# Calculating the resistance needed

I am trying to calculate the resistance and power Rating for a resistor. I have a 12V 2A power supply and I'm trying to divide it across different motors that are rated for 12V at 0.32A. I understand ohms law so that i need a resistance of 37.5 ohm but I don't understand the power Rating of resistors and what it means. Does the power rating need to be 7W?

Thanks

Harry

• You really don't need any resistors here. – Eugene Sh. Feb 27 '18 at 14:34
• What are the resistors for? – Trevor_G Feb 27 '18 at 14:34
• Read THIS – Trevor_G Feb 27 '18 at 14:35

You dont need any resistor if you just wanna drive those motors with 12V supply. 0.32A is the current that your motor draws at steady state for 12V input. You just have to connect both motors in parallel to the 12V supply. They will draw 0.32 + 0.32 = 0.64A in total from the supply, while your supply is able to source up to 2A. However the motors will draw high peak current initially while starting. Later on back-emf is generated, which reduces the peak current to the steady value of 0.32A.

When a motor is rated at 0.32A, it means that the motor will draw this amount of current when it is operated at its rated load, the rated mechanical output of the motor. At other loads, the motor draws as much current as needed to meet the demand; current varies with the applied load. Note that exceeding the rated load might result in motor damage from overheating.

So, just wire the motor(s) to the power supply, no resistors are necessary. The motor current is limited by its internal resistance and back emf.

BTW, the power dissipated in a resistor is the product of the voltage across it and the current through it. For example, if you connect your 37.5 ohm resistor across your 12V supply, the power in the resistor will be 3.84W

If a motor is rated to run on 12 V then you can just put 12 V across the input and it will work correctly. There is internal resistance of in the coil of the motor and the operation of the motor will introduce it's own impedance that limits the current to what's on the plate.

In your case just put them in parallel across the voltage source.