I'm trying to do.I have 1 12vdc timer 2 external limits & a dpdt relay.I have everything operating but can't reverse the polarity.I have wired as follows: power from timer goes to top limit NC then to 3 Earth from timer goes to Bottom limit NO then to 7
    5 bridges to 2 & onto motor 6 bridges to 1 then on to motor 4 goes to earth as does 8 There is a diode between wires to 7 & 3 to prevent backflow.
    It works ok, but I can't discover how to reverse polarity between cycles.my idea was for the relay to be energised between cycles.Can anyone suggest where I have stuffed up
    Thanks Brian
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    Welcome to AAC!

    A thread belongs to the OP (original poster). Trying to take over someone elses thread is called hijacking, which is not allowed at All About Circuits. I have therefore given you a thread of your very own.

    This was split from http://forum.allaboutcircuits.com/showthread.php?t=24338

    Did you note the thread in question is around 2½ years old? The OP is long gone, but a few of the people who answered are still here.
     
  3. strantor

    AAC Fanatic!

    Oct 3, 2010
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    the terminal numbers don't mean anything to us, as not all relays are the same, including terminal numbers. It would be most helpful if you draw a circuit of what you have wired up (with relay internal contacts shown, not just terminal numbers).
    also, refer to this drawing (collaboration between praondevou and myself) from the epic chicken coop thread.
    [​IMG]
     
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  4. SgtWookie

    Expert

    Jul 17, 2007
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    Gosh, I sort of remember the "epic chicken coop" thread, but I didn't participate in it.

    The trouble with the circuit is that the relay coil will have to be energized the entire time the door is either open, or shut. That precludes the system from being operated from an economical solar panel system.

    On the other hand, were the electromechanical relay to be replaced with a MOSFET H-bridge that were controlled by 4000-series logic, the power consumption could be practically negligible.

    Another way to make it power-thrifty would be to use two individual SPDT relays, with their coils' grounds controlled by the limit switches. That requires one of the relay coils to be energized only when the motor is operating; the relay disengages when the limit is reached. That method would require two timers; one for the door open time, the other for the door close time.
     
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  5. bergybob

    Thread Starter New Member

    Dec 28, 2011
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    Hi, Thanks for your imput to fix my relay problem.I'm not sure what I have forgotten to do but it goes up & down ok but it won't reverse the motor.It must have a relationship with the normally open or closed curcuits.
    Any observations would be appreciated.
    I think I have attatched a jpeg freehand drawing for your perusal

    Regards,
    Brian
     
  6. bergybob

    Thread Starter New Member

    Dec 28, 2011
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    Just located this photo of my last effort.Clearly something malfunctioned !
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    Ouch! That's why you use a fuse in the line. Better to blow a 50 cent fuse than fifty bucks worth of parts.

    Here's one I did awhile back, with some changes for you (battery, solar cell):

    [​IMG]

    There is no fuse shown in this diagram either. It should be right above where the battery+ and diode cathode (the banded end) join. 5A slow blow will likely be more than sufficient to run the motor without blowing, unless something is jammed or there is a miswire.

    D3 keeps the solar cell from discharging the battery when the sun goes down.

    As you can see, I used two SPDT relays instead of one DPDT relay. You can get those at any auto parts store. Have to be careful though; many relays are just SPST, either NO or NC - and some are DPDT.

    Have a look at this page:
    http://www.the12volt.com/relays/relays.asp

    Note Figure 1. A SPST relay normally will not have terminal 87a; you don't want one of those. You want 5 terminals showing at the bottom.

    Figure 2 shows the diagram of when the relay coil is not energized.
    Figure 3 shows the diagram of when the relay coil IS energized.

    I'm pretty tired and can't think of a good way to do it using a DPDT relay at the moment.
     
    Last edited: Dec 29, 2011
  8. strantor

    AAC Fanatic!

