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using two momentary switches with DPDT Relay for f/r dc motor control

Discussion in 'Sensors and Actuators' started by Magoogle, Jan 5, 2015.

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  1. Magoogle

    Magoogle

    62
    1
    Jul 31, 2012
    I have a DC motor and I have 2 momentary switches as well as a DPDT Relay.

    I would like to use each switch to control the direction of the motor but only when the switches are depressed. When no switch is depressed I want the motor to be idle.

    Is this possible with my setup?

    I am repairing a lift chair for my grandma and the components that were damaged are no longer available.
     
  2. Gryd3

    Gryd3

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    Jun 25, 2014
    There is a circuit that is commonly used for automotive electric locks or windows.
    It boils down to using a 2 relays. The common terminal on each relay goes to each side of the motor. The Normally Closed contacts are typically both connected to ground, and the normally open contacts are then connected to power. When one of the relays get energized, there is a current path in through the energized relay, through the motor, and the return path is the relay in the neutral position. This allows for bi-directional control, and if both relays happen to be energized at the same time, the current flow will stop and no ill-effects will be observed.

    Here is an example schematic from a GM:
    The alarm arm/disarm wires would be from a Double Throw switch, or two momentary switches.
    The black and light blue wire would go directly to the DC motor.

    *As far as only using the components you have, I would need to know the current draw of the motor, and the current capability of the momentary switches. It is not possible to do what you want with a single relay if each switch will only be controlling the relay.
     

    Attached Files:

  3. Magoogle

    Magoogle

    62
    1
    Jul 31, 2012
    Thank you for your post Gryd3

    the handheld remote is a small board similar to a tv remote control board. I doubt they would handle more than an amp or so. The power-supply output for the motor is 24v at 2 amps.
     
  4. Gryd3

    Gryd3

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    Jun 25, 2014
    If that's the case, I would urge you to use the double relay method as pictured above for GE's electric locks. I can draw a different picture up if it helps. You just need to ensure the buttons or switches you want to use are rated high enough to actually energize the relay. I would not attempt to use the little rubber buttons in a TV remote, but an additional transistor and diode per relay can help with this.
     
  5. Magoogle

    Magoogle

    62
    1
    Jul 31, 2012
    roger thanks I have 2 standard spst automotive relays laying around also. Ill give this a shot.
     
  6. Gryd3

    Gryd3

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    Jun 25, 2014
    two SPDT relays need to be used. Perhaps you called them SPST incorrectly?
    You need to have the common terminal, Normally open, and normally closed terminal for the dual relay setup to work correctly. A SPST relay is a simple On/Off relay and only has one of the two NO or NC terminals.

    Edit: You know what... the image I linked is poorly done... Let me draw a more correct one for you.
     
  7. Gryd3

    Gryd3

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    Jun 25, 2014
    This is better...
    Each terminal on the motor is connected to a relay that is normally holding it at ground.
    Once a push button is pressed, one of the relays flip over and provide power to the motor allowing the current to flow. If both buttons are pushed, both relays switch over connected both sides of the motor to Power... there is no longer a current path so it stops. Unlike an H-Bridge, you don't need to worry about shoot-through or shorting it out by accidentally going two directions at once.

    *Take special note of the wires in the image! Dots mean the wire is connected, if there is not dot, the wires are simply going across each other.
    Relay.png
     
    KrisBlueNZ likes this.
  8. Magoogle

    Magoogle

    62
    1
    Jul 31, 2012
    hmm maybe the relays I had don't work right or are incorrect.. I put it together the way your first image said and the buttons click the relays on and off but provide no power to the motor.
     
  9. Gryd3

    Gryd3

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    Jun 25, 2014
    Sorry I should have posted that in bold once I realized the mistake.. the relays in the first image don't provide a ground path for the motor which is why I posted the second image.
     
  10. Magoogle

    Magoogle

    62
    1
    Jul 31, 2012
    well.. still not sure.

    I have a PCL-202D1s relay (8 terminal) and 2 standard 30amp 12v 5 terminal relays.. No idea if these are SPDT or not.

