Connect with us

Reversing polarity

Discussion in 'General Electronics Discussion' started by Ernie Woodburn, Jan 7, 2018.

Scroll to continue with content
  1. Ernie Woodburn

    Ernie Woodburn

    6
    0
    Jan 7, 2018
    Hi all. Brand new here and looking forward to discussions with you all.

    I have this circuit wired up to reverse polarity which works great but what I need is to latch the limit switches sw1 & sw2. I need the motor to run until s2 is pressed and then reverse direction until sw 1 is pressed and visa versa.

    They are just single press limit switches. Any help will be appreciated.
     

    Attached Files:

  2. AnalogKid

    AnalogKid

    2,412
    686
    Jun 10, 2015
    A wiring diagram and a schematic are two different things. Please draw a schematic using standard relay symbols so we can see what you are trying to do. Pin numbers on a socket don't really tell much.

    ak
     
  3. duke37

    duke37

    5,364
    769
    Jan 9, 2011
    BISTABLE

    My scanner has gone into a sulk.
    You can do this with a pair of NOR gates 4001 or a pair of NAND gates 4011 or a 555.
    The 555 may be prefered since it might be more robust.
    Connect p2 to 12v via 10k.
    Connect p4 to 12v via 10k
    Output p3.
    One relay will be connected to ground and the other to 12V.
    The relay is 80Ω so will tax the 555 and a fet driver will be advantageous. Use a nchannel for one relay and a pchannel for the other relay.
    Ground p2 to go one way
    Ground p4 to go the other way.
    The direction will hold if the inputs are not grounded.

    I intend to use a similar circuit to change the traverse on a coil winder.
     
  4. Ernie Woodburn

    Ernie Woodburn

    6
    0
    Jan 7, 2018
    are you saying I can't do this with a SPDT automotive relay?
     
  5. BobK

    BobK

    7,682
    1,686
    Jan 5, 2010
    Yes, you can do it.

    Just look for a wire that is only activated in the forward direction, and insert the forward limit switch in that wire, then do the same for reverse.

    Bob
     
  6. AnalogKid

    AnalogKid

    2,412
    686
    Jun 10, 2015
    To be clear, you want the system to run back and forth continuously?

    ak
     
  7. kellys_eye

    kellys_eye

    4,275
    1,146
    Jun 25, 2010
    Yes you can. But you need limit switches at the end stops of the travel and a switch to determine the direction of motion.
     

    Attached Files:

  8. Ernie Woodburn

    Ernie Woodburn

    6
    0
    Jan 7, 2018
    I did find that diagram but the relays don't have the numbers on it. So yes, I want to flip a main switch and have an electric arm (window motor) travel up until it hits the limit switch and then travel down until it hits that limit switch and so on.
     
  9. hevans1944

    hevans1944 Hop - AC8NS

    4,542
    2,116
    Jun 21, 2012
    If I recall correctly, you cannot simply use a limit switch that reverses the motor direction when the limit switch is actuated, and then continue in the reversed direction toward the other limit switch, because as soon as the motor drives the mechanism off the first limit switch, de-actuating it, the motor will immediately reverse direction again, back to the original direction as soon as the first limit switch de-actuates. So the most it can do is oscillate a short distance back and forth as it alternately actuates an de-actuates the first limit switch. It never reaches the second limit switch at all.

    You need a 3PDT relay wired to self-latch through a set of SPDT contacts on the relay when the appropriate limit switch is actuated. This relay serves as a "one bit" memory to remember which way the motor will turn when the mechanism is between the limit switches. If the relay is de-energized, it will be wired to turn the motor in a direction that will energize (and latch) the relay when the limit switch for that direction is reached. If the relay is energized, it will be wired to turn the motor in the opposite direction to interrupt the self-latching circuit and de-energize the relay when the limit switch for that direction is reached. The remaining pair of DPDT contacts are used to implement the motor direction function.

    The relay "remembers" which direction the motor was turning and remains either de-energized, or energized and latched on, until a limit switch is actuated. Actuating the limit switch either interrupts the latching circuit of the energized relay, thus de-energizing the relay, or it energizes the relay coil and enables the relay to self-latch. The motor therefore reverses direction and continues in the reversed direction each time a limit switch is actuated. The process continues indefinitely as the motor drives the mechanism between the two limits.

    3PDT relays may be harder to find than a SPST relay and a DPDT relay. If you want to use two DPDT relays, or a SPST and an DPDT relay, wire their coils in parallel so they both operate at the same time. Use one set of SPST contacts on one of the relays to implement the self-latching function. Use the DPDT contacts on the other relay to implement the motor direction function.

