DC motor autostop circuit

Discussion in 'The Projects Forum' started by 5&under=sorcery, Sep 29, 2010.

  1. 5&under=sorcery

    Thread Starter New Member

    Jul 4, 2010
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    I got my electric gate project running. Runs forward, reverses, closes. I designed and built an autostop circuit using the normally open (closes at gate position full open or full closed) contacts in the (gate ram) microswitches. One for full open, one for closed. The gate is now under manual control via the hand held transmitter buttons. Push-run, release-stop. Two buttons, open and close. The autostop circuits I built worked fine but I don't like them. They used accessory relays and added another layer of complexity and potential breakdowns. Since the contacts on the microswitches close upon full open or closed that is the natural signaling method to use. I'd like to put those switches in parallel with one leg of the DC circuit and have the current to the motor shunted away so the motor will stop and the users can't continue to drive the motor after the gate is fully open or closed. I was thinking maybe a large capacitor across the positive lead and ground that would discharge across a diode as soon as the ram started reversing from it's previous position and the micro contacts open again. The motor is tiny and only works because it is WAY geared down. Anyone have a better idea? It's 12 volt DC, max 15 amp.
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    You need something like this:

    [​IMG]

    Note that the limit switches are NC. This is more reliable than using NO switches, as if the contacts fail open, the motor won't run.

    The way it's wired causes a braking action on the motor as soon as the limit switches contact the end.

    One brief pulse on either S1 or S2 causes it's respective relay to latch.

    Just noticed that as it's wired now, it's possible to latch BOTH relays at the same time, causing the motor not to run; in order to re-set the latched relays you would either have to press one of the limit switches or cycle the power.
     
  3. 5&under=sorcery

    Thread Starter New Member

    Jul 4, 2010
    28
    0
    OK, never mind. I started drawing again. Sometimes it pays to get away from a piece of engineering for a bit and let the mind leave the rut of what is blinding it to other approaches. The solution was a 1 pole, one throw relay, breaking only one side of the circuit. Doesn't matter which side breaks, result is the same, the motor stops. The micro switches carry only the coil current. By tying everything together properly it is always the positive side of the circuit energizing one side of the coil when the ram reaches either full open or full closed, pulling the normally closed relay contacts carrying the motor current open. Much less complex than my previous design.
     
  4. 5&under=sorcery

    Thread Starter New Member

    Jul 4, 2010
    28
    0
    Sgt Wookie, that is actually somewhat similar to what I drew originally. Thank you.
     
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