1. Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.
    Dismiss Notice

Electric components to toggle DC motor polarity

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

  1. dcwatson84

    dcwatson84 Member

    Joined:
    Jul 21, 2016
    Messages:
    61
    Likes:
    0
    The timer has second resolution. In fact my method relies on that principle as well, but works electrically. The timer needs to "nudge" the door off the switch, since the switch is attempting to shut off the main relay. Once the door is off the switch, the timer can shut off, then the door hits the next switch, this time with the timer off, so it shuts off the main relay again, ending the cycle.
     
  2. alec_t

    alec_t Well-Known Member Most Helpful Member

    Joined:
    Jul 10, 2011
    Messages:
    9,255
    Likes:
    1,218
    Location:
    Cardiff, Wales
    No. The 'DPDT' part is electrically separate from the coil. It consists of two (that's the 'DP', double-pole, bit) electrically-separate but mechanically coupled switches that operate in synchronism. Each switch is a double-throw (that's the 'DT' bit) type, where the Common terminal of the switch connects to one or other of the NC (normally-closed) and NO (normally-open) terminals. 'Normally' means when the coil is not powered.
    Here's an example.
     
    Last edited: Aug 9, 2016
  3. dcwatson84

    dcwatson84 Member

    Joined:
    Jul 21, 2016
    Messages:
    61
    Likes:
    0
    normall
    Gotcha. So the schematic would still show a single coil, but would have two internal switches. That should be enough for me to draw up my current design.
     
  4. dave

    Dave New Member

    Joined:
    Jan 12, 1997
    Messages:
    -
    Likes:
    0


     
  5. alec_t

    alec_t Well-Known Member Most Helpful Member

    Joined:
    Jul 10, 2011
    Messages:
    9,255
    Likes:
    1,218
    Location:
    Cardiff, Wales

    There are various symbols for a DPDT relay.
    One of the tricky things about a relay cicuit design for motor reversal is avoiding one of the relays being on all night/day and gobbling power.
     
  6. dcwatson84

    dcwatson84 Member

    Joined:
    Jul 21, 2016
    Messages:
    61
    Likes:
    0
    Gotcha. I'm using a latching-relay that only needs power to flip the switch either way. Once the switch is open, the system consumes no power because the entire circuit is open. The timer then closes the relay again, turning the system on, until it again turns itself off at the end of the cycle. The timer is simply a trigger to get the system on. From there the system knows what state it's in and when to end it's cycle.

    I still plan to provide a schematic, just haven't been able to get it drawn right yet.
     
  7. ChrisP58

    ChrisP58 Well-Known Member

    Joined:
    Apr 27, 2012
    Messages:
    1,081
    Likes:
    131
    Location:
    Provo, Utah, USA
    I'm curious to see your schematic.

    Personally, I can't figure out how to do it with a single SPST latching relay without having some additional relays or circuitry. At the very least you need something to make a meta-state between the up and down states to prevent the relay from buzzing. By that I mean, if the contacts of your latching relay determine which coil to be energized next, you need that transition to wait until the coil has been de-energized, before the contacts change to select the other coil. Otherwise you will create a buzzer.

    I believe that you really need an impulse relay like that mentioned by K.I.S.S. in post #99. That kind of relay acts like the button on a ball point pen. You have to push, and release, the button for each change of state. The two steps of the operation are very important for the stability of the system.
     
  8. dcwatson84

    dcwatson84 Member

    Joined:
    Jul 21, 2016
    Messages:
    61
    Likes:
    0
    I guess I can't explain at that level why it's working, but it does work. The SPST relay is latched on, but the coil is not energized I guess? The hitting of the top/bottom switch closes a contact which causes the main relay to switch off. I have a drawing which I'll post tomorrow night.

    Keep in mind there is also the H bridge DPDT relay. The swapping of that current (positive to negative) is actually what flows through to the main relay and switches it off. This is so that the relay *cannot* switch off until the H bridge is swapped, since I want to leave the system in the swapped state for the next run.
     
  9. dcwatson84

    dcwatson84 Member

    Joined:
    Jul 21, 2016
    Messages:
    61
    Likes:
    0
    Here's my current schematic. I'm sure people will point out many issues, but if something looks off just ask me first and I'll explain. If I have some relay contacts backwards in my drawing, then please refer to the flow to see whether the contact is supposed to open or close the relay. One thing I have not shown here (but should have) is that all the relays have a single positive contact for the coil. So just assume that the two relays have that positive connected directly from the battery. The system is shut on/off by controlling the negative current via the main relay.

