How it works
Series resistors add. For parallel resistors, take the reciprocal of the sum of reciprocals.
Rseries = R1 + R2 + …Rparallel = 1 / (1/R1 + 1/R2 + …)
To build a target value, the calculator combines two E24 standard values in series or in parallel and finds the combination closest to the target.
Worked example
Two 10 kΩ resistors in parallel give 5 kΩ. For a target of 7.5 kΩ, the calculator searches for a combination of standard values within 1 % of the target.
Design tips
- When the two values differ greatly, the parallel combination is close to the smaller one. To fine-tune with a large resistor, a series combination works better.
- Two equal values in parallel share the power and also average their tolerance.
- Combinations need more parts and more board area. First check whether a single standard E96 value solves the problem.
Limitations
- The error shown is the theoretical error of the standard values. Add the real tolerance (for example ±1 %) when judging the result.
- Pairs of standard values are shown as a practical option; they are not a substitute for a precision or custom-value resistor.
Frequently asked questions
Is a 1 % error from a standard-value pair good enough?
For general circuits it is sufficient. For precision references or filters, calculate the real tolerance together with the error and check the error budget.
How is power shared in a parallel pair?
Two equal resistors share the current and so share the power equally. With different values, the smaller resistor carries more of the current.