Resistance is an important concept in electrical work. By changing the amount of resistance in a circuit, it's possible to change the voltage within that circuit. This ensures that every component within the circuit gets just the right amount of electricity to power it without causing damage. Adding resistance allows you to step down the voltage of a 12V circuit to only 9V, but you have to make sure that you don't overdo it; adding too much resistance will lower the voltage too much, potentially causing problems for those components that are starved for power.

As electricity flows through a material, it encounters resistance. This means that not all of the voltage passing through the material makes it through as output voltage, as some portion of it is absorbed by the material itself and turned to heat. This is actually how electric heaters work; high-resistance materials have electricity run through them, causing the materials to heat up and emit that heat into the surrounding air. When electrical current needs reduction in an electrical circuit, resistance is added in the form of resistors. A resistor is a high-resistance material encased in a protective coating (often epoxy) to prevent them from radiating heat while still providing resistance within the circuit. Resistors are made to provide a specific amount of resistance, measured in ohms, and are color-coded for easy identification. The color code used depends on the type of resistor you use.

If you need to step down a 12V current to 9V within a circuit, you'll have to first determine how many resistors you need and how many ohms of resistance they should provide. First, determine exactly how much you need to step down the voltage by subtracting your desired output voltage from the input; in this case, you have:

To determine how many ohms of resistance you need to step down these three volts, you'll need to know how many amps are in your circuit as well; this can vary from one circuit to another and will depend on the materials used, your power source and how you've designed the circuit. Calculate the ohms of resistance (​R​) you need by using the formula:

with ​V​ equaling the volts you're stepping down (3) and ​A​ equaling the amps in your circuit. Once you know your resistance, you can decide whether you want to use a single resistor or if you want to break it down across multiple resistors.

Once you've calculated your resistor needs, it's time to install the resistors into your circuit. If using a single resistor, you simply need to install it between your power source and the device or load that requires a 9V current. If using multiple resistors, they will go in the same location (between the power source and the load). Install the smaller resistor first, stepping down your voltage from 12V to 11V. Once you've added the first resistor to your circuit, install the larger resistor to step the voltage down again. This resistor will take the remaining 11V current and reduce it to the 9V output you desire.

Once your resistors are installed in your circuit, be sure to test its voltage with a multimeter. The input voltage on the circuit should still be 12V, but the output voltage should drop to 9V as the current runs across the resistors. If the voltage drops as expected, finalize the circuit and solder everything into place. If the output voltage is wrong, however, recheck your calculations and change your resistors until you get the proper voltage shift.