A multimeter, also called a multitester or ohm-voltmeter, is a multifunction, multi-range measuring instrument. A typical multimeter can measure DC current, DC voltage, AC voltage, resistance, and audio level. Some models can also measure AC current, capacitance, inductance, and certain semiconductor parameters. It is a common tool for electricians.
Quick rules and mnemonics
1. Always check the function and range before measuring; do not measure without checking.
2. Do not change the range while measuring. Especially when measuring high voltage (for example 220 V) or large currents (for example 0.5 A), avoid switching the selector during measurement to prevent arcing and damage to switch contacts. After finishing a measurement, set the range selector to the off position.
3. Keep the meter dial level and align your eye with the pointer when reading the scale.
4. Choose an appropriate range. If the expected magnitude is unknown, start with a higher range, then reduce the range step by step until the pointer deflects to about two-thirds of full scale.
5. Do not measure resistance on a live circuit. When checking large-value capacitors in equipment, discharge the capacitor by shorting it before measuring.
6. When measuring resistance, first set the selector to the resistance range, short the two test leads, and adjust the Ω zero potentiometer until the pointer reads zero ohms. Readjust the ohm zero point each time you change the resistance range.
Measuring AC voltage
Set the range switch to AC and verify the correct unit and scale.
Connect the test leads across the two points of the circuit to be measured; observe polarity for parts that have positive and negative.
When measuring high voltage, move the lead to the high-voltage jack. Before changing range settings, disconnect power.
Usage precautions
For an analog multimeter, insert the test leads with the red lead to positive (+) and the black lead to negative (-), then confirm the pointer is at the zero position. The pointer should align with the left end mark on the scale; if it does not, perform a zero adjustment. Before measuring current and voltage, estimate the expected range and start at a higher range before selecting a more appropriate lower range to avoid excessive current that could damage the meter.
When measuring, consider the effect of the meter's internal resistance. For voltage measurements, connecting the meter places its internal resistance across the circuit and allows current to flow through it, which affects the measured value. Using different ranges changes the meter's internal resistance and thus changes the measurement error for the same test point.
When testing transistor circuits, using a DC range with an input resistance of about 20 kΩ/V is usually appropriate; this value is often marked on the meter scale. Transistor circuits may also require measuring low voltages, for example 0.1 V, so choose a meter that provides a 1 V range when needed.
