Introduction: Custom Resistor Values
Sometimes schematics call for an uncommon resistor value. Instead of ordering a new resistor online, consider creating a custom resistor by combining multiple resistors together in series.
Resistance adds in series, this means that the total resistance of a series of resistors equals the sum of all the contributing resistors. The images above show how to wire resistors in series. When two resistors, R1 and R2, are wired in series, their combined resistance equals R1 + R2. This principle applies to more than two resistors, three resistors in series, R1 R2 and R3, have a combined resistance of R1 + R2 + R3.
As an example I wired together a 3.3KOhm and a 120KOhm resistor in series on a breadboard in fig 4, the total resistance between the yellow wires is 3,300+120,000 = 123,300Ohms. In figure 5 I added a 4.7KOhm resistor for a total resistance of 3,300+120,000,+4,700 = 128,000Ohms.