# Reverse Polarity on a motor

#### Bouffant1984

Joined Jan 26, 2007
6
I am a computing student and for a project my group are extending out of our normal field of knowledge and have built a motorised chair that rotates through 180 degrees. The chair moves by a belt/pulley mechanism from a (geared down) motor. The chair is to be rotated left and right by switching the polarity of the current running through the motor. It all mechanically works at the moment and will move the chair with someone sitting in it quite happily when connected up to the power. This is to be activated by two footswitches, one to go left and one to go right. They are just simple on/off switches and we can't change them. My question is how do we wire it all up electrically to get these footswitches operating the chairs movement? I think we need to use relays or something but I am not really sure as its been years since I have done anything like this, my knowledge is at a complete beginner level. Also we are dealing with a current of about 0.5  1 amp DC. Sometimes it peaks at 2 amps for a split second when the motor starts up. If you have any idea how to make this circuit I would greatly appreciate any help. I dont think this is that complex but please understand I have not done this stuff before.

Any help would be much appreciated

#### kubeek

Joined Sep 20, 2005
5,757
I would suggest using an H-bridge made of four MOS-FETs, two P and two N type.

What type exactly are the switches? DPST momentary switch would be the best solution.

#### Bouffant1984

Joined Jan 26, 2007
6
Im sorry im I dont really understand what any of that means! The switches arent anything special they are just foot pedals with an on/off mechansim - but we cant change them, we have to use them.

#### Ron H

Joined Apr 14, 2005
7,014
I think I can tell you how to do this, but I need to know what voltage your motor runs on.

#### thingmaker3

Joined May 16, 2005
5,084
Type "h bridge" into your favorite search engine. You'll get several versions of basic h-bridge theory and many example schematics. It can be done with relays, with mosfets, even with bjt transistors if the motor voltage and current ratings are within the transistor specs. A set of 2n5884s and 2n5886s would, for example, suffice for your motor if the voltage is less than 80vdc.

What kind of switches are your switches? ("Nothing special" and "simple" are not switch types.) Are they single-pole double-throw? Double-pole single-throw? Something else? Are they push-on push-off or are they momentary contact? If they are momentary contact, are they normally open or normally closed? If you can't change them then it is most critical to know just what they are!

One more question, just to be certain an h-bridge is the way to go... Is your motor dc or ac?

#### Bouffant1984

Joined Jan 26, 2007
6
The motor is running on 13.8v DC. The switches are momentary contact ones that are normally open.

here is an image of them:

#### kubeek

Joined Sep 20, 2005
5,757
And how many wires/contacts does one switch have? 1 or 2 contacts (2 or 4 wires)?

#### Bouffant1984

Joined Jan 26, 2007
6
And how many wires/contacts does one switch have? 1 or 2 contacts (2 or 4 wires)?
1 contact per switch so 2 wires

#### Ron H

Joined Apr 14, 2005
7,014
I have a schematic for you, but I'm having trouble uploading it. I'll try again later.

#### Ron H

Joined Apr 14, 2005
7,014
I have a schematic for you, but I'm having trouble uploading it. I'll try again later.
OK, I'll try again.
EDIT: I uploaded it, but it came out tiny. I deleted it, and now once again I can't get it to upload.
EDIT:
One more time...
This should work. The logic prevents a dead short should both switches be pressed. The R-C-D networks build in some dead timeto prevent shorts during direction change. These may not be necessary, due to the inherent properties of the relays.

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#### Ron H

Joined Apr 14, 2005
7,014
I guess I should have done a new post instead of an edit. This should bump it so our OP can find it. See my previous post.

#### Bouffant1984

Joined Jan 26, 2007
6
ok thanks v much

#### Bouffant1984

Joined Jan 26, 2007
6
Hey that's brilliant, it works really well you truly are a pro!

#### Ron H

Joined Apr 14, 2005
7,014
Hey that's brilliant, it works really well you truly are a pro!
Gnarly!