    Electrical Components...

    1x Battery
    1x Timer
    1x SPST Latching Relay
    1x DPDT Latching Relay
    2x NC1/NO1 Limit Switches

    Schematic Labels...

    T=Timer
    B=Battery
    M=Motor
    H=H Bridge

    Heres the flow of things...

    1) When no switches are being touched and the main relay is closed the motor is running in a given direction dependent on the state the H bridge is in. Both switches have a N.C. Neg current passing to either side of the bridge coil which means they aren't changing the state once it's in motion.
    2) When the door travels far enough to hit a switch, that switch opens its N.C. gate to the H bridge, which causes the current from the other to swap the bridge coil.
    3) At the same time the switch also closes its N.O. gate passing current to the main relay coil. This current is (by design) positive for an instant, having no effect. However as soon as the H bridge is swapped an instant later the current becomes negative, which opens the main relay.
    4) At this point the system is a) in the swapped state ready for the next cycle, and b) consuming no power. The catch is that it is also c) in a state where any current through the limit switch wants to hold the main relay open. The system will sit like this until the timer comes on again to close the main relay.
    5) When the timer comes on it runs negative current to the main relay and successfully closes it (because no current is coming through the switch from step #4 yet).
    6) Once the main relay is closed the switch is still attempting to open it. However the timer is still holding current to the coil for just a few seconds which means the switch cannot change the relay state.
    7) Once the door is raised/lowered off the switch (~1sec), the timer can shut off, leaving the system essentially at #1 in the flow.
     

    Attached Files:

    Last edited: Aug 10, 2016
  10. alec_t

    alec_t Well-Known Member Most Helpful Member

    Joined:
    Jul 10, 2011
    Messages:
    9,255
    Likes:
    1,218
    Location:
    Cardiff, Wales
    :confused: I don't see how the SPST relay ever gets energised if the lower terminal of its coil (if that's what the hatched thing on its symbol represents?) is connected only to N.O. contacts.
    It would help us to understand what's going on if you use conventional symbols for the relay(s) and show both their coil connections and their contact connections.
     
    Last edited: Aug 10, 2016
  11. dcwatson84

    dcwatson84 Member

    Joined:
    Jul 21, 2016
    Messages:
    61
    Likes:
    0
    Sorry, I don't know the conventional symbols for most of this stuff. As for your question, the SPST relay is first energized by current from the timer. Also note my satement in the post that the postive terminal of the coil is connected directly to the battery. So all it needs is negative current from the timer. It doesnt need to remain energized because it's a latching relay. The N.O. gate on the switch becomes closed when the switch is hit by the door.

    Er, also maybe I left out an important detail. These are dual coil latching relays. So they have a contact for both close/open actions. The contacts you see on the coils are 1 for open and 1 for close. The positive terminal is omitted/assumed. The open/close contacts are not labelled though, as I didn't know what a proper symbol might be.
     
    Last edited: Aug 10, 2016
  12. dcwatson84

    dcwatson84 Member

    Joined:
    Jul 21, 2016
    Messages:
    61
    Likes:
    0
    I got the switches today and assembled it on a breadboard and it worked great. System is activated by closing the main relay, you press one switch, the H bridge flips and the system shuts off. Activate it again and the current is now reversed. You hit the other switch, and back to the beginning. So there it is... 1 timer, 2 switches, 2 relays = coop door. I think my design is far more minimal than any others, and it required only 4 new electrical components. I'd still love any comments on the schematic though so that I at least know if im doing anything terrible.
     
  13. alec_t

    alec_t Well-Known Member Most Helpful Member

    Joined:
    Jul 10, 2011
    Messages:
    9,255
    Likes:
    1,218
    Location:
    Cardiff, Wales
    Good to hear it's working.
     
