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Question on reversing polarity for DC Motors
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JagV12:
Timing is definitely not the best way to go. Limit switch is way better in this situation. Micro-switch or Reed switch & magnet will do. I had a 12V wiper motor to extend-show and retract-hide a 1 meter wide Hand fan (through a chain reduction gear). It was wired like this :

(See attachment. Hey, how can I insert the attached image here ?)

SW2 and SW3 are NC (Normally Closed). If SW1 is in the down position, B is +12V and A is 0V. Motor will rotate "forward" until SW3 opens. If SW1 switches to up position, A is +12V and B is 0V. D2 is passing current until SW3 closes again and motor rotate "backward" until SW2 opens. If SW1 switches to the down position again, D1 will pass the current until SW2 closes and motor will rotate "forward". Of course 1N4148 are probably not suitable for the motor startup current, choose the diodes and switches accordingly. In case of Reed switch & magnets, relays will be necessary...
Zip file is to be opened with Proteus ISIS for simulation (and fun !)

EDIT :New versions without diodes
Zero999:

--- Quote from: JagV12 on January 28, 2019, 08:35:55 am ---Timing is definitely not the best way to go. Limit switch is way better in this situation. Micro-switch or Reed switch & magnet will do. I had a 12V wiper motor to extend-show and retract-hide a 1 meter wide Hand fan (through a chain reduction gear). It was wired like this :

 (See attachment. Hey, how can I insert the attached image here ?)

SW2 and SW3 are NC (Normally Closed). If SW1 is in the down position, B is +12V and A is 0V. Motor will rotate "forward" until SW3 opens. If SW1 switches to up position, A is +12V and B is 0V. D2 is passing current until SW3 closes again and motor rotate "backward" until SW2 opens. If SW1 switches to the down position again, D1 will pass the current until SW2 closes and motor will rotate "forward". Of course 1N4148 are probably not suitable for the motor startup current, choose the diodes and switches accordingly. In case of Reed switch & magnets, relays will be necessary...
Zip file is to be opened with Proteus ISIS for simulation (and fun !)

--- End quote ---
That will work and is a common limit switch configuration. Yes, of course don't use the 1N4148 (I'd recommend Schottky diodes such as the 1N5822) and the switches should be rated to carry the full motor current. I agree, using relays is a good idea and is a must if the motor draws over a few Amps. If SW1 is replaced with two relays, the control and limit switches and associated wiring only needs to carry a small current.

Adding in-line images is a bit of a pain. Attach them and post as usual. Go to your post, open the image you've just attached. Copy the address to the clipboard (the exact procedure depends on your browser). Edit your post and paste it between the [img][/img] tags.

Here's an example, using your schematic.


Note, please use png format for schematics, because it saves space, results in a clearer image and makes it easier for others to edit, if necessary. You can save even more bandwidth by converting the image to indexed (256 or fewer colours) which will not affect the image quality of schematics, since they hardly use that many colours anyway.
Raj:
contactors maybe? for heavy motor ,it's easiest
Zero999:

--- Quote from: Raj on January 28, 2019, 02:04:12 pm ---contactors maybe? for heavy motor ,it's easiest

--- End quote ---
Yes, contactors are a good idea if failure to stop could cause a lot of damage and mandatory if it could result in injury.
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