Understand True RMS Digital Multimeters in 1 Minute

A True RMS (Root Mean Square) digital multimeter is a highly accurate, easy-to-use, and feature-rich electrical measurement tool with a wide range of applications. It has become widely used in fields such as high-precision analog circuit measurement, variable frequency drives, brushless motor DC to AC inverters, and more. This tool brings great convenience to both our work and daily life.

In certain special circuits, the output waveform is not a sine wave, but rather a pulse, square wave, triangle wave, etc. In these cases, using an average-reading multimeter will produce incorrect measurements. Only a True RMS multimeter can correctly measure the effective values in these circuits, such as AC voltage, current, and power.

CEM “Quick Facts”

True RMS Digital Multimeter

A True RMS digital multimeter is an electrical measurement tool capable of measuring AC and DC voltage, current, resistance, capacitance, and other parameters. Unlike traditional average-reading multimeters, it uses a more precise measurement method known as RMS (Root Mean Square) detection technology. RMS detection refers to the square root of the ratio of the sum of the squares of the measured electrical signal over the measurement time period.

Principle of the Instrument

A multimeter's ability to measure voltage, current, and resistance is achieved through the conversion circuitry. Measurements of current and resistance are based on voltage measurements, meaning the digital multimeter is essentially an expansion of the digital DC voltmeter. The converter transforms the continuously changing analog voltage into a digital signal, which is then counted by an electronic counter to obtain the measurement result. This result is displayed through a decoding display circuit.

A True RMS digital multimeter uses an internal square wave generator to convert non-sinusoidal electrical signals to similar square wave signals, which are then processed digitally. By performing operations like squaring, integrating, and square-rooting the digital square wave signals, the true RMS value is calculated.

Key Selection Points

1. Accuracy:
   The higher the accuracy, the more precise the measurements. For measuring small values, a high-accuracy multimeter is preferred.

2. Resolution:
   The smallest measurable change in value is displayed on the screen. High resolution is crucial for weak signals or precision measurements.

3. Signal Characteristics:
   True RMS measurement is suitable for complex circuits, while average value measurement is ideal for pure sine wave signals. True RMS multimeters can accurately and real-time measure the effective value of various AC waveforms without considering waveform parameters and distortion, especially for distorted signals like square waves, triangle waves, and sawtooth waves. Only a True RMS instrument can accurately measure these signals.

4. Safety:
   Overload, short-circuit protection, and CAT safety ratings are essential for ensuring safety. Features like automatic power off and audible alarms are also necessary for safe operation.

5. Functionality and Expandability:
   In addition to basic measurements, modern multimeters offer additional functions like capacitance and inductance measurement. Consider data recording, transmission capabilities, and connectivity to other devices as well.

6. Measurement Settings:

   • Auto-range: Automatically selects the appropriate measurement range.
   • Manual range: Allows you to select the measurement range yourself.

Important Notes

1. Select the Correct Measurement Range
   Before using the digital multimeter, ensure that the correct measurement range is selected.

2. Avoid Exceeding the Range
   When measuring voltage and current, ensure that the measured values are within the multimeter’s range. If unsure, start by selecting the highest range and then adjust to a more suitable range based on the measurement results.

3. Safety Operation
   Always ensure the circuit is disconnected before using the multimeter. Avoid measuring live circuits, especially when measuring high voltages or large currents. Never change the measurement range while the circuit is live to prevent damaging the multimeter with electrical arcs.

4. Correct Use of Probes
   Do not touch the metal parts of the probes with your hands while measuring to avoid electric shock and to maintain measurement accuracy. When measuring resistance, avoid touching both ends of the component, as the resistance of the human body in parallel with the measured resistance will lead to inaccurate results.

5. Keep the Instrument Clean and Dry
   Avoid moisture or water ingress when using or storing the multimeter. Keep the instrument clean and dry to extend its lifespan.

6. Regularly Check the Instrument's Condition
   Before use, inspect the multimeter for damage, including the insulation of the connectors. If you notice any abnormality, stop using it and have it repaired.

7. Avoid Magnetic Interference
   When using the multimeter, try to avoid interference from strong external magnetic fields, as this can cause deviations in measurement results.