How Do You Measure Voltage, Current and Resistance?
Voltage, current and resistance are all measured with a multimeter, but the connections differ. Voltage is measured in parallel across two points. Current is measured by breaking the circuit and connecting the meter in series. Resistance is measured on a de-energized circuit, in parallel across the component. A wrong connection can give a false reading or blow the meter's fuse.
The basic way to measure voltage, current and resistance in a circuit is the multimeter; but each quantity is connected differently. The most critical difference: voltage is measured in parallel, current in series.
Measuring Voltage (In Parallel)
Set the multimeter to the voltage range (V⎓ for DC, V∼ for AC) and touch the probes in parallel to the two points you're measuring. You don't need to break the circuit.
Measuring Current (In Series)
To measure current, you have to break the circuit and connect the multimeter in series; the current must pass through the meter. Don't forget to plug the red probe into the separate current (A/mA) input on most meters.
Measuring Resistance (De-energized)
Resistance is always measured with the circuit de-energized and, if possible, with the component removed from the circuit. Otherwise the other paths in the circuit corrupt the reading.
| Quantity | Connection | Circuit state |
|---|---|---|
| Voltage | Parallel | Energized |
| Current | Series | Energized |
| Resistance | Across the ends | De-energized |
Common Mistakes
- Measuring current in parallel: connecting the meter in the current range in parallel makes a short circuit and blows the fuse.
- Forgetting the probe in the A input: trying to measure voltage with the probe in the current input can damage the meter.
- Measuring resistance energized: measuring resistance with the circuit on is inaccurate and harmful to the meter.
Frequently Asked Questions
How do you measure voltage?+
Why is current measured in series?+
What to watch when measuring resistance?+
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