Voltage Divider Rule

When the output of a circuit is not loaded, the voltage divider rule can be used to calculate the circuit’s output voltage. Assume that the same current flows through all circuit elements (Figure 2–5). Equation 2–6 is written using Ohm’s law as V = I (R1 + R2). Equation 2–7 is written as Ohm’s law across the output resistor.

Figure 2–5. Voltage Divider Rule

Substituting Equation 2–6 into Equation 2–7, and using algebraic manipulation yields Equation 2–8.

A simple way to remember the voltage divider rule is that the output resistor is divided by the total circuit resistance. This fraction is multiplied by the input voltage to obtain the out-put voltage. Remember that the voltage divider rule always assumes that the output resistor is not loaded; the equation is not valid when the output resistor is loaded by a parallel component. Fortunately, most circuits following a voltage divider are input circuits, and input circuits are usually high resistance circuits. When a fixed load is in parallel with the output resistor, the equivalent parallel value comprised of the output resistor and loading resistor can be used in the voltage divider calculations with no error. Many people ignore the load resistor if it is ten times greater than the output resistor value, but this calculation can lead to a 10% error.