    Oct 3, 2010
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    which direction is reverse? is reverse when the relay is activated? What happens to the NC contact of the timer when it is activated? I'm thinking it may connect to ground; that would explain why (1) your motor wouldn't reverse and (2) why all your wiring would burn up. It's a long shot, but from looking at your drawing, I don't see why it wouldn't work.
     
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  9. Adjuster

    Well-Known Member

    Dec 26, 2010
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    Fixing the wiring as suggested should work, provided you have something like a car door motor which is designed to reverse. If there is any doubt about what type of motor you have, why not test the it with the supply connected manually both ways around.

    While DC motors with permanent-magnet fields will reverse perfectly well with circuits of this kind, wound-field types will not. In the latter case, the armature and field magnetisation would reverse together, giving no net effect. In principle at least it might be possible to reverse a wound-field motor by reversing the armature or the field or the field only, but this would require access to the winding connections inside the motor. It is also possible that larger motors may be optimised for running in the designed direction, and may be unhappy running in reverse.

    If your motor does not run backwards on a reversed supply, it may be easier and safer to look for an alternative than to fiddle about with it.
     
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  10. Adjuster

    Well-Known Member

    Dec 26, 2010
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    Fixing the wiring as suggested should work, provided you have something like a car door motor which is designed to reverse. If there is any doubt about what type of motor you have, why not test the it with the supply connected manually both ways around.

    Edit: Don't try this if there is any chance that you have a "brush-less" motor with electronic drive. These normally do not reverse. Some may short-circuit, which may be what happened to you before.

    While DC motors with permanent-magnet fields will reverse perfectly well with circuits of this kind, wound-field types will not. In the latter case, the armature and field magnetisation would reverse together, giving no net effect. In principle at least it might be possible to reverse a wound-field motor by reversing the armature or the field or the field only, but this would require access to the winding connections inside the motor. It is also possible that larger motors may be optimised for running in the designed direction, and may be unhappy running in reverse.

    If your motor does not run backwards on a reversed supply, it may be easier and safer to look for an alternative than to fiddle about.
     
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  11. SgtWookie

    Expert

    Jul 17, 2007
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    One thing that's still killing the DPDT relay idea is the need to keep the coil constantly energized. That's going to be a continual drain on the battery. Sooner or later there will be several days in a row with reduced sunshine, and the battery will be dead. I can't see a good way around that drain, either - without getting crazily complicated. :rolleyes:

    Using a pair of SPDT relays like I've shown in reply #7 DOES work without draining the battery unnecessarily, is pretty simple, and if properly wired, there are no combinations of the relays being open/closed that will harm anything/cause a short/etc.
     
  12. strantor

    AAC Fanatic!

    Oct 3, 2010
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    did you examine his circuit closely? The way he did it, it would not need to stay energized. It's pretty cool actually, I am curious why it doesn't work.
     
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  13. bergybob

    Thread Starter New Member

    Dec 28, 2011
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    I like your thinking. Yes I wired it so the relay is active from 9pm until 6am and I have included a small LED indicator light activated by the bottom limit.This may have caused the shorting ,I had not been able to identify the reason for the problem because I did have the system working manually for only short periods(including the reversal of the motor) whilst I adjusted the limits.The problem occured when I set the automatic timer to run a complete test.I still can't fully grasp why the burning occured even if the relay was earthed.Would it occur over time or would it occur when the relay was deactivated .My thinking was that the diode was too small & could not withstand the prolonged current overnight & somehow allowed current to breech the diode .The major issue I confront is ,having rewired it identically to before including a new motor new relay & new bigger diode, I clearly have a wire in the wrong location as it wont reverse.
     
  14. SgtWookie

    Expert

    Jul 17, 2007
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    I haven't gotten much sleep the last few days; I see it now.

    There needs to be some Zeners/TVS diodes added, a slow-blow fuse, and a diode added across the relay coil; otherwise the one existing diode stands a good chance of getting zapped. That may be why the whole system had a melt-down before. The fuse might could be a resetting type, but that would most likely leave the battery dead; as overloads don't normally heal themselves.

