Voltage Divider Utility Calculator

Instructions

This utility calculates any one of the four variables involved in a simple two-resistor voltage divider, given values for each of the other three variables. The variables are:

  • Vin - The (input) voltage that appears at the top of the divider, measured at the top of Rupper, with respect to circuit ground.
  • Rupper - The resistance (in ohms) of the upper resistor
  • Rlower - The resistance (in ohms) of the lower resistor
  • Vout - The voltage that appears at the junction of Rupper andRlower, measured with respect to circuit ground.

The Calculator

Voltage Divider

Enter Variable values (set unknown variable = 0)

VIn:
Rupper:
Rlower:
Vout:



Example

Normally, one wants to compute one of the two resistances, given a specified Vin, a desired Vout, and one of the two resistances. In this case, you enter the two voltages and the known resistor and the utility will solve for the unknown resistor

However, consider the following case:

You are troubleshooting a circuit and you find that the Vout of a voltage divider for a 5 Volt Vin , designed with two 10k resistors is only measuring .45V (you expected to see 2.5V).

In this case, you suspect you might have used an incorrect resistor value. You would input one of the designed resistor values, input the two voltages, and solve for the resistor value. Examining the calculated value could suggest that you may have misread a resistor. For example in a voltage divider consisting of two 10k resistors to divide 5V to 2.5 V, you find that the output was actually 0.45 V. You think you might have a wrong value for one of the resistors. So, you:

  • input the 5V as Vin, and
  • input the 0.45V as Vout, and
  • input the designed 10,000 ohms (no commas) as Rupper, and
  • set Rlower to zero (using the "0" button at the right-hand edge of the field), and
  • Click on the Solve button to solve for the unknown Rlower
and, Voila! - you get a value of 989 ohms for Rlower, suggesting that you might have mistakenly substituted a 1k ohm resistor for a 10k resistor! (In fact, if you had entered the Vout with enough precision, say 0.45454545, you would get a nice round 1,000 for Rlower).