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  #1  
Old 12-07-2009, 11:49 PM
ammich1 ammich1 is offline
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Default relay to reverse polarity of 12v current on timer

A similar thread was started on 06-05-2009, 08:27 AM. I have the same question and there is not an answer.


I am trying to make an automatic control system for a 12v dc motor.
The system will open a hinged door in the AM with a 120v timer. The timer will only be energized for 30 minutes each time it is activated. The door will close when the timer goes off again in the PM. The motor is connected to limit switches to stop the motor at each end.
How do I reverse polarity with a dpdt relay? I am having a hard time getting my brain to think this though.
Thanks.
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Old 12-08-2009, 02:04 AM
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mik3 mik3 is offline
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To be able to reverse the direction of the motor and stop it completely you will need two SPDT relays or a SPST (at least) relay and a DPDT relay.

If you use two SPDT relays connect:

the motor positive on relay1 C and the motor negative on relay2 C

the positive supply on relay1 and relay2 NO

the negative supply on relay1 and relay2 NC

When relay1 is energized the motor rotates in one direction. When relay2 is energized the motor rotated in the other direction. If both relays are off or on the same time then the motor stops.


If you use a SPST (at least) relay and a DPDT relay have a look at figure 3 here:

http://www.distel.co.uk/DC_MOT_CON1.htm

Replace SW1 with the SPST relay and you have full control.
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Old 12-08-2009, 02:34 PM
ammich1 ammich1 is offline
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Thanks for the reply. I need to get the relays and then I will give it a try.
Just for my info...is there any way to reverse a motor using 2 power sources(like 2 120v timers), diodes and 2 limit switches?
Thanks
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Old 12-08-2009, 02:46 PM
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mik3 mik3 is offline
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Post the datasheet of the timer.
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Old 12-08-2009, 03:46 PM
ammich1 ammich1 is offline
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The 120v timers are plain plug into the wall 24 hour vacation timer. You push the nib down for the times you want power supplied. I was thinking that one timer could be + polarity and the other timer - polarity. This would change the motor direction depending on the time of day. Sorry, no high tech stuff here. I am having a hard enough time with this simple circuit to start including fancy stuff.
Thanks
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Old 12-08-2009, 04:21 PM
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mik3 mik3 is offline
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The output of the timer is 120V, isn't it?

So you will need relays to control the 12V motor.
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Old 12-08-2009, 04:32 PM
ammich1 ammich1 is offline
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Yes, the timers o/p is 120v but I have 12v dc adapters that I would plug in, so it is now basically a 12v timer.
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Old 12-08-2009, 06:11 PM
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You can use two timers to control the motor but it is not the best solution since you need two relays.
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Old 12-08-2009, 10:36 PM
KMoffett KMoffett is offline
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This might work for you. Set the timer for power-off for all the time you want the door open, and power-on for all the time you want the door closed.

Ken
Attached Images
File Type: gif MotorLimitSwtReverserTimer.gif (12.7 KB, 75 views)
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  #10  
Old 12-08-2009, 11:27 PM
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KMoffet,

It would be better to control the relay coil with the limit switches and not the motor because if the motor needs large currents then the required limit switches will be bulky and expensive.
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