    It's been awhile since I've played with DC motors, but I am not aware of any circuit that will cause a DC motor to reverse direction using only a SPDT set of relay contacts. Please post such a circuit here, if you find one, so that my training can be complete,
     
  10. Kiwi

    Kiwi

    335
    83
    Jan 28, 2013
    Automotive terminal numbers for 5 terminal changeover relay;
    Contacts
    30 - common
    87 - normally open
    87a - normally closed

    85 - coil negative
    86 - coil positive
     
  11. hevans1944

    hevans1944 Hop - AC8NS

    4,542
    2,116
    Jun 21, 2012
    Okay, for those of you who need a "wiring diagram" see the image below. The motor must have "floating" connections. Ignore the apparent open connection at the bottom of the motor "-" terminal: this was supposed to be a connection between the motor "-" terminal and terminal "30" of the bottom automotive relay.

    upload_2018-1-8_9-48-15.jpeg

    The circuit as shown assumes that the three relays (they all operate together) are NOT energized, that 12 V is applied as shown between 12 V and the ground symbol, and that the motor is running in a direction that will eventually cause NO limit switch LS1 to actuate. When that happens, LS1 NO contacts close to apply 12 V to all three relay coils and the the coils are subsequently latched on from the 12 V supply through terminals "87" and "30" and NC contacts of limit switch LS2. The motor direction reverses, driving the mechanism toward NC limit switch LS2. By the time it reaches and actuates LS2, LS1 has already de-actuated and its contacts are open. When LS2 is actuated, its NC contacts open, releasing the self-latching circuit and de-energizing all three relay coils. The motor again reverses and the process repeats indefinitely.
     
  12. Ernie Woodburn

    Ernie Woodburn

    6
    0
    Jan 7, 2018
    Thank you. If I did that properly, I couldn't get it to work. I assume that 87a on the top relay goes nowhere and that 30 on the bottom relay does NOT connect to 87. My initial diagram works well but I just need the 85 and 86 (sw1 & sw2) to latch so I think I will simply buy2 latching switches and that should do the trick.
     
  13. hevans1944

    hevans1944 Hop - AC8NS

    4,542
    2,116
    Jun 21, 2012
    Sorry about your luck. I have used this circuit for many years and it has always worked. It appears you may have a limited understanding of electricity and circuits, and perhaps are having difficulty following and connecting simple wiring diagrams.

    The bottom two relays are wired as a DPDT polarity-reversing switch. When both of the bottom relays are de-energized, as they will be when 12 V is initially applied, the motor will turn in one direction. When both of the bottom relays are energized, as they will be when 12 V is applied and limit switch LS1 closes, the motor will reverse and turn in the other direction. Obviously, initially the motor must turn in a direction that advances the mechanism toward actuating LS1. I have shown LS1 and LS2 as SPDT switches, which is standard practice for microswitch limit switches, but LS1 is wired as a NO switch and LS2 is wired as a NC switch when these switches are not mechanically actuated at the limit positions..

    The top relay is wired as a self-latching relay that is energized (latched) in one motor direction, and de-energized (un-latched) in the other motor direction. The limit switches are wired normally open (NO) for LS1 and normally closed (NC) for LS2. Both switches are shown in their de-actuated positions, as they would be when the mechanism is between limits.

    The motor runs as soon as 12 V power is applied. The direction that the motor moves the mechanism must be the direction that actuates LS1. When LS1 actuates, all three relays are energized and the motor reverses, driving the mechanism toward LS2. When the mechanism reaches and actuates LS2, it interrupts the latching circuit to the relays and the motor again reverses, driving the mechanism toward LS1. This action repeats until power is removed or the motor fails.

    No need to assume anything. That is exactly what the diagram I posted indicates. Only the NO contact "87" and the common contact "30" is needed to latch the top relay in the energized position through NC contacts on LS2.

    The relay armature or common contact "30" on both of the bottom relays connects only to the motor terminals, just as it did on your two-relay circuit. Unless I forget when hand-drawing a schematic, crossing lines are NOT connected unless a solid dot appears at the crossing. I like to avoid this even happening by breaking and then slightly off-setting the crossing line, placing two solid dots where it connects. I also put a dot where a connecting line terminates between the ends of the line to which it is connected. Strictly speaking, this is not necessary, but putting the dot down emphasizes that a connection is intended.

    The difference between my circuit and yours is this: my circuit always applies power to the motor if 12 V is present. Only the polarity applied to the motor changes when this pair of relays is energized (actuated) or de-energized (de-actuated).

    In your circuit, the motor windings are shorted together and connected to ground if your two relays are de-energized. If somehow both relays are energized, the motor windings are shorted together and connected to 12 V. The motor runs if, and only if, just one of the two relays in your circuit is actuated. If both relays are de-energized or both relays are energized, the motor terminals are shorted together by relay contacts. Nothing wrong with that, and it is an effective way of bringing a brushed DC motor to a rapid stop if your relay contacts can tolerate the arcing that occurs.

    Well, you go ahead and do that and report back here with your success. I, for one, would like to learn that trick.

