Need to reverse a motor with temperature.

Discussion in 'Homework Help' started by David Hennessy, May 2, 2015.

  1. David Hennessy

    Thread Starter New Member

    May 2, 2015
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    Hi guys, hope you could help me out as this is for an important school project.

    So here is my problem;
    I need to move a motor one direction when it's hot, move it opposite direction when it gets cold, now that part is OK, It's working that way with a simple thermistor circuit and a DPDT relay, but i need the motor to stop when limit switch is pressed.
    Currently the two limit switches that i have inserted only turn the motor off when it is rotating clockwise and not anti clockwise. Here is how I wired it:

    [​IMG]

    In otherwords it is semi working, when the motor turns clockwise Limit S2 turns it off, but when it turns anti clockwise Limit S1 does nothing, although Limit S1 turns it off when motor turns clockwise aswell, which is pointless.

    Not sure if i'm explaining myself but hope there is someone here that could help me out.
    Thanks in advance.


    This is the DPDT relay i'm using http://www.rapidonline.com/design-te...-relay-60-4690
    And the temp circuit i'm using is http://www.rapidonline.com/pdf/70-0315-Thermistor-temp-project.pdf although that works perfectly.
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,347
    Hello,

    How about placing the limit switches ditectly at the motor.
    Use diodes accross the switches, so the motor will work again when reversed.
    The limitswitches and diodes must be able to withstand the motor current.

    Bertus
     
  3. David Hennessy

    Thread Starter New Member

    May 2, 2015
    7
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    Thanks for the answer i think that should work :) I don't have access to a diode atm so i will buy one on tuesday and post here if it works :)
     
  4. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,347
    Hello,

    As I said, you will need diodes for both directions, so one accross each limitswitch.
    Try to make a schematic, so we can check if you understand what I mean.

    Bertus
     
  5. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,553
    2,375
    When you draw it up you only need two conductors out to the motor from the limit switches.
    Max.
     
  6. MrAl

    Well-Known Member

    Jun 17, 2014
    2,439
    492
    Hi,

    If the limit switches cant take the current of the motor, then another relay will handle that, or perhaps two.

    But my question would be, how does the motor start again once the limit switches turn it off? The other limit switch should be closed, which would allow the motor to turn in the other direction to get away from the first limit switch that recently opened.
     
  7. David Hennessy

    Thread Starter New Member

    May 2, 2015
    7
    0
    I got two power supplies, first one powers the sensing circuit, the second one powers the motor which is always turning until the limit switch is pressed, and reverses when the first power supply energizes the relay. (not sure how to explain it properly)
     
  8. David Hennessy

    Thread Starter New Member

    May 2, 2015
    7
    0
    Yea i get the idea of what you mean, so the diode blocks the current when motor is running in a certain direction and allows the limit switch to work(?)

    Would this be how you wire it? or the diodes go on the other side of the limit switch?
    [​IMG]
     
  9. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,553
    2,375
    Each N.C. L.S. has a diode rectifier in parallel, when the switch opens the diode is forward biased only when the polarity reverses.
    It is an old ploy.
    Max.
     
  10. David Hennessy

    Thread Starter New Member

    May 2, 2015
    7
    0
    Oh, that makes sense :D

    I got it working now without the diodes, was setting the circuit up like above just to insert the diodes when i buy them and it worked, maybe it's because instead of wiring the limit switch from both the NC pins in the diode i wired one from NC and another from NO (by accident :p) [​IMG]

    Just wondering, if i leave it like this it won't damage the motor or anything right?

    Again, thank you all so much for the help :D
     
  11. MaxHeadRoom

    Expert

    Jul 18, 2013
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    It shouldn't.
    Just inconvenient.
    Max.
     
  12. David Hennessy

    Thread Starter New Member

    May 2, 2015
    7
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    Ok thank you all very much, i'll add the diodes anyways to get a good grade on it ;)
     
  13. David Hennessy

    Thread Starter New Member

    May 2, 2015
    7
    0
    It finally hit me, there was a problem that the second limit switch didn't work and then...bam i understood what you meant there. Thanks Max, Bertus. Appreciate it.
     
    Last edited: May 5, 2015
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