Every resistor changes the story of a circuit.
A resistor opposes current. Its value, measured in ohms (Ω), determines how much current can flow at a given voltage. Some resistors are fixed; others change with position, temperature or light.
Predict current and voltage in practical circuits.
Change resistance and inspect live measurements.
Interpret open circuits, shorts and out-of-spec readings.
Learning objectives
- Identify fixed, variable and sensor-dependent resistive components.
- Read resistor markings, values, tolerance and wattage.
- Apply Ohm's and Watt's Laws to series and parallel networks.
- Use voltage, current and resistance readings to evaluate circuits.
- Explain how potentiometers, thermistors and photoresistors behave.
- Separate a measured symptom from a confirmed root cause.
Training model only: the circuits on this page are idealized DC circuits and do not replace vehicle service information.
Ten short lessons, one connected concept
Open any lesson to view the key idea and an automotive example. Practice the calculations in the workbench afterward.
Make the electrons work for you.
Choose a circuit. Adjust the 12 V source and resistor values; watch current, voltage drops and power recalculate instantly.
Ideal, closed DC circuit. Component sizes and wire shapes are illustrative.
Resistor 1
Resistor 2
I = V ÷ R = 12 ÷ 120 = 0.100 A
Power figures are calculated dissipation, not recommended resistor wattage ratings. In a real build, allow appropriate power derating and account for heat, tolerance and fault conditions.
Decode the four color bands.
Use the first two bands as digits, the third as a multiplier, and the fourth as tolerance. This exercise uses the standard four-band resistor code.
One idea, three applications.
Potentiometer example: unloaded 5 V supply, 5 kΩ track and ideal wiper. Thermistor example: illustrative 10 kΩ NTC at 25 °C with β = 3950 K. Photoresistor values are illustrative—not universal specifications.
What does the meter actually tell you?
Set a meter function and a fault condition. Your job is to decide what the reading establishes—and what it does not establish.
Diagnose the dim indicator.
A simple 12 V indicator circuit has a 120 Ω load. A technician measures 12.0 V across the entire operating circuit but finds that an unexpected 120 Ω series resistance has developed in a connector.
Resistor fundamentals: unlocked.
You've explored resistance, decoded a component, taken virtual measurements and traced a circuit fault.