DC polarity reverse

Discussion in 'General Electronics Chat' started by xxxyyyba, Jan 18, 2015.

  1. xxxyyyba

    Thread Starter Member

    Aug 7, 2012
    249
    2
    Hi!
    Here is simple circuit for DC polarity reverse.

    1.jpg
    When one press switch 1 (switch 2 is off), S1 will close, S2 will open and S3 will close. We have +12V on output:

    2.jpg

    When one press switch 2 (switch 1 is off), we have -12V on output:

    3.jpg

    My question is, how would one realize switch 1 in practice?
     
  2. franticAAC

    New Member

    Jun 25, 2012
    28
    1
    a simple relay with 2 contacts normally open and two contacts normally closed (2NO & 2NC).
     
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  3. profbuxton

    Member

    Feb 21, 2014
    233
    68
    Just use a double pole double throw switch.
     
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  4. Reloadron

    Active Member

    Jan 15, 2015
    963
    232
    I am not sure what you are asking but as suggested for mechanical switches use a DPDT switch as shown below.

    Switch Polarity Reverse.png

    One problem with the switches in your drawing is if the wrong switch is switched out of sequence bad things can happen. If for example S2 and S3 are both closed the 12 volt source will become very unhappy very fast. Even in my drawing the switch must be a break before make design which most are anyway. Even when electronically switching polarity, as in a H Bridge considerations must be made to make sure we don't directly short our supply.

    Ron
     
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  5. xxxyyyba

    Thread Starter Member

    Aug 7, 2012
    249
    2
    Thanks for replies. I want to control DC motor for moving satellite dish. I used 2 DPDT Double Break switches and 2 "ordinary" switches. It is absolutely necessary (you will see why) that these switches have some kind of spring inside them so when one press them they will close but as soon as one move hand away of them they come in "original" state. If you know what I mean :)
    Here is schematic:

    22.jpg

    Under normal circumstances (satellite dish didn't reach maximal left or right position) S1 and S4 will be closed and direction of moving satellite dish will be controlled using S2 and S3. If we press S2 motor will have -12V:

    33.jpg

    If we press S3 motor will have +12V:

    44.jpg

    Of course one shouldn't press both S2 and S3 simultaneously.
    S1 and S4 will be mounted next to satellite dish so when satellite dish reaches for example maximal left position there will be physical contact between it and S1 and S1 will open. When satellite dish reaches maximal right position there will be physical contact between it and S4 and S4 will open.
    Now if satellite dish reach maximal left position, S1 will open (and let's say that for moving satellite dish to left one has to press S2):

    66.jpg

    Now satellite dish can't go left anymore but we can move it in opposite direction if we press S3:

    77.jpg

    Because of it's internal construction, S1 will close as soon as physical contact between it and satellite dish disappear:

    88.jpg

    Similarly, if satellite dish reach maximal right position, S4 will open (and let's say that for moving satellite dish to left one has to press S3):

    99.jpg

    Now satellite dish can't go right anymore but we can move it in opposite direction if we press S2:

    10.jpg

    Because of it's internal construction, S4 will close as soon as physical contact between it and satellite dish disappear:

    88.jpg

    What do you think? Where can I find these switches?
    Btw it would be better if I used "maximum east/west position" instead of "maximum left/right position".
     
    Last edited: Jan 19, 2015
  6. Reloadron

    Active Member

    Jan 15, 2015
    963
    232
    OK, this would have gone better if you explained the project in the first post. You are talking about employing "Limit Switches" so when the motor reaches full travel in either direction the motor will stop. The below circuits are similar to what you have.

    Limit Switches.png

    There are two examples of using limit switches. SW1 should be a On-Off-On DPDT switch with the center position Off. Depending on your needs you may want to use a Momentary Switch for SW1 as in Mom On/ OFF / Mom On which means when On the switch must be held, these switches are spring loaded. SW 2 and SW 3 are limit switches. I would use what I call Micro Switches. The link explains them and shows the different styles, you will see how they can serve as limit switches.

    Something you need to consider is these drawings are rough drawings. The switches are not specified as to their ratings. I have no idea what the current draw is of your motor. You need to make sure any components used can handle your DC currents for the motor! If the motor is high current then these switches would be used to turn on and off large relays or contactors to handle your motor current.

    I don't know because I can't spec out switches without knowing all the motor data. Also based on your location I have no idea what suppliers you have available. As for me? For stuff like this I just call my local McMaster Carr Supply, if I call today I have the parts tomorrow or I can just drive over and pick them up.Additionally you need to figure out which limit switches would work for what you have. Any industrial supply distributor should have this stuff you need.

    Try these naming conventions:
    Manual Reversing Motor Switches Springs Back (Momentary) or Stays Switched (Maintained)
    Limit Switches can be pin plunger, roller plunger, roller lever, adjustable roller lever, adjustable rod and wobble stick to name a few.

    Ron
     
    Last edited: Jan 19, 2015
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  7. xxxyyyba

    Thread Starter Member

    Aug 7, 2012
    249
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  8. Reloadron

    Active Member

    Jan 15, 2015
    963
    232
    OK, that is likely a good choice. Windshield wiper motors like that are gear head motors and can deliver considerable torque. I suggest you read this link before we go any further. These motors can draw some high DC Current as can be seen in the link. Without a load 4 and 5 Amps isn't unusual. The problem becomes switching those higher currents of DC. I suggest using a high current H Bridge circuit to handle the higher currents. The Pololu High-Power Motor Driver 18v15 is the one I used in the attached drawing and linked to. This way you can use off the shelf simple switches and components. Here is how I would go about doing it:

    H Bridge Motor Driver.png

    I would use an ATX (computer) power supply if you don't already have one. While I show F1 as 15 Amp I would start with a 10 Amp fuse. You may want to also fuse the motor. The data sheet for the H bridge covers things well. I just noticed I forgot to place wire on the LED cathodes, sorry. Resistors R1 and R2 will be determined by the LEDs you choose. Typically they will be around 200 Ohms or so. SW1 is a Normally Open small push button switch and serves as a reset switch. SW2 is the direction switch and I suggest a spring loaded momentary switch as in (MOM) ON-OFF-(MOM) ON as we covered earlier. SW3 and SW4 are the limit switches to prevent motor over travel.

    Ron
     
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  9. xxxyyyba

    Thread Starter Member

    Aug 7, 2012
    249
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    I'm not sure I can find high current H Bridge in my place :( It's not necessary, it's only suggestion, right? I have computer power supply. DC motor doesn't need constant voltage, right? It will work with pulsating voltage like one below (full wave rectifier output voltage, without smoothing capacitor)?

    pulse.jpg
     
    Last edited: Jan 19, 2015
  10. Reloadron

    Active Member

    Jan 15, 2015
    963
    232
    You can try switches, the H Bridge would just be my choice. Just make sure the switches are rated for the DC current you will be switching. There is also the option of relays but again making sure the relay contacts can handle the DC current. I would think an H bridge module like the one I suggest can be ordered online from anywhere on the planet but do understand some parts can be hard to get in some locations. You may want to contact the company I linked to and look at International Orders. I feel for what you want to do the H-Bridge is the best choice. Maybe another member has some thoughts?

    Ron
     
  11. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,548
    2,373
    Many of the DC drives use a SCR controlled bridge and operate on the resulting 120hz ripple signal.
    No capacitor required.
    Max.
     
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