Reversing polarity for DC motor

Discussion in 'The Projects Forum' started by riggz, Oct 25, 2007.

  1. riggz

    Thread Starter Member

    Oct 25, 2007
    10
    0
    This is kind of confusing, but here goes....

    I have installed new power mirrors on my vehicle. They are electronically adjusted by one master switch. The switch can adjust the mirror up/down and left/right. The problem I have is, the old mirrors and the new mirrors are not exactly wired the same. The new mirrors will adjust up/down correctly, but when the switch is trying to adjust the mirror left, the mirror moves right. Same thing for trying to adjust right, it moves left. There are 3 wires going to each mirror. 1 common wire, 1 wire to the up/down motor, 1 wire to the left/right motor.

    The motors are not accessible and the switch can't be modified. Here is what happens when the switch is set to each position:

    Up: Yellow (+), Green (-), Purple ( )
    Down: Yellow (-), Green (+), Purple ( )
    Left: Yellow (+), Green ( ), Purple (-)
    Right: Yellow (-), Green ( ), Purple (+)

    What I need to do is whenever the purple wire is active [ (-) or (+) ], reverse it with the polarity of the yellow wire.

    I'm familiar with relays but that's about it. If anyone can help point me in the correct direction I would greatly appreciate it!

    Thanks.
     
  2. niftydog

    Active Member

    Jun 13, 2007
    95
    0
    A bridge rectifier will rectify either a positive or negative voltage into a positive voltage. So, you could use one to turn the signal from the purple wire into an always-positive signal and use that to control a relay to switch the wires with each other. However, what is not clear from your post is what the ( ) indicates - whether it's 0V or ground or just totally disconnected from the circuit.

    The problem is this; say you connect a bridge rectifier between yellow and purple. It will create a positive voltage whenever there's a voltage difference between yellow and purple, provided that voltage is sufficient to bias the diodes on. The trouble is, if ( ) indicates 0V then there will be a voltage difference between yellow and purple all the time!.

    You might be lucky in that the "rest" state voltage [when purple = ( )] might be half that of the "active" state voltage [when purple = (+) or (-)].

    I think what we'd need to go further is actual voltage measurements on the wires made with respect to ground.
     
  3. riggz

    Thread Starter Member

    Oct 25, 2007
    10
    0
    Sorry, the ( ) is 0v or nothing. Neither ground or positive 12v.
     
  4. niftydog

    Active Member

    Jun 13, 2007
    95
    0
    Ok, so what is (-) then if not 0V?

    So, is it more like this?;
    ( ) = not connected
    (+) = 12V
    (-) = 0V
     
  5. riggz

    Thread Starter Member

    Oct 25, 2007
    10
    0
    ( ) Is nothing. I guess you could call it 0 volts. (-) would be a ground.
     
  6. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    So () means "open." ;)

    You want something that will swap yellow and purple polarities whenever current is flowing in purple, but not when current is flowing in green.

    I concur that a bridge rectifier on yellow and purple (from your switch) could route to a relay (double pole, double throw) to swap the yellow and purple (into the mirror).

    mirrors.jpg
     
  7. niftydog

    Active Member

    Jun 13, 2007
    95
    0
    ^ That's exactly what I was getting too! ^
     
  8. riggz

    Thread Starter Member

    Oct 25, 2007
    10
    0
    Ok, I'm having a bit of trouble understanding the diagram you drew up. Can you explain it in newb terms? :)
     
  9. niftydog

    Active Member

    Jun 13, 2007
    95
    0
    The rectangle with the diagonal line through it is the relay coil.
    The square on 45 degrees with a squiggle in it (diode symbol) is a bridge rectifier.
    To the right side of those is the switch terminals of the relay.
     
  10. riggz

    Thread Starter Member

    Oct 25, 2007
    10
    0
    Ok, I'm with ya so far. 1 more thing (I think).... what's the current flow through the recitifier for each situation?
     
  11. niftydog

    Active Member

    Jun 13, 2007
    95
    0
    It will be whatever the relay coil draws, which depends on which relay you select.
     
  12. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    When purple is at ground and Yellow is at +12V, the rectifier will provide +11.3V to the coil. When purple is +12V and yellow is at ground, the rectifier will provide +11.3V to the coil again. In either case, the relay is energised, swapping the purple and yellow out to the mirror motors.

    When purple is open, no current comes from the bridge rectifier at all, and the relay is de-energised. Yellow remains yellow going to the mirror.
     
  13. riggz

    Thread Starter Member

    Oct 25, 2007
    10
    0
  14. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    They will work.
     
  15. riggz

    Thread Starter Member

    Oct 25, 2007
    10
    0
    excellent. I will pick these up later today and give it a shot.
     
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