Name of safety switch (stop at center) to reverse dc motor? Reduce chance of stupidity (mine).

Discussion in 'Electronics Resources' started by fitzhugh, May 1, 2015.

  1. fitzhugh

    Thread Starter New Member

    May 1, 2015
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    I have added a brushed dc motor and controller from a treadmill to an old shopsmith. I am hoping to find a simple switch that prevents the user (me) from bumping (or stupidly throwing) it all the way from one direction to the other by physically having to throw the switch, release it, and throw it a second time to get it all the way over.
    Any idea what I would call such a switch? A dpdt what?
    All my searches take me either to large knife switches for power distribution (no halting function) or switches to avoid turning starter when car motor is running, or engaging blades on riding mower when in reverse.

    My google powers are weak.

    Btw, I don't need to actually wait for the motor to stop. I hope to revisit this when I have time and use it as a chance to learn some really basic aspects of circuits, as the fan on the back of the motor has tabs for an optical encoder (not present). Adding a circuit to check that, or the hall sensor tach off ebay I added, before switching relays? I'll come back for that when I get my project list to a manageable level.

    Thank you for any suggestions
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    One, it is not safe or good on the drive to reverse it when it is still turning in the opposite direction, the motor presents a virtual dead short to the drive at this point.
    Commercial drives such as KB/Baldor use a special toggle switch that has a physical stop to prevent going from fwd to rev it forces a stop at centre, also not only is a resistor placed across the motor at the centre off point in order to brake, but also the drive inhibit terminals are closed, essentially resetting the drive, forcing the drive to go through an acceleration process even though the speed pot may be in a high setting.
    If you do this with relays etc, then it requires some special wiring.
    T.M. drives do not usually have a inhibit because they are not required to reverse, they do however, often have a requirement that forces the pot to be taken to zero first if the drive is powered on/off to avoid a sudden high rpm when power is applied.
    Max.
     
  3. MaxHeadRoom

    Expert

    Jul 18, 2013
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    I should add you can get the switches from a local Baldor motor rep. they are not cheap.
    You may be able to order them from KB also.
    I have one I could sell you.;)
    Max.
     
  4. fitzhugh

    Thread Starter New Member

    May 1, 2015
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    Yes, that is exactly what I'm aiming to avoid (switching directions when the motor is still turning). I didn't explain well.
    As you said it never was intended to run in reverse on the treadmill and no provisions or safety features were included in the controller to accomodate such use. The switch will be between the motor and the controller.
    ... replacing what I'd typed but forgot to post...
    well, closest I've found are ones Honeywell makes: - on-off-on dpdt 'three positions lock' locking lever (they have levers that lock in all three positions, or just the center, or .... lots of options) but they cost upwards of $50 per switch. They are 'mil grade' however, while I just need 'really not very likely at all to fail' grade. NKK makes them and they run under $10 for volumes of one.
    Example part number from nkk: M2023LL2W30 (have not looked at data sheet to determine if that is right, but it is in the right series. M20
    The problem is, best I can tell, there is nothing that actually keeps you from moving it the other way. I want a center lock, on off on switch that lets you push it to the center but not further. Seems I'm on the right track.
     
  5. MaxHeadRoom

    Expert

    Jul 18, 2013
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    If you want the one I have for $10.00 +p/p it comes with a brake resistor, it is the Baldor drive switch and does not allow the straight switching from left to right without a pause in centre.
    Max.
     
  6. LDC3

    Active Member

    Apr 27, 2013
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    This should work as a toggle to give you a forward and reverse control lines. The switch is a (on)-off-(on). R1, R2 and C1 is to debounce the switch. Starting from both outputs low, moving the switch to forward will activate the forward control line. When you move the switch to reverse, the forward control goes low; the reverse control line remains low. Moving the switch to reverse a second time will make the reverse control line high. You will have to adjust the resistor (R4) and capacitor (C2) at the BPT to delay the falling edge on the clock line. The only problem I encountered was when I started the simulation and the JK states were unknown.

    Toggle-Switch.png
     
  7. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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  8. MaxHeadRoom

    Expert

    Jul 18, 2013
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  9. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    [​IMG]

    This is only one of many possible supplier/manufacturers. I just linked it to show that locking toggle switches is probably the proper term that needs to be searched for.
    Sorry if my input is not helpful
     
  10. fitzhugh

    Thread Starter New Member

    May 1, 2015
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    No, it is very helpful, answers my question directly and gives me a start.

    I'm happy to get all the help I can and different approaches is great.

    I'll have to look at the details of each later when I can access my desktop, the phone doesn't cut it.
    The circuit LDC3 posted sounds like a good chance to learn. I'll have questions, in sure, but want to do my homework first.
    In the mean time, Thank you each!
     
  11. strantor

    AAC Fanatic!

    Oct 3, 2010
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    why not have two cheapo 2-pos DPDT toggle switches; one for FWD and one for REV? You could place them physically apart from each other for maximum idiotproofness. The forward switch will not work if the reverse switch is in the reverse direction, and vise versa.
    switches.png

    You could flip the reverse switch upside down so that it makes more sense ("down" = "reverse ON")
     
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