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Mechanical engineer needs help with electronics!!!!

Discussion in 'General Electronics Discussion' started by Monkeypuzzeled, Jun 14, 2011.

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  1. Monkeypuzzeled

    Monkeypuzzeled

    3
    0
    Jun 14, 2011
    Hello Electronics Point Forum!:D

    First post and I'm in need of help:confused:. To be honest, I'm more used to things I can hit with hammers and am harking back over half my life to my GCSE electronics memories (which are vague to say the least).

    I think I'm right on the circuit I've attached as a pdf. But I can't be sure, and need to be 100% confident before I move forward. And it's bound to need tidying and I'm going to have to have missed something!

    So, down to business. I am working on a model car door window mechanism that needs to drop 15mm when one switch is momenterally pressed and returns the 15mm when another switch is momenterally pressed (the two press to make switches in the pdf).

    As ever in life I have no budget, so it's on the cheap cheap cheap.

    Here's what I'm thinking:-

    The motor I have is a two poll 12v dc that I have geared to two rack and pinions. So I need a circuit that will turn on in either polarity and stop when limit switches are reached (The two push to break switches).

    Two SPDT relays, a motor, two diodes, four microswitches and a battery seems to be the best I can come up with?

    Would I need the diodes? Is there a chance of the current after the motor would latch the resting relay (would I need resistors?)? Can anybody out there help me? Please please please, slap me with a kipper if i'm being a fool and I have it totally wrong. But please post a circuit that will work for me (and tell me where I'm wrong or I'll never learn!)!!

    Right, sorry for the long blurb, and thanks in anticipation,

    Monkey
     

    Attached Files:

  2. poor mystic

    poor mystic

    1,067
    31
    Apr 8, 2011
    Hi I think the diodes would be better directly across the inductors so that any potentials that the inductors generate at switch off, which will be in opposition to the previously applied voltage, are shorted back into the inductor instead of arcing out over your switch contacts. Spark quench circuits can get surprisingly sophisticated but the diode is the simplest and most universally applicable.
    Well, I've gimped your drawing and changed the diodes around to where I think they should be - the image is attached for criticism.
     

    Attached Files:

  3. Monkeypuzzeled

    Monkeypuzzeled

    3
    0
    Jun 14, 2011
    Cheers Mystic

    Thanks for your input Mystic.

    I see you've removed the latch connections (where the diodes used to be), was that a mistake?
    Should I use DPDT relays for the self latching of the relays and keep the two circuits seperate?? :confused:
    Does the circuit seem sound otherwise? Attached is the rough layout including a solenoid shot bolt that'll stop the door opening if the glass is up, but release when down?!

    Any more for any more?

    Thanks again,
    Monkey
     

    Attached Files:

    Last edited: Jun 14, 2011
  4. poor mystic

    poor mystic

    1,067
    31
    Apr 8, 2011
    I do beg your pardon. I now see that your diodes have a latching function, whereas I had only looked for spark-quenches.
    Yes by all means your circuit looks good, and the only thing I think you need to add is diodes in the place I put them, to reduce contact wear due to arcing from the inductor.
    So here's the amended diagram as attached, once again for criticism.

    There is something else I've thought of - sometimes when you need a slow release on a relay you can replace the back-emf diodes with non-polar capacitors - usually small ceramic discs. This allows ringing currents to flow in a tank through the relay winding and makes the relay a bit more slow to release.
    It all depends on need.
     

    Attached Files:

  5. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    Looks good, except it'll lock up (never going anywhere until the power is turned off) if both buttons are pushed. Maybe the use of DPDT relays can solve that issue.
     
  6. poor mystic

    poor mystic

    1,067
    31
    Apr 8, 2011
    Well spotted Resquline!
    an alternative might be to use a rocking momentary SPDT switch which does not allow both sides to be made at once.
     
  7. Monkeypuzzeled

    Monkeypuzzeled

    3
    0
    Jun 14, 2011
    Thanks to all!!

    Thank you both so much for your input. I'll be going ahead with DPDT relays and additional diodes to stop arcing!

    Your help has been invaluable and you've given me the confidense to push ahead!

    :D
     
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