  14. Les Jones

    Les Jones Well-Known Member

    Joined:
    May 15, 2015
    Messages:
    1,446
    Likes:
    187
    Location:
    Lancashire UK
    Hi dcwatson84,
    I do not undersatnd the "schematic" in post #128. Is the item marked "H" and listed as an H bridge in your text actually a single coil DPDT latching relay conected like this ?
    DPDT_relay.jpg
    (Sorry to other forum members for using the very old convention for wires crosssing without making contact. I thought it may be clearer to the OP.)
    If so there is never any current through the coil as both ends are connected to the same point when the NC contacts of both limit switches are closed and if either is open then no current can flow. Also (Assuming that the NO contcts on the limit switches close when the limit is reached.) The coil of the SPST relay can only be energised when either limit switch is actuated. If these NC contacts work the other way round (I.E they are open when the limit is reached.) then the motor will be shorted out beween the limits.

    Les.
     
    Last edited: Aug 13, 2016
  15. dcwatson84

    dcwatson84 Member

    Joined:
    Jul 21, 2016
    Messages:
    61
    Likes:
    0
    Yes its DPDT and that is how it's hooked up. However (see my post from Wed) It's a dual coil latching relay. So each contact is only energizing the coil in one direction, and unfortunately I omitted the positive contact. So technically I think it has two coils, each with a permanent positive contact, and what I've shown is just the negative contacts, because those are the ones that I use to affect the state. Also, the coil does not have to be energized for current to flow through the relay. I must be using wrong terminology because I keep having to specify that - but every definition I look up for latching relay says that, but should I be using another term to make the operation more clear? With that in mind - in your drawing the current would flow regardless of whether the coil was energized. And that is indeed how it works. Current will flow through my H bridge even if the coil contacts are not connected to anything.
     
    Last edited: Aug 13, 2016
  16. Les Jones

    Les Jones Well-Known Member

    Joined:
    May 15, 2015
    Messages:
    1,446
    Likes:
    187
    Location:
    Lancashire UK
    In a single coil latching relay when the current flows in one direction (Say + to the top and - to the bottom.) the contacts would be pulled towards the coil and remain in that position when the current stopped flowing. I the polarity of the current through the coil was reveresed (- to the to + to the bottom.) the contacts would be pushed away from the coil and remain in that position when the current stopped. For a two coil latching relay think of the second coil as being to the right of the contacts (As in my drawing in post #133) Current through the left hand coil would pull the contacts towards it and they would stay in that position when the current stopped. Current through the right hand coil would pull the contacts towards it and they would stay in that position when the current stopped.

    Les.
     
  17. dcwatson84

    dcwatson84 Member

    Joined:
    Jul 21, 2016
    Messages:
    61
    Likes:
    0
    Yep, that's exactly how it functions in the circuit as well, I just didn't describe it in that detail, I assumed . As I said the only thing that's really omitted in the schematic is the positive coil contact. I didnt really know how to indicate that it was dual coil. I guess I couldve drawn two coils? But I wasnt sure if that was a standard convention.

    Knowing that, is the schematic a little more sensible? Or are there still questions or comments as to how/why it's working?
     
    Last edited: Aug 13, 2016
  18. Les Jones

    Les Jones Well-Known Member

    Joined:
    May 15, 2015
    Messages:
    1,446
    Likes:
    187
    Location:
    Lancashire UK
    Now that you say that the latching relay is a 2 coil type and explain that the conections that you show going to each end of a single coil in fact each go to one end of the 2 coils and the other end of the 2 coils are connected to battery positive the reversing part makes sense. From your "schematic" I can see that the motor will start in the correct direction but as soon as the door moves away from the limit switch it's NO contct will open causing the SPST relay to drop out and the motor will stop. It will never start again unless the door is moved to one of the limit switches.

    Les.
     
  19. dcwatson84

    dcwatson84 Member

    Joined:
    Jul 21, 2016
    Messages:
    61
    Likes:
    0
    The NO switch path isn't holding the relay closed, it's used to *open* the relay when the switch is hit. The relay is being held closed by current from the timer for the first couple seconds. Maybe I have the SPST coil reversed in the drawing.
     
    Last edited: Aug 13, 2016
  20. dcwatson84

    dcwatson84 Member

    Joined:
    Jul 21, 2016
    Messages:
    61
    Likes:
    0
    If I redrew the schematic, what's the correct way to depict dual coil latching relays so that that piece is clear from the drawing? Because they are both dual coil + latching.
     
  21. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

    Joined:
    Jan 4, 2007
    Messages:
    21,187
    Likes:
    644
    Location:
    Ex Yorks' Hants UK

Share This Page