# Simple (?) Automatic Polarity Reversal Circuit For Dual DC Motor Control

Discussion in 'The Projects Forum' started by Toby-Won, Feb 3, 2016.

1. ### Toby-Won Thread Starter New Member

Feb 3, 2016
4
2
Looking for input from someone a bit more technical than me...

I am working on a project that is runs two DC motors from only one switch. The issue is that regardless of the polarity directed into motor #1, motor #2 can only accept one polarity direction (please refer to my attached file).

I have searched for several weeks, but am having no luck. Most likely my inexperience in electrical systems is not taking me in the right direction.

Any help regarding this would be appreciated.

A few details...
Power Supply: 12 VDC
Motor #1: 12 VDC / 10A, bi-directional rotation
Motor #2: 12 VDC / 5A, unidirectional rotation only
Switch: Reversible - DPDT

• ###### Automatic Polarity Reversal.pdf
File size:
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Jul 18, 2013
10,872
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(What goes here?) ?
A bridge rectifier.!
Max.

Jun 22, 2012
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4. ### shortbus AAC Fanatic!

Sep 30, 2009
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1,698
So you would put the wires from the switch on the AC inputs of the bridge rectifier?

5. ### wayneh Expert

Sep 9, 2010
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That's a good simple solution, but unfortunately wastes about 10% of the power used by M2. A complex circuit, and active rectifier, could eliminate that loss but is likely not warranted.

Jul 18, 2013
10,872
2,528
Yes it is done quite frequently to ensure correct polarity.
The other simple way but more parts is two DC D.P. relays with a series connected diode to each coil, each one reversed biased to the other.
Max.

shortbus likes this.
7. ### TheNight Silence New Member

Feb 4, 2016
13
0
i think simply u can but two diodes on the two terminals of M2 as follow
the 1st diode the cathode is connected to the +ve terminal of M2,and the other the anode is connected to the -ve terminal of M2

Jul 18, 2013
10,872
2,528
That will only run for one polarity, not both.
Max.

9. ### Toby-Won Thread Starter New Member

Feb 3, 2016
4
2
Thanks for the information.

I did some research regarding the bridge rectifier and feel that this may be the simplest solution for this project. Fortunately, the input to M2 can be reduced by up to 15% without any adverse issues, so the losses mentioned above should be manageable.

I have a few different rectifiers on order and will try them out tomorrow.

Again, thank you for setting me it the right direction - I appreciate the responses.

10. ### k7elp60 Senior Member

Nov 4, 2008
481
69
Her is a complete schematic for you, Just make the sure the diode rating for the bridge rectifier is = to or greater than the motor current under load

11. ### crutschow Expert

Mar 14, 2008
13,526
3,393
To reduce the voltage drop, you could use some large (≥ 10A) Schottky type rectifiers.

You could also use a 3PDT center-off switch with the 3rd pole controlling M2.
Connect the 3rd wiper to V+ and both throws in parallel to M2's plus terminal.
Connect M2's minus terminal to V-.

Last edited: Feb 4, 2016
12. ### wayneh Expert

Sep 9, 2010
12,406
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Just to clarify, that should be done whether you use the other ideas or not. In other words, you could just use 2 diodes, just as you do when "OR"-ing power supplies together.

13. ### Toby-Won Thread Starter New Member

Feb 3, 2016
4
2
Thanks for the schematic... will come in handy!

14. ### Toby-Won Thread Starter New Member

Feb 3, 2016
4
2
All... The bridge rectifiers I received this morning work as you said (not that I had doubts).

Just wanted to say thanks again!

ebeowulf17 and wayneh like this.