    The PCL relay will not work? I thought it would do the duty of 2 relays in a single unit
     
  11. Gryd3

    Gryd3

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    Jun 25, 2014
    The PCL relay can be thought of two relays in one... but we need each relay to behave independently... The PCL could be wired to alternate the current flow through the motor, but an additional relay would need to be wired in series to apply and disconnect power to the circuit... as the PCL can't do that.
    The 5 terminal relays you have sound like Double Through Relays... Should be two terminals for the coil, and 3 for the switching side. (Common, NO, NC)
    Grab a multi-meter and start probing if you are unsure which terminal is which.

    You will measure a resistance across two terminals. This will be the coil, that will be connected to the momentary switch.
    You will then measure almost 0 resistance across two other terminals, this is the Normally Closed pair. The last terminal is the Normally Open terminal. From here, put voltage on the pair of coil terminals and re-measure the other 3. The last terminal should now be connected to one of the normally closed terminals. Make special note that this terminal will be marked as 'common' because it will switch back and forth between the Normally Open and Normally Closed.
    If this is what you have, then you are good to start creating.
    Are there any markings on the 30A relays? I'd like to get you a pinout and redraw a diagram for you.
     
  12. Magoogle

    Magoogle

    62
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    Jul 31, 2012
    Here are the 2 relays. They are taped together
     

    Attached Files:

  13. Gryd3

    Gryd3

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    Jun 25, 2014
    eww... Looks like the two complementary terminals are tied together :s
    These relays will not work for you unfortunately.
    However, I'm certain I can whip something up that will use the PCL relay and one of those other relays that will function for you. You will need a diode as well.
    I will leave this up to you. Personally, I would use the two double through relays, but that would require you to go and buy them.
    Alternatively, you can hook both types of relays together... essentially the resting position of the PCL relay will be for one direction of the lift.
    When button 1 is pushed the 'regular' relay switches on, powering the lift to go up.
    When button 2 is pushed the 'regular' relay AND the PCL relay will switch. This will power the lift in the opposite direction.

    With the 'lets make it work with what I have' method, both relays MUST switch at the same time for the lift to travel in reverse. The other described method will only require one relay to switch for either direction and is simpler to wire and troubleshoot.

    How do you wan't to proceed?
     
  14. Magoogle

    Magoogle

    62
    1
    Jul 31, 2012
    which ever is best with what I have. I can solder anything up based on the diagram you provide. Honestly this would be really easy with a simple dpdt switch but grandma not good with new things. Trying to use that old remote.
     
  15. Gryd3

    Gryd3

    4,098
    875
    Jun 25, 2014
    You got a pic of your PCL relay?
    I'll draw up a schematic in the mean time.
     
  16. Gryd3

    Gryd3

    4,098
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    Jun 25, 2014
    K. Here is a basic circuit diagram . Built with as few components as I can think of.
    Untitled.png
    Switch one will only trigger the 'power' relay due to the diode causing the lift to move in direction 1.
    Switch two will trigger the 'power' relay and the 'direction' (PCL) relay causing the lift to move in direction 2.
    If the diode is accidentally orientated wrong, the button functions will be reversed.

    If you like I can show a more detailed image with I see the PCL relay you are talking about.
     
  17. Magoogle

    Magoogle

    62
    1
    Jul 31, 2012
    here ya go. Thanks again
     

    Attached Files:

  18. Magoogle

    Magoogle

    62
    1
    Jul 31, 2012
    I should have a diode laying around somewhere. Ill look for it.
     
  19. Magoogle

    Magoogle

    62
    1
    Jul 31, 2012
    If I cant find it Ill have to go to town anyway to get one so if that happens might as well tell me what I should get to make this work better.
     
  20. Gryd3

    Gryd3

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    Jun 25, 2014
    Well.. as a permanent solution. If you need to leave the house to buy a diode, it would be best to pick up two double through relays that are rated for 24V or higher. Then wire it up using second image I posted.

    The relays you have are only rated for half the voltage of the motor power supply, so they will most likely work short-term, but will likely die or fail in the short-term requiring you to do more work again. Again, sorry I missed the voltage of those relays. I finally got home and am able to sink more time and thought into my posts.

    If we were talking about restoring function for a temporary emergency move than I would personally use the relays you have fully expecting them to die anywhere between the first and perhaps 2nd or 3rd trip... considering you want to repair a piece of hardware due to lack of available replacement parts, I highly recommend the dual relay option and diagram 2.
     
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