Correctly using a multimeter not only allows for quick identification of faults but also prevents damage to electrical devices and the multimeter itself. Below are some tips for using a multimeter.
Check the range before measuring, do not measure without checking
Do not change the range while measuring, switch to the off position after measuring
The dial should be level, and readings must align correctly
The range must be appropriate, and readings must align correctly
Measure resistance without power, discharge capacitors before measuring
Zero the meter before measuring resistance, re-zero when changing ranges
Remember that black is negative, and the black lead connects to + in the meter
Measure current in series, measure voltage in parallel
Ensure polarity is correct, develop the habit of using one hand
Multimeters can generally be divided into analog and digital types.
Analog multimeter:
The structure of an analog multimeter: There are many types of analog multimeters, but the basic structure is similar. The main components of an analog multimeter are the meter head, the selector switch (also known as the range switch), and the measurement circuit.
Meter head: It is the display device for measurements; the meter head is essentially a sensitive ammeter.
Selector switch: It selects the type and range (or multiplier) of the quantity being measured.
Measurement circuit: It converts different types and sizes of measured quantities into DC current that the meter head can accept.
Meter head:
Selector switch:
Digital multimeter:
Range dial:
Ports:
Using the multimeter—zeroing
Correct wiring:
The red probe connects to the “+” polarity socket, and the black probe connects to “—” or “＊” or “COM” polarity socket.
When measuring DC, pay attention to the positive and negative polarities to avoid reversing the pointer.
When measuring current, the meter should be in series with the circuit being measured; when measuring voltage, the meter should be in parallel across the circuit.
When measuring transistors, remember that the red probe connects to the negative terminal of the internal battery; the black probe connects to the positive terminal of the internal battery.
Correctly selecting the measurement range:
When measuring voltage, set the selector switch to the corresponding voltage range; when measuring current, set it to the corresponding current range, etc.
When selecting the current or voltage range, it is best to keep the pointer in the two-thirds position of the scale; when selecting the resistance range, it is best to keep the pointer in the middle of the scale.
When measuring, if you are unsure of the range of the value being measured, first turn the selector switch to the maximum range, and then gradually reduce it to the appropriate range based on the pointer’s deflection.
Common measurement methods—measuring AC and DC voltage
Measuring current:
Measuring resistance:
Select range and zero: When measuring resistance, first select the appropriate resistance range, then short the probes, and adjust the “Ω” zero adjuster to make the pointer return to 0 (re-zero should be performed each time the range is changed).
Measuring and reading: Connect the probes across the resistor, and take the reading. The resistance value is the reading on the resistance scale multiplied by the current selected resistance range multiplier.
Range: Choose a range that keeps the pointer between 1/3 and 2/3 of the full scale to reduce measurement errors.
Probe polarity: The battery in the multimeter is active when measuring resistance, with the battery “+” connected to the “-” on the panel. When using the multimeter to determine the polarity of diodes or rectifying components, attention must be paid to probe polarity, as current flows from the black probe, through the external components, and returns to the red probe.
Measuring resistors in a circuit: Must measure with the power off. If unsure whether parallel resistors exist in the circuit, first disconnect one end of the resistor from the circuit, and if there are capacitors in the circuit, discharge them before measuring.
Checking the leakage resistance of electrolytic capacitors: Set the switch to the R×1K range, the red probe must connect to the negative terminal of the capacitor, and the black probe connects to the positive terminal of the capacitor.
Measuring the DC current gain hFE of transistors
Select the ADJ range, short the probes, and adjust the ohm zero adjuster so that the pointer points to 300hFE.
Select the hFE range, insert the leads of the transistor being tested into the corresponding E, B, C sockets of the transistor tester, and the deflection indicated by the pointer is approximately the transistor’s DC gain.
Insert N-type transistors into the NPN socket, and P-type transistors into the PNP socket.
Measuring the reverse cutoff current Iceo and Icbo of transistors:
Iceo: The reverse cutoff current between the collector and the emitter (base open).
Icbo: The reverse cutoff current between the collector and the base (emitter open).
Rotate the switch to the R×1K range, short the probes, and adjust the zero ohm potentiometer to bring the pointer back to zero (at this time, the full-scale current value is about 90μA).
Separate the probes, insert the transistor being tested into the socket as shown, and the scale indicated by the pointer multiplied by 12 is approximately the transistor’s reverse cutoff current value.
If Iceo > 90μA, use the R×100 range for measurement (at this time, the full-scale current value is about 900μA).
Diode lead polarity identification:
A diode is a semiconductor device with significant unidirectional conductivity or nonlinear volt-ampere characteristics. Typically, the forward resistance of small power germanium diodes is 300~500Ω, and the reverse resistance is tens of kilohms; the forward resistance of silicon diodes is about 1kΩ or larger, and the reverse resistance is above 500kΩ. The greater the difference in forward and reverse resistances, the better.
Range selection: When measuring, generally select the R×1K or R×100 range, do not use the R×1 or R×10K ranges. Because using the R×1 range results in excessive current, which may burn out the diode, while using the R×10K range results in too high voltage, which may cause the diode to break down.
Polarity judgment: Connect the probes to the two terminals of the diode, compare the resistance values measured twice, and the terminal connected to the black probe is the positive terminal of the diode, because in the multimeter’s resistance measurement circuit, the red probe is connected to the negative terminal of the internal battery, and the black probe is connected to the positive terminal of the internal battery.
Capacitance and inductance measurement:
Capacitance measurement: See the diagram for the measurement method, select the C.L.dB (10V AC) range, and read the capacitance measurement value on the C(μF) 50Hz scale.
Inductance measurement: See the diagram for the measurement method (just replace capacitance with inductance), select the C.L.dB (10V AC) range, and read the inductance measurement value on the L(H) 50Hz scale.
Precautions for using a multimeter:
Before using the multimeter, you must familiarize yourself with the operating methods, measurement principles, types of measurements, and ranges, and check that the selector switch and sockets are correct.
The multimeter must be placed horizontally during use, and before use, the mechanical zero should be adjusted.
After measuring, set the range selector switch to the highest position to prevent accidental damage to the meter during the next measurement.
If the multimeter will not be used for a long time, the battery should be removed to avoid deterioration and leakage due to prolonged storage, which could damage the circuit board.
When measuring voltage in circuits with inductive reactance, the multimeter must be disconnected before cutting off the power to prevent damage to the voltmeter due to high voltage generated by self-induction.
Source: Compressor Network
Edited by Chemical 707
Editor: Pingping
Reviewed by: Little Crown
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