Two DPDT switches (non-momentary) for one motor

Discussion in 'General Electronics Chat' started by eriktoft, Jul 12, 2012.

  1. eriktoft

    Thread Starter New Member

    Jul 11, 2012
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    Hi,

    I have a motor (12V DC) that I need to be able to operate both backwards and forwards from two locations. The simple way is to connect it with two DPDT switches with momentary on in each end and off in the middle, i.e. also called (on)-off-(on). See attached drawing.

    The reason for using momentary switches is that there would otherwise be a risk of short circuiting if one of the switches are left in on-position. However, I almost always need the motor to go to preset limits in both directions which takes a couple of minutes (the limits are built-in functions in the motor, so no need to set those). So I was wondering if there was a cheap and easy way of inserting a kind of timer or similar that would only leave the connection on for a couple of minutes only (and yes, there would still be a risk of short circuiting within those two minutes, but it would be unlikely to happen)?

    Or in other words, how can I get an effect as if using non-momentary DPDT switches but without risking short circuiting?

    Any ideas would be welcome. In (the likely) case the above is not clear, perhaps this will help: I need it to operate a curtain, i.e. moving the curtain up and down, but I need to be able to do that from two different locations. Preferably it would be good if I could also stop the engine somewhere in the middle - as I would be able to do with a switch simply be releasing it. The best/cheapest/easiest way would, I think, be to have two DPDT switches with a spring or something with a couple of minutes delay, i.e. if it is left in the on-position for more than two minutes it would automatically spring back. But such a thing seems not to exist... or?

    Would be grateful for any inputs.

    Erik
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    Try it like this, instead:

    [​IMG]

    There is no combination of switches that will cause a short.
    Both switches will reverse the direction of the motor.
    You won't need a timer.
    The disadvantage is that you won't be able to stop the motor in the middle; that would require another pair of SPDT switches, and more wiring. If you really want to do that, we can.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
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    Here's the same thing, with the capability of stopping at any position added:

    [​IMG]

    As before, there is no combination of switch settings that could cause a short.
    The two SPDT switches in the middle allow you to stop anywhere, and start back up again, from either switch station.

    As before, no timers are necessary or desirable.
     
  4. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,388
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    SgtWookies wireing ideas are very good. To get the timing function as simply as possible you could just use a time delay relay such as one of these. Looking at one that may work (SST12QEA) I found but one distributor and it was $66, so this isn't a cheap solution.

    (I didn't think it would be that cheap as I'm the guy who designed the timing circuit for the MIL-PRF timers and I know what the board itself (without the relay) costs.)

    So you may be better off doing the '555 timer thing. Once you select a relay (12V automotive style relays would be good here) you can see if the 555 can drive it directly or if you need a transistor to switch it.
     
  5. eriktoft

    Thread Starter New Member

    Jul 11, 2012
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    That is absolutely fantastic SgtWookie. So simple! Luckily I did not go with timers, relays etc. Just one additional question though: Are the SPDTs really needed if I have on-off-on DPDT switches: I would then think that when one of the switches are in the off-position, there would be no power coming to the motor? Or perhaps the issue is that it is not supposed to be on-off-on.

    On the other hand: I assume if one of the DPDT switches are left in the center position, it would mean that there would be no power coming to the motor regardsless of what I do on the other switch, or I am wrong? Perhaps that is what the SPDTs are for, i.e. if using the SPDTs there is not really any need for DPDT with on-off-on, but just on-on?

    Or am I again misunderstanding something?

    Thank you so much.

    Erik
     
  6. eriktoft

    Thread Starter New Member

    Jul 11, 2012
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    Thanks for this too. With SgtWookies solution I do not need a timing. But I have added a few questions for him/others on the DPDT and SPDT switches.

    Erik
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    You need ON-ON DPDT switches. If you use on-off-on switches, you won't be able to start the motor from one station if the other station is in the OFF position.

    Same with the SPDT switches; they also need to be ON-ON.
     
  8. eriktoft

    Thread Starter New Member

    Jul 11, 2012
    4
    0
    Thanks a lot for this. I think I am good to go. If I do not post anything here, everything is working smoothly.

    Erik
     
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