Introduction

This page contains a calculator that determines the values of two standard value resistors Ra and Rb which, when connected in parallel, will result in a net resistance R that will be within very close tolerances of almost any value you want.

The program works by first selecting a standard resistor (Ra) that is close, but larger, than the specified resistance. It then determines the value of another standard resistor (Rb) that can be placed in parallel with the first one (Ra) to obtain the resistance required. Because we are using standard resistors, the final value will not be exact, but it will be within 2 or 3 percent, even when you are using 10 percent resistors.

Input Requirements

Using the select buttons and text areas to the left, enter the target resistance value, the tolerance of the resistors you wish to use, and the voltage across the two resistors. If you don't know the voltage right now, that's OK. It is not needed to calculate the final resistances. It is, however, a good idea to supply a value that is close to the expected voltage so that you can determine the power dissipated in each resistor and size your resistors properly.

For precision resistors less than 10 Ohms, refer to the HamCalc Copper Wire programs for data on copper wire resistors.

Required Resistance, R,
between points X & Y
Ohms K Ohms M Ohms

Resistor Tolerance
 5 Percent 10 Percent

Voltage, X to Y
u Volts m Volts Volts K Volts M Volts

Output Results

The value selected for Ra is x. This is the next highest x standard resistor, greater than the required resistance, R.

The value of a parallel resistor to obtain the required resistance, R, precisely is x. The x standard resistor value closest to the precision value for Rb, is x.

Therefore, using Ra = x and Rb = x yields a parallel resistance, between points X and Y, of x. This deviates x from the required value of x.

With x between points X and Y, the current through Ra is x and the power dissipation is x. The current through Rb is x and the power dissipation is x. Choose the resistor power ratings of your resistors based on this data.