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View Full Version : relay to reverse polarity of 12v current on timer


ammich1
12-07-2009, 11:49 PM
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.

mik3
12-08-2009, 02:04 AM
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.

ammich1
12-08-2009, 02:34 PM
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

mik3
12-08-2009, 02:46 PM
Post the datasheet of the timer.

ammich1
12-08-2009, 03:46 PM
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

mik3
12-08-2009, 04:21 PM
The output of the timer is 120V, isn't it?

So you will need relays to control the 12V motor.

ammich1
12-08-2009, 04:32 PM
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.

mik3
12-08-2009, 06:11 PM
You can use two timers to control the motor but it is not the best solution since you need two relays.

KMoffett
12-08-2009, 10:36 PM
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

mik3
12-08-2009, 11:27 PM
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.

ammich1
12-08-2009, 11:50 PM
I have the thing working!!! Thanks, so much!
Now that it is working, I have some more questions? Was I to use the 2 SPDT relays(method 1 from first reply) instead of 1 DPDT for safety? If I would have used the DPDT relay the circuit could have given the motor power to reverse and go forward at the same time? Am I understanding this right?
Are you sure you can't use diodes, 2 timers, 2 wall warts, and 2 limit switches to reverse a motor? I wasted alot if time trying to get that to work. It really hurt my brain.

Thanks again. My next project is to make a solar water heater for less than a gallon of water. I only need it to keep the water above freezing. Any suggestions on where I should get some ideas?

ammich1
12-09-2009, 12:00 AM
Thanks for the suggestion with the 120v relay. That would certainly have worked for me. The motor I am running is tiny. I might have some reservations about having the coil energized for 12 hours each day.

mik3
12-09-2009, 12:08 AM
Are you sure you can't use diodes, 2 timers, 2 wall warts, and 2 limit switches to reverse a motor? I wasted alot if time trying to get that to work. It really hurt my brain.


You can use two timers, two wall warts and two limit switches but you have to make sure the timers will be never turned on the same time because the wall warts will be destroyed. It will be good to put a fuse on each positive side of the wall warts to protect them in case both timers turn on.

ammich1
12-09-2009, 01:29 AM
I must have been wiring it wrong. I was using diodes in place of fuses. It would reverse when I unplugged one wall wart and plugged in the other, but the motor turned very very slowly. I was only getting about 3 volts to it. You don't happen to have that wiring diagram archived anywhere do you?
Thanks.

mik3
12-09-2009, 01:39 AM
I don't have the diagram but if you post your circuit we might help you.

ammich1
12-09-2009, 02:38 AM
Okay. I'm not sure how to post a circuit. Is there a place to draw a circuit here?

CDRIVE
12-09-2009, 04:03 AM
Are you sure you can't use diodes, 2 timers, 2 wall warts, and 2 limit switches to reverse a motor? I wasted alot if time trying to get that to work. It really hurt my brain.


I'm afraid that you can't. Your wall wart polarities are opposite of each other, which precludes the use of blocking diodes. The wall warts cannot be simultaneously connected to the motor, whether the other timer is on or not. If you already tried this you may have already popped one of the wall warts.

ammich1
12-09-2009, 02:27 PM
Thanks for the info. I'm glad to know it won't work, since I was unable to make it work. The wall warts seem to be okay, but I'll mark them so I know which ones might have future problems.
Am I on track with my thinking about the use of the 1 DPDT relay versus 2 SPDT relays. Is it just so both cannot be energized at the same time?

CDRIVE
12-09-2009, 07:51 PM
Why not use the circuit posted by KMoffet?

http://forum.allaboutcircuits.com/showpost.php?p=194030&postcount=9

Since you don't want to deal with a 120VAC relay coil you could power a 12DC 4PDT model from radio shack with another wall wart. If your motor draws any significant current I would modify the circuit as per Mik3's suggestion.

mik3
12-09-2009, 09:12 PM
Okay. I'm not sure how to post a circuit. Is there a place to draw a circuit here?

You can draw the circuit on a paper, scan it and post it here as a photo.

ammich1
12-10-2009, 12:45 AM
Thanks for all of the help. I have completed the project and it is partially installed, just some minor tweeking with the placement of the limit switches to get it finished. I ended up using a combination of all the supplied info.
As far as my questions about the diodes and different relays I was just trying to learn a little more so that I would not have to bother you all the next time I attempt some simple automation.
By the way, I guess I'm in the dark ages here. I don't own a scanner. Maybe I'll have one the next time I have some wiring problems.
Thanks again!