The formula for calculating the output voltage of a voltage divider with two resistors connected to a battery is
Equation 1: Vout= Vbattery*R2/(R1 + R2)
The circuit diagram below shows a voltage divider connected to a 10 V battery. With the 2000 and 3000 ohm resistors, the output voltage is calculated from equation 1 as
Vout = 10*3000/(2000 + 3000) = 6 Volts
The idea can be extended to any number of resistors to obtain any fractional output voltage desired. The formula comes about by dividing the applied voltage across the total series resistance to obtain the current. In the example above the current is 2 mA (10V/5000 Ohms). The current multiplied by the resistor will give the voltage drop across the resistor.
Voltage Buffers
Often when a voltage divider is used to supply a different voltage than a battery voltage, a voltage buffer is placed at the output voltage node of the voltage divider. A voltage buffer, because it does not draw any current (or very little), will not change the output voltage of the voltage divider. Voltage buffers are often made from transistor devices such as JFETs and MOSFETs.
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