    Maybe you could rig a pair of toggle switches with a stiff wire connecting their levers, so when the mechanism reached and flipped one toggle/limit switch, the stiff wire would flip the other toggle/limit switch in the opposite direction. Since only one toggle/limit switch would be flipped "ON" at any given time, they could each be wired to operate your two relays independently. You could probably rig this up with two household switches mounted at each end of the mechanism. Drill a hole through the switch operating lever and insert a coat-hanger wire to mechanically connect the two switches.
     
    Last edited: Jan 10, 2018
  14. Ernie Woodburn

    Ernie Woodburn

    6
    0
    Jan 7, 2018
    I will disassemble and rewire. Perhaps I missed a connection it did operate but did not reverse when it hit the limit switch. It tried to keep going.

    Although all help is very much appreciated, sarcasm and criticism is not. Although you are correct in that my experience with relay logic is low, most of my hobby work is done with PLC's and ladder logic. Hard wired relays are kind of obsolete so I haven' done much with them. I've done this sort of thing on a PLC a thousand times for large foundries but sometimes it's just more cost effective to use cheap relays.

    Coat hangers....seriously?
     
  15. Bluejets

    Bluejets

    4,268
    907
    Oct 5, 2014
    Go easy on the professionals , they take a lot of time and effort to help. It is not their problem if you cannot follow.
     
  16. Ernie Woodburn

    Ernie Woodburn

    6
    0
    Jan 7, 2018

    You guys are great.
     
  17. hevans1944

    hevans1944 Hop - AC8NS

    4,542
    2,116
    Jun 21, 2012
    Sounds like it was going in the wrong direction when it hit the limit switch. In other words, it should have been going in the opposite direction and hit the other limit switch instead. If it didn't reverse, then the top relay was not energized by the NO contacts of limit switch LS1 and actually it was NC limit switch LS2 that was actuated. Actuating LS2 DOES NOTHING unless the top relay is latched on.

    Why not? Cheap, readily available, stiff enough to do the job.

    Relays are far from obsolete, although many of their functions have been replaced with solid-state switches. And almost ALL of the old relay-logic is truly obsolete and its functions are now performed with PLCs or other embedded microcontrollers. Still, the time it takes a PLC to traverse a ladder diagram is significant and the delay brings it's own problems when attempting to implement control functions. I am especially distrustful of using a PLC to implement anything related to human safety without careful attention to fail-safe design practices. But that's a separate issue.

    To troubleshoot the three-relay solution to the problem, try disconnecting the coil of the top relay and energizing the two bottom relay coils directly from the 12 V supply manually through a toggle switch. Leave the wiring for the top relay contacts and the two limit switches in place for this test. Verify that the motor reverses direction as the two bottom relays are alternately energized and de-energized by operating the toggle switch. Note: the motor runs constantly when 12 V power is available; it is only turned off by removing the 12 V power.

    If the above test procedure for motor reversal operation works, leave the mechanism positioned between the limit switches, with neither switch actuated and 12 V removed. If the motor does not reverse when the two bottom relays are alternately energized and de=energized, correct the wiring associated with the contacts on those two relays and repeat the test until the motor reverses whenever the test toggle switch is operated.

    Now that you have verified that the bottom two relays will reverse the motor when energized and de-energized, re-connect the top relay coil back in parallel with the bottom two relay coils, along with the circuit containing the two limit switches and the top relay contacts.

    Apply 12 V power and observe which way the motor runs and which direction the mechanism moves. The mechanism MUST move in a direction that will eventually actuate LS1. If the mechanism is being driven away from LS1 toward LS2, reverse the two connections to the motor. None of the relays should be energized until LS1 is actuated. If the mechanism moves in the wrong direction, i.e., toward LS2 during initial power on, reverse the two motor leads before continuing.

    I have seen mechanical reversing mechanisms that operate pretty much the same way as I described for the two light switches connected with coat-hanger wire. Typically, a nut traverses a rotating lead screw until it hits a limit stop. The limit stop is mechanically connected to, and moves, a lever that reverses the lead screw rotation, driving the nut in the opposite direction. When the nut reaches the other limit stop, it pushes it in the opposite direction and this motion is communicated to the direction control lever by a connecting metal bar, again causing the lead screw rotation to reverse. The only difference between this operation and two light switches acting as limit switches with a coat-hanger wire connecting them is electrical.

    Oh, did you detect a hint of sarcasm in some of my comments? Which ones perzactly? As for criticism: it goes with the territory. If you post circuits here, they will be examined and criticized by someone if there is anything anyone perceives is wrong with your circuit. Take that with a grain of salt and weigh it against what you know versus what you believe. Not everyone who posts here knows WTF they are talking about, but its a free and open forum so they will post anyway. After all, facts and logic never stop a True Believer of any faith. You gets what you pays for... usually.
     
    Tha fios agaibh likes this.
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-