Resistors in parallel

- The values produced by placing resistors in parallel including a reference table

When designing or building an electronic circuit, it may be necessary to use two or more resistors instead of one. Either the correct value of resistor may not be available, or it may be necessary to dissipate a higher power than that which a single resistor can dissipate on its own.

Resistance of resistor combinations

Resistors can be placed in series or parallel. When placed in series the total resistance is equal to the sum of the individual resistors:

It is also worth noting that the same current flows through each resistor, but the voltage across each resistor is proportional to the resistance of that particular resistor.

For resistors placed in parallel, the arithmetic is a little more complicated because the reciprocal of the total resistance is equal to the sum of the reciprocals of the constituent resistors:

When there are only two resistors R1 and R2 in parallel this can be simplified to:

If both these resistors have the same value it can be seen that the overall value of the resistance is half the value for the individual resistor.

When resistors are placed in parallel the voltage across the resistors is the same, but the current through each one is inversely proportional to its resistance.

Parallel resistor value table

While the values for resistors in parallel can be calculated relatively easily in most cases a table giving suitable values that are in the most case preferred is given below. Column A gives the required resistor value, and column B gives the first choice combination for the parallel resistors. A second choice is shown in column C. The values should be a close tolerance as possible, typically 0.5%.