12v linear actuator control circuit help needed

Discussion in 'The Projects Forum' started by JohnF, Mar 26, 2010.

  1. JohnF

    Thread Starter New Member

    Mar 23, 2010
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    I have a 12 volt Dayton linear actuator #4Z842 (obslolete). I am looking to put sealed limit switches towards each end of planned travel. The actuator label says 12 volt 22 amp. I read a post in here where it was stated look for a switch with that rating but I'm coming up empty handed in my search. I have the wiring diagram but can not source the darn switches. I can't even find anything in Honeywell. What I'd like to use is a sealed micro or snap switch but at this point I'd consider anything sealed that covers this power rating. Help PLEASE! :confused:
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    You don't use the limit switches to switch the motor current.

    You use the limit switches to control power relays.

    Something like this:

    [​IMG]

    The relays can be SPDT automotive-type 30A power relays. The other switches can be rated for double the relay coil current.
     
    Last edited: Mar 26, 2010
  3. John P

    AAC Fanatic!

    Oct 14, 2008
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    The guy didn't way how he wants to control this thing, but I don't like the aspect of that design that means you have to hold the button down until motion is complete. There could be an extra contact on the relays that hold their power on until the limit switch operates. Some power contactors do this with some contacts being rated for high power and others not.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    The schematic I posted above is similar to the way that automotive power windows and seats work. As long as you depress either S1 or S2 to run the motor in one direction or the other, the motor runs until the limit is reached or the switch is released. The motor is automatically braked when S1/S2 are released. Pressing both S1 & S2 simultaneously will simply cause the motor to brake; both terminals connected to +12v, which is the same as connecting both to ground.

    Since the only specifications the OP gave were controlling the limits of travel using limit switches on a 22A motor, I just put up something I had already prepared that would work with low-power limit switches and widely available relays.

    If this is an application where humans might be near the actuator when in operation, the controls should still require manual input to control the actuator.

    There IS a very simple way to make that circuit self-latching by adding only a single pair of diodes. However, that should only be used if there is no way that humans could come in contact with the device while in operation.

    To keep the schematic simple, no reverse-EMF diodes for the motor were shown.
     
  5. JohnF

    Thread Starter New Member

    Mar 23, 2010
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    Thanks for the info. This is in fact an application where human contact is not possible (covered) so it could be a self-latching design. If my understanding of that, is that it stays energized until limit is reached. Is this correct? The design engages and disengages a reverse drive motor with a total of about 1.5" of travel. The original design had 6" travel. In the meantime I have stumbled across linear actuator controllers. They are only about $50 and seem to cover and solve a whole bunch of design problems. I'm thinking for the money "Why re-invent the wheel".
    Thanks again for your help!
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    Yes.

    Here is the latching version:

    [​IMG]

    Note that reverse-EMF protection diodes for the motor are still not shown for simplicity.

    Your time is certainly worth something. Purchasing a COTS solution (Commercial Off-The-Shelf) is often the most cost and time-effective way to solve a problem.

    If simply getting the job done is your goal, the COTS would likely be the way to go. However, keep in mind that when it fails, you may be going through this whole scenario again.

    If you build your own controller from automotive relays and locally available limit switches/pushbuttons, you can simply purchase a few extra parts to have spares on hand for when it eventually breaks. Being an electromechanical device, it will eventually fail.
     
  7. JohnF

    Thread Starter New Member

    Mar 23, 2010
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    I already ordered the COTS solution but am keeping your schematics for future reference and "just in case". Thanks again for your help.
    John
     
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