changing polaritys

[Justin Barnett]

how could i hook up a switch on a electric motor so it will change the polarity i want to make a normal box fan be able to go backwards. In other words where would i put the switch?

 

[Reloadron]

Reversing the DC polarity to a DC motor is a relatively simple process using a DPDT switch. I tried to load an image link but am having problems. The below should give you an idea of a few things to consider.

**broken link removed**

However, things may not be quite as simple as the switch circuit makes it look. Small box fans as frequently found in home computers can't be reversed. They are not designed to run in reverse and when connected with reverse polarity will not run at all or some may emit smoke. Fans like this are designed to move air in one direction, right down to the blade pitch of the blades. Most fans like this are designed to run on a full 12 Volts or PWM (Pulse Width Modulation) which precludes using reverse polarity.

The first thing you need to do is determine if the fan in question can run on reverse polarity. Even if it does you need to determine if it will move air when running reverse polarity.

What exactly are you trying to do? Based on your post I assumed a DC motor?

Ron

 

[Vizier87]

google for H-bridges, you can make 'em using a combination of switches. L293 chip also serves that purpose, but given your status, I don't think you want it to be electronic-controlled.

 

[Vizier87]

here's a good diagram:
**broken link removed**

 

[Reloadron]

This is more what I had in mind for a simple way to reverse the polarity. However, again as to fans things don't always go as expected. Now if you want to go the electronic route then consider an H bridge but to keep it simple a DPDT switch will work. Additionally I wouldn't go from forward to reverse with the motor running.

Ron

 

[Justin Barnett]

thanks i guess its worth a try im going to go out to radio shack later to get a DPDT switch

 

[Vizier87]

The problem with DPDTs is that you can't switch it off, you can only have it in one direction or the other, but not to a stop. Well if that is what you want, go ahead. No problemo.

 

[crutschow]

The problem with DPDTs is that you can't switch it off, you can only have it in one direction or the other, but not to a stop. Well if that is what you want, go ahead. No problemo.

You can solve that by using a DPDT switch with a center off position.

 

[Vizier87]

You can solve that by using a DPDT switch with a center off position.

oh.. there is one? I didn't know that. Great then, that's even better.

 

[crutschow]

**broken link removed** are some examples of DPDT, center off toggle switches.

 

[Vizier87]

wait a mo.. that is a switch.. I thought you referred to DPDT relays.. well this serves the purpose anyways..