how does a rear windshield motor reverse polarity?

Discussion in 'The Projects Forum' started by eire, Apr 21, 2014.

  1. eire

    Thread Starter New Member

    Apr 21, 2014
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    Anyone know how a rear windshield motor runs for a few seconds and at the end of its 180' rotation, changes dc polarity to go back? Is it a common switch or what? I am working on a yard ornament and would like it to do that.. for holloween! can you tell I'm bored..
     
  2. wayneh

    Expert

    Sep 9, 2010
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    I believe the reversal is mechanical, not electrical.
     
  3. MaxHeadRoom

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    Jul 18, 2013
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    I agree, traditionally it has been mechanical reversal, but with modern auto's, anything is possible.
    Max.
     
  4. BillB3857

    Senior Member

    Feb 28, 2009
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    I think it is the same way a piston reverses direction in an internal combustion engine.:)
     
  5. alfacliff

    Well-Known Member

    Dec 13, 2013
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    the ones I've seen use a reversing switch or mechanical linkage. gthe ones with the switch dont reverse current, they usually use an spdt switch to use one of two windings.
     
  6. MaxHeadRoom

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  7. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    They still work that way. Though mostly now days the linkage is under the cowl panel and can't be seen. The only ones I've ever seen that reversed direction in the "motor" were the really old vacuum wipers.
     
  8. BillB3857

    Senior Member

    Feb 28, 2009
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    We had a 1973 Ford Country Squire that reversed the motor to park the wipers below the cowl. Normal operation kept them above the cowl, but when turned off, they reversed and a special linkage took them lower.
     
  9. ErnieHorning

    Member

    Apr 17, 2014
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    I believe all of the ‘Hidden wiper” designs in the 70’s would reverse the motor. A cam inside the gearbox would lift the electrical contacts, which would stop the motor at the park position.
     
  10. ErnieHorning

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    Apr 17, 2014
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    A simple switch is all that's needed. You could also do this with a relay that has a DPDT switch configuration.

    [​IMG]
     
  11. MaxHeadRoom

    Expert

    Jul 18, 2013
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    The advantage of continuous run in one direction is the elimination of the high inrush as you reverse the motor, especially on immediate reverse where the motor has not yet stopped in the opposite direction, very frequent operations will greatly heat the motor.
    Max.
     
  12. GopherT

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    Nov 23, 2012
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    Welcome to the forum, Ernie.
     
  13. ErnieHorning

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    Apr 17, 2014
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    Now days I would use a FET H-Bridge with PWM and a microcontroller. But…

    Using the following circuit with carefully placed switches, the motor would slow down to almost a full stop before it could be reversed. The DPDT switch could again be part of a DC relay that could be controlled by 555 timer circuit.

    A stable power supply voltage would be critical for this to work reliably but it would be a very simple circuit.

    [​IMG]
     
  14. ErnieHorning

    Member

    Apr 17, 2014
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    Thanks.I normal frequent Christmas Light forums but I’m heavily into electronic design, both in my day job and my hobbies.If you do a Google search using my username, you’ll likely find all of those places and some of the things I’ve done.
     
  15. NFA Fabrication

    Member

    Aug 12, 2012
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    Here is a typical wiper motor. Picture the ball on that crank arm being attached to a link. That is how a typical wiper system has a push/pull to make the wipers go back and forth. And for curiosity, has the automotive ban been lifted? I would love to see the directions this forum could go if so!

    https://www.youtube.com/watch?v=rvX4LaVic1U#t=138
     
  16. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
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    Ernie, do you have a specific Halloween prop in mind? This reference, http://www.halloweenmonsterlist.info, has tons of projects, and some using a windshield wiper motor.

    I used to run a Haunted House fundraiser and have built many such devices. You can also google my user name, check my posts here or go to my website http://djsfantasi.com/ to see some of my devices.

    NFA, this isn't really an automotive post.
     
  17. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Manual Mechanical switches would work, but the OP design called for continuous automatic reversal?
    Max.
     
  18. eire

    Thread Starter New Member

    Apr 21, 2014
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    So I looked over the one on the back of a salvage yard and it seems to have wires going into the housing, a diode on the surface and wires then going to the motor.. must be magic inside the housing. not sure if I want to buy one just to rip it apart..
    front wipers use constant one direction and mechanical arms.. not the back. they figured a smooth working reverse polarity somehow?
     
  19. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
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    Do you have the make or model number of the wiper motor? What car and year did it come from?

    Are you stuck on using this motor or can you use a front wiper motor? There are many references online for a front motor. Look at Scary Terry's web site.
     
  20. ErnieHorning

    Member

    Apr 17, 2014
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    I did some searching on the net for rear wiper motors. It looks like they still turn one direction and has the same wig-wag output, it’s just more compact and there’s no need for linkage since there’s only one wiper.

    If you still want to change directions of a DC motor and do it totally automatic, here’s a simple way to do it. The Halloween guy’s may already know how to do this but it seems pretty simple. This should be applicable to linear or circular motion. I tried to find something on the net but couldn’t find one that was simple and had a decent schematic, so I did my own.

    It uses one DPDT relay as a temporary latch and two momentary limit switches. These could be straight push switches or the edge style that you roll over (micro switch). When it starts out, the relay is off and supplies power to the motor that must make it travel in the direction of Switch 1. When the Switch 1 limit is pressed, it supplies power to the relay which causes the top relay switch to latch the relay in a permanent on condition. This also causes the bottom switch to reverse the power to the motor. When limit Switch 2 is pressed, it removes power to the relay and it returns to turning the motor in the original direction. The will repeat endlessly.

    [​IMG]
     
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