Polarity-Reversing Cable for DC Power Supply

This is a very simple Instructable aimed at a specific problem. What to do if a DC power supply has the wrong output polarity – say, the plug tip is positive and the barrel is negative, but you need the opposite?

I recently needed a power supply for a Casio CTK-491 musical keyboard. I had one with the required 9 VDC / 1 Amp output, and the correct plug size - 5.5 mm outside diameter (OD) / 2.1 mm inside diameter (ID). However, the Casio jack requires the opposite polarity from that of the power supply.

I could have simply cut off the power supply plug, reversed the leads, soldered and insulated the joints, and marked the change on the power supply. However, I wanted a more flexible solution, especially since I might use that power supply for other applications requiring the original polarity.

Instead, I chose to make an “extension cable” that would go from the DC power plug to the keyboard jack that would also provide the needed polarity to the keyboard.

Step 1: Pick an Approach and Get Parts

There are a few approaches to doing this simple project. Barrel-style plugs and jacks are available with or without lead wires already attached. You may have one or both types in your parts collection. The options are:

Solder a non-wired plug or jack to its wired counterpart. Easiest and recommended.
Connect a non-wired plug to a non-wired jack with a two-conductor wire
Solder two wired connectors by their leads

I made two extensions for the purpose of this Instructable. One extension used a non-wired plug with a wired jack; the other used two wired connectors. I bought the following parts online on ebay.com:

Plug/Jack Pairs without leads: http://www.ebay.com/itm/281909198563

Plug/Jack Pairs with leads: http://www.ebay.com/itm/272180991208

These are for 5.5 OD / 2.1 mm ID plugs/jacks. Your application may use another size. The total cost for these was about $10, but that did give me additional spare parts for my ever-growing collection. :)

You'll of course need a few tools/supplies:

Soldering iron and solder (all Options)
Wire strippers (all Options)
Multimeter / continuity tester (all Options)
Needle-nose pliers (Options 1 and 2)
A length of two-conductor stranded 18 gage wire (Option 2)
Electrical tape or heat-shrink tubing and heat gun (Option 3)

Please take the appropriate precautions when working on this electrical project.

Step 2: Making a Cable - Non-Wired and Wired Connector

The simplest option (Option 1) is connecting a non-wired plug or jack to its pre-wired mate:

Check the polarity on the pre-wired connector. For those I bought, the red wire connected to the center (tip) and black to the exterior (barrel).
Unscrew the plastic sheath from the non-wired connector, and slide it down the wire of the pre-wired connector.
Solder the wires to the non-wired connector, making sure the polarity is reversed. (In my case, I soldered the red wire to the barrel and the black wire to the tip. The pre-wired connectors I got had short, pre-tinned ends, but you may want or need to strip a little insulation and tin the conductors, especially if you're recycling a wired connector from your parts bin.)
Use needle-nosed pliers to crimp the stress relief "wings" onto the wires to help prevent pulling on the solder joints.
Check the polarity and continuity between the plug and the jack.
Slide and screw the plastic sheath back into position.
Connect the extension cable to the power supply, and plug the power supply into an AC outlet. Make sure there are no signs of shorting.
Check the voltage and polarity at the extension plug before use.

The process is similar if you're joining two non-wired connectors (Option 2), except you're soldering at both ends after supplying and preparing the wire. (For this connector size, 18 gage stranded wire is appropriate.)

Step 3: Making a Cable - Two Wired Connectors

If you're connecting two pre-wired connectors (Option 3), you'll be soldering and insulating the wires end-to end:

Refer to the diagram throughout the steps below.
Check the polarity on the pre-wired connector. For those I bought, the red wire connected to the center (tip) and black to the exterior (barrel).
On each connector wire, split the two conductors at the end apart from each other, just far enough down the wire for you to work comfortably. If you're using shrink tubing for insulation, include enough room to slide a length of tubing on each conductor on one of the connectors. The tubing must be a little longer than the solder joint will be.
Strip and tin the leads, long enough that you can wrap them together inline. (I also recommend trimming and stripping the leads so that the two solder joints end up staggered / not adjacent to each other, as shown. That reduces the chance they may short together. Of course, make sure to do this trimming so that the correct wires are soldered together for reversing polarity.)
If you're using heat-shrink tubing, slide the tubing that will cover the solder joints down the split wires on one of the connectors. These pieces must be longer than the final solder joints will be. You may first want to slide a third, larger piece down the wire, if you plan to cover the entire splice in heat-shrink as well.
Twist the tinned wires together. In my case, I connected the red wire to the black on both sides to reverse the polarity.
Solder the twisted wire joints.
Insulate the solder joints. Apply electrical tape if you're using that. If you're using heat-shrink tubing, slide the tubing over the solder joints and secure using a heat gun. Do the same if you're using a larger piece to cover the entire splice.
Check the polarity and continuity between the plug and the jack.
Connect the extension cable to the power supply, and plug the power supply into an AC outlet. Make sure there are no signs of shorting at the joint.
Check the voltage and polarity at the extension plug before use.