    The LED being on all night will drain the battery unless you use a very low current LED.

    Here's a revised schematic:

    [​IMG]
     
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  15. strantor

    AAC Fanatic!

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    I think this will be my new chicken coop door mechanism of choice.
     
  16. SgtWookie

    Expert

    Jul 17, 2007
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    Well, there are a few things about the single DPDT relay version that I'm not fond of.

    1) The limit switches are carrying the full load of the motor, rather than a relatively light load like the coil of a single relay. This means that the limit switches must be rated for the motor current, and will fail much more quickly than they would switching just a relay coil.

    2) The timer SPDT switch is ALSO carrying the full load of the motor, and is the primary "make" of the circuit. While you may be tempted to dismiss this, the starting current of the motor will be several times that of the run current. The stress on the timer's contacts will result in an earlier failure than if it were simply switching relay coils on.

    3) D1 is in the current path to the motor while it's running in one direction. This can reduce the voltage to the motor by about 1v; ~8.3%. While one might argue that 8.3% is not very much, I'll assert that in a battery powered application, it is important to make the circuit as efficient as possible within a reasonable budget.

    I still suggest that the dual SPDT relay solution is more satisfactory, as it addresses all of these concerns.
     
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  17. bergybob

    Thread Starter New Member

    Dec 28, 2011
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    Thanks all for you help & advise.I think I may have stumbled on the problem.It seems the diode has had a surge and it is allowing current both ways thus preventing the relay from activating.I will replace the diode when I can get one & hopefully Problem Solved.I have added a 5amp fuse to the battery to prevent the same again.
    Cheers
    Brian
     
  18. SgtWookie

    Expert

    Jul 17, 2007
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    Brian,
    Have a look at my re-draw of your circuit. You really do need to add the pair of Zener diodes across the motor, and the diode across the relay coil. If you do not add these, you will sooner or later (probably sooner) have the same problem of that diode getting "zapped" - it'll either burn open, or act as a short as the one you had that failed.

    Also, the Zeners and diode will help to make your relay and switch contacts last longer; as otherwise their will be a lot of arcing as they open up.
     
  19. bergybob

    Thread Starter New Member

    Dec 28, 2011
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    Thanks for the diagram looks good .I've never heard of a zener diode !
    I did try to insert a diode between the coil but it just fried (must have been the wrong way around) so I gave the idea away.
    I will investigate them when the shops reopon after new years
    .
    I truly appreciate all the help given by this forum.I had run out of ideas.
    It was supposed to be a simple project for over the christmas period but it ended up keeping me up nights trying to resolve issues.I am confident I have now got it working but clearly there are a couple of mods to make it better.
    Thanks again & Happy New Year to All
    Brian
     
  20. SgtWookie

    Expert

    Jul 17, 2007
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    You are welcome - thank you for bringing your idea to our attention. :)

    Zener diodes were first postulated by Dr. Clarence M. Zener in 1934. He was a theoretical physicist who worked at Bell Labs for many years, among other places. Dr. Zener was certainly ahead of his time. Bell Labs named this particular type of diode after him. Zener diodes are designed to conduct normally when forward-biased; when reverse-biased, they break down at a specific voltage. They are designed precisely to do that. They are quite useful.

    Yes, you must have had it in there backwards. You want the cathode (the end with the band) to be towards the more positive voltage. That way, the only time it will conduct is when the power is turned off to the relay, and the coil in the relay is still charged. The diode provides the discharge path. Without a diode, the voltage can build to very high levels.

    That sounds good. :)

    It can be a big help to have other people take a look at what you're doing when you get "stuck" on something. When you've been working solo on something for awhile, you tend to overlook things that you are "certain" are OK - but in the end, it's one of those things that turns out to be the problem. Or, their may be a different way to do something that is easier, more efficient, less costly, whatever.

    And to you & yours, too! :)
     
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