How to Figure out needed Voltage, Resistance and Current

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This formula will tell you what resistance you must provide to reduce the voltage of the power source to the needed\safe component voltage. You need to the know the manufacturers specification for 'Forward Voltage' and 'Amperes' of the component you want to power. 

 R = (*bV-*fV) / I

Lets take an LED light. Mine are 3.2V Forward Voltage, and will draw at least 24 mA. We want to illuminate it with a 9V battery.

  1. Subtract The forward voltage of the LED 3.2V from the Battery's 9V. Gives us 5.8V. That is the excess voltage, that will damage your LED. 
  2. Now we divide the excess voltage by the amperes\current. First Convert the mA's to Amps, take 24 mA and divide by 1000, equals .024 amps.  Now divide 5.8V / .024A
  3. Your answer is 241.6666 Ohms
So using a 241 Ohm resistor with a 9 volt battery, will reduce the voltage to 3.2, and provide enough current. 

  • R = Resistance
  • ** bV = Battery\Power Source Voltage
  • ** fV = Forward Voltage: What the component requires, like a standard LED requires a minimum of 3.2V
  • I = Amperes
  • mA = Milliamps  
  • Ohms = Resistance

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