Series, Parallel, and Divider Circuits
A guide to resistor combinations and divider behaviour, focused on how series values add, parallel values reduce, and divider outputs depend on the ratio between resistors.
Key formulas
The key pattern to remember
Series resistances add directly. Parallel combinations reduce the total resistance below the smallest branch value. Divider outputs depend on the ratio between components, not just their absolute size.
Remembering those three ideas makes it much easier to sense-check the answer before you trust the exact figure.
Worked example
Two 100 ohm resistors in series give 200 ohms. The same two resistors in parallel give 50 ohms. Those opposite directions help explain why the physical arrangement matters so much.
A simple divider built from equal resistors gives roughly half the input voltage at the midpoint.
Common mistakes
- Adding resistances directly even when they are in parallel.
- Expecting a parallel result to be larger than the branch values.
- Using the divider formula without checking which resistor is the lower leg.
- Ignoring LED forward-voltage assumptions when choosing a resistor.
Apply the topic straight away.
Resistance Series Calculator
Add resistor values in series to find the total equivalent resistance for a simple chain of components.
Resistance Parallel Calculator
Combine resistor values in parallel to find the equivalent resistance seen across the network.
Resistor Divider Calculator
Use the Resistor Divider Calculator for practical circuit and electronics work involving resistor divider.
LED Resistor Calculator
Use the LED Resistor Calculator for practical circuit and electronics work involving led resistor.