Power adaptors come in a wide range of voltage levels. The 12-volt adaptor is one of the most common sizes. What would you do if the device you needed to power was a 9-volt device and you only had a 12 volt adaptor. Even the plug would fit the device it's just that the voltage levels were different. It is possible to lower a 12 volt power adaptor to a 9 volt level.
Find out how much current the device needing the power will need. This is usually located on a label somewhere on the device. If the current is not provided, look for the voltage and power requirements. Divide the power by the voltage and the result is the current in amperes. The voltage has to measured in volts and the power must be in watts.
Divide 3 volts by the current found in Step 1. The results is the size of the resistor needed in ohms. For example, if the current 0.5 amps then the resistance would be 6 ohms. The 3 volts is the amount the voltage needs to drop to be compatible with the electric device.
Take the power adaptor and cut one of the wires going to the plug. Strip 1/2 inch of insulation off of the two ends that were cut. Connect one wire to one side of the 6-ohm resistor and connect the other wire to the other side of the resistor. Secure both wires and wrap the wire connections and the resistor with electrical tape.
Plug the adaptor plug into the device and test the device.