The multimeter, also known as a multifunction meter, three-in-one meter, or universal meter, is a versatile and multi-range measuring instrument. Generally, a multimeter can measure direct current, direct voltage, alternating voltage, resistance, and audio levels. Some models can also measure alternating current, capacitance, inductance, and certain parameters of semiconductors (such as β).
1. Structure of the Multimeter (Model 500)
The multimeter consists of three main parts: the meter head, measurement circuit, and selector switch.
(1) Meter Head
The meter head is a highly sensitive magnetic electrodynamic DC ammeter, and the main performance indicators of the multimeter largely depend on the performance of the meter head.
The sensitivity of the meter head refers to the value of the direct current flowing through the head when the pointer deflects to full scale. The smaller this value, the higher the sensitivity of the meter head. The internal resistance when measuring voltage should be larger for better performance.
The meter head has four scale lines, which function as follows:
The first line (from top to bottom) is marked with R or Ω, indicating the resistance value. When the selector switch is in the ohm position, read this scale line.
The second line is marked with ∽ and VA, indicating alternating and direct voltage as well as direct current values. When the selector switch is in the alternating or direct voltage or current position, and the range is set to any position other than alternating 10V, read this scale line.
The third line is marked with 10V, indicating the 10V alternating voltage value. When the selector switch is in the alternating or direct voltage position and the range is set to alternating 10V, read this scale line.
The fourth line is marked with dB, indicating the audio level.
(2) Measurement Circuit
The measurement circuit is used to convert various measurements into a small direct current suitable for the meter head. It consists of resistors, semiconductor components, and batteries. It can unify various measurements (such as current, voltage, resistance, etc.) and different ranges into a certain limit of small direct current through a series of processes (such as rectification, shunting, voltage division, etc.) for measurement by the meter head.
(3) Selector Switch
The selector switch is used to select different measurement circuits to meet various types and ranges of measurement requirements. There are generally two selector switches, each marked with different positions and ranges.
2. Symbol Meanings
(1) ∽ indicates AC and DC.
(2) V–2.5KV 4000Ω/V indicates that for the AC voltage and 2.5KV DC voltage range, the sensitivity is 4000Ω/V.
(3) A–V–Ω indicates that current, voltage, and resistance can be measured.
(4) 45–65–1000Hz indicates that the operating frequency range is below 1000 Hz, with a standard frequency range of 45–65Hz.
(5) 2000Ω/V DC indicates that the sensitivity for the DC range is 2000Ω/V. The symbols on clamp meters and analog meters are similar to the above symbols.
3. Mnemonics for Using the Multimeter
Correctly using a multimeter can not only quickly and accurately identify faults but also prevent damage to electrical devices and the multimeter itself.
(1) Check the Range Before Measuring
Before measuring, always double-check whether the measurement category and range selector switch are set correctly. For safety, this habit must be developed.
(2) Do Not Change Range During Measurement
Do not randomly adjust the selector switch during measurement, especially when measuring high voltage (like 220V) or high current (like 0.5A), to avoid arcing and damaging the switch contacts. After measurement, return the range selector switch to the “•” position.
(3) Keep the Dial Level, Align Reading
When using the multimeter, keep it level and ensure your line of sight is directly in front of the pointer when reading.
(4) Choose Appropriate Range, Pointer Should Deflect More Than Half
When selecting a range, if the size of the measurement is uncertain, choose a larger range first. Then, based on the deflection angle, gradually switch to a smaller range until the pointer deflects to about 2/3 of the full scale.
(5) Do Not Measure Resistance While Powered, Discharge Capacitors Before Measuring
Never measure resistance while the circuit is powered. When checking large capacitors on electrical devices, discharge the capacitor before measuring.
(6) Zero Out Resistance Before Measuring, Reset Zero When Changing Ranges
When measuring resistance, first turn the selector switch to the resistance position, short the two probes, and adjust the “Ω” zero potentiometer until the pointer reads zero ohms before measuring. Each time you change the resistance range, you should readjust the ohm zero point.
(7) Remember the Black is Negative, Connect Black to “+” Internally
The red probe is positive, and the black probe is negative, but in the resistance range, the black probe connects to the positive terminal of the internal battery.
(8) Connect in Series for Current, Connect in Parallel for Voltage
When measuring current, the multimeter should be connected in series with the circuit being measured; for voltage, it should be connected in parallel across the circuit.
(9) Do Not Reverse Polarity, Develop a One-Handed Habit
When measuring current and voltage, pay special attention to ensure the red and black probes are not reversed, and develop a habit of one-handed operation to ensure safety.
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