Circuit lab 03 · Beginner to intermediate
Measuring and fault-finding
A multimeter is the tool you will use most. Learn to measure voltage, current and resistance on a virtual meter that behaves like a real one, including its mistakes. Then track down the faults in nine broken circuits.
Three measurements, three rules
Voltage: across
Touch the probes to two points with the circuit on. The meter shows the difference in voltage between them. It has a very high resistance (often 10 MΩ), so it barely disturbs the circuit.
Current: in series
Break the circuit and let the current flow through the meter. In current mode the meter is almost a short circuit, so never place it across a battery or a component: it can blow the meter’s fuse.
Resistance: power off
Switch the circuit off. The meter sends its own small current through the part. Other parts connected in parallel affect the reading, so for an exact value lift one leg out of the circuit.
Safety
Practise on battery and USB-powered circuits only. Measuring mains needs a meter rated for it (a suitable CAT rating), proper probes and training. Before measuring current, check which socket the red lead is in: leaving it in the current socket and then measuring voltage is a classic way to blow a fuse.
Practice bench
Click a test point to place the active probe (red first, then black), or use the menus. Complete the four tasks; each ticks itself when the meter shows the right measurement.
Tasks
Find the fault
Each case is a circuit with one hidden fault. Read the report, take measurements, then choose your diagnosis. Good technicians reach the answer in a few measurements; there is no penalty for taking more.
How the virtual meter works
Simplified model
The circuit is solved with nodal analysis, the same method circuit simulators use, and the meter is added to the circuit exactly as a real one would be: a 10 MΩ resistance between the probes for volts, a 2 Ω “burden” resistance in series for current (protected by a fuse that blows above 200 mA), and a 1 mA test current for resistance. These are typical figures; real meters differ.
LEDs use the same model as Circuit lab 01 (off, or a fixed forward voltage), and they look open on the resistance range because the meter’s test voltage is below their forward voltage. Batteries have a small internal resistance; a worn-out one has a large one.