I have a 18 volt dc motor that is being supplied with 6 v or -6 volts. How would I amplify the 6v to 18 or in reverse -6 to -18 v using a 12 v battery as an additional power source.

 

So, do you want to power an 18V motor from a 12V battery?

How much current does your motor require?

 

If using an additional 12v battery then just series them up for 18v?
Max.

 

6+ 12 v is 18 v. I can't put them in series because when the motor goes in reverse the ground and source swap so the it would be -6 + 12 v only 6 volts and it would be in the wrong direction. When forward I get 6 volts want 18. When in reverse I get -6 want -18.

 

Connect your 12V battery in-series with your existing 6V supply. Now you have 18V. Since it seems you need to reverse the motor, use an 18V H-Bridge, with the motor connected across the sides of the bridge. Your positive logic inputs (presumably 5V) can be referenced to 0V...

 

If connected in series isn't there a risk, if the battery becomes discharged, of it receiving a reverse charge from the 6V power-supply?

 

The 6 v supply alternates to -6. Its like an ac supply, but there is no fixed frequency.

 

the 6 v supply used to power the motor hence the 6 and-6 volts for direction. Now I need more power so I have to increase the voltage to 18 for forward and -18 for backwards. The 6/-6 v supply isn't a battary it is the power coming from a seperate chip that I can't modify.

 

Can't you just buy an 18V supply? Probably cheaper (certainly simpler) than building a circuit to combine and reverse the two supplies you have.

 

But how to swap the polarity of the 18 v battery so the motor can go in reverse? I can build and solder a circuit but Im clueless as to the design for this.

 

As Mike said (post #5), use an 18V H-bridge.

 

But how to swap the polarity of the 18 v battery so the motor can go in reverse? I can build and solder a circuit but Im clueless as to the design for this.

A simple DPDT relay would work just fine for the power handling and reversing process.

 

Thanks. Would it be possible to build a circuit like it using transistors instead?

 

Would it be possible to build a circuit like it using transistors instead?

Google 'MOSFET H-bridge schematic'