I am confused about something. I need to drop 120 volts down to 12 using a resistor. I am trying to power a 12 volt motor with an amperage draw of .3 amps. I need help in calculating how many ohms of resistance I need to make this drop.
DROP THAT IDEA! You will likely get killed!
Get a switched power supply converting 120 Volt AC to 12 volt DC, able to output at least 5 Amps to survive the start current of the motor.
I am trying to make my own power supply
my father just told me that a transformer if the way to go in this process
but I have no knowledge on the subject of transformers. How you do calculate the voltage drop of transformers based on the amount of coils.
That, and a rectifier (or four, in a "bridge"), and a filter capacitor.
You can also find 12 V power bricks dirt cheap at flea markets, garage sales and thrift stores.
i am already using a rectifier
I just want to know how to drop the high voltage to a low one here
Use a 120 VAC to 12 VAC transformer (9 VAC will work, too).
The purpose of the voltage drop is to?
The purpose of the voltage drop is to?
See post #1
Besides being super dangerous there's another problem - The resistance of a motor isn't constant (and neither is the current). That makes it impossible to get constant 12V across the motor.
It's also super inefficient. 12V x 0.3A = 3.6 Watts for the motor. The resistor will have 108V x 0.3A = 32 Watts. And that means you need a big power-resistor, and the resistor will get hot!
When it starts-up it draws more current (lower resistance) and/or if there's a heavy load on the motor it draws more current. If there's no load on the motor and it's just spinning, current is lower.
Ohm's Law describes the relationship between voltage resistance, and current. (Current = Voltage/Resistance).
If you're working with low current "signals" you can make a Voltage Divider. For example if you want to feed 12V from a car into an Arduino input, a voltage divider is one way of converting that signal to 5V.
Voltage dividers are NOT for "power".
Assuming the 12v motor is dc, much safer and better approach is use rechargable battery for your supply.
Then use a mains charger suitable for your battery.
The size and type of battery would depend on your application.
@ 300mA, a standard AA battery (8 x 1.2V NiMh) 1500mAh would run for approx 4 hours or more depending on what loading you apply.
A good choice would be AA Eneloop cells, most will have chargers available as a combo pack.
Thanks for the reminder.
this is not really a project that is necessary but one for learning purposes
thank you all for your help
You were completely and correctly answered in #2. 
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