The multimeter measurement method refers to a method of measuring the voltage, the current, and the magnitude of the resistor in a circuit with a multimeter to determine the fault. Therefore, the multimeter measurement method is further divided into a resistance measurement method, a voltage measurement method, and a current measurement method. It is the most used method of troubleshooting electronic products. In addition, the quality of electronic components is often measured using a multimeter.
Resistance measurement
The resistance measurement method is to use the multimeter ohmic block to analyze the fault location by checking whether the DC value between the circuit under test and the ground is normal. Resistance measurement methods are available in both "on-line" and "off-weld" methods.
(1) Method essentials:
1 Use the “Ω×1†block of the multimeter to detect the resistance of the path. If necessary, the test point should be scraped off, and then soldered and then tested to prevent the contact resistance from being too large, causing measurement errors. When detecting the connector, the contact resistance can be measured by swinging the connector. If the resistance value is not fixed, it indicates that there is a bad contact failure.
2 Use the multimeter's “Ω×1k†or “Ω×10k†block to detect the capacitance value of the capacitor and the degree of leakage.
3 Use the multimeter “Ω×1k†block to detect the small power transistor, use the “Ω×100†block to detect the medium power transistor, and use the “Ω×10†block to detect the high power tube.
4 Use the “Ω×1k†block to detect whether the meter indicates the quality of the meter.
See Chapter 3 for the test methods of common component performance.
(2) Note:
1 Can not detect various resistances when the appliance is powered.
2 pairs of capacitors should be discharged first, then remove one end for inspection.
3 For the detection of the resistive component, if the resistor is connected to other circuits, one end of the resistor to be tested should be removed and then tested.
4 should pay attention to the multimeter test pen polarity and electrical block selection, so as not to misjudge.
5 When using the "online" measurement method, the measured resistance value is affected by other parallel branches, and should be considered when analyzing the test results to avoid misjudgment.
2. Voltage measurement method
Detecting the external AC voltage of the electrical equipment and the internal circuit DC voltage is the basis for analyzing the cause of the fault. Therefore, in the process of adjusting the electrical and electronic circuits, the relevant voltage should be measured first, so that problems are often found and faults are detected.
The voltage measurement method is to measure the voltage of each part of the tested electrical or electronic circuit and compare it with the normal value to find out the location of the fault.
During the overhaul process, even if the circuit fault location has been determined, it is necessary to further measure the voltage of each pin of the transistor, integrated circuit, etc. in the relevant circuit or the voltage value of the main node in the circuit to see if it is normal. This can be helpful in finding damaged components and analyzing the cause of the failure.
For a circuit that is inconvenient to measure current, the voltage across the resistor through which the current flows is often measured, and then indirectly estimated by Ohm's law (this method is also called indirect current test method). Such as the semiconductor triode current negative feedback amplifier circuit and the emitter output ICQ of the emitter output, this method is commonly used for measurement. However, this method has errors. The reason is that the actual resistance has a certain error with the nominal value; the resistance of the resistor will change after long-term use, especially the carbon film resistance.
Measuring the voltage of the input or output signal of the electronic circuit of the detected electrical appliance to check whether the dynamic working state is normal is also in the category of voltage measurement.
(l) Method essentials:
1 Select the appropriate range AC voltmeter or DC voltmeter (universal multimeter), 狈 0 amount of the corresponding power supply voltage.
2 Use an electronic voltmeter with appropriate range and high input impedance to measure the operating voltage of the node or device electrode.
3 Select the appropriate range and frequency range of electronic voltmeter to measure the input and output signal voltage of the relevant circuit.
4 Find the fault location against the normal value. Where the voltage deviates from the normal value, it is often the location of the fault.
(2) Note:
1 should pay attention to the DC voltmeter "+" "one" polarity to determine the direction of the voltage or current.
2 should pay attention to the relative measurement point of the working voltage: generally the ground wire (that is, the casing), some are selected for zero volts, some are between the electrodes, need to be classified, carefully tested.
3 electronic voltmeter access, should pay attention to the "high" potential end and the "low" potential end of the order. When printing, the ground potential terminal (ie ground) should be connected first, followed by the "high" potential terminal. After the measurement is completed, the "high" potential terminal should be removed first, and then the "low" potential terminal should be removed.
4 should pay attention to the influence of the voltmeter input impedance and frequency range on the test results.
3. Current measurement method
The current measurement method is a method of determining the failure of an electric appliance or a circuit by detecting the current value of an electric appliance or a circuit using a current block of a multimeter. Many circuits use the value of the current to determine the operating point. Therefore, measuring the magnitude of these currents is an important method to judge whether the circuit works normally.
(1) Method essentials:
Disconnect the device under test from the printed circuit board copper foil or wire to form a measurement port, connect the current meter with appropriate range, and measure the current to compare with the normal value to determine the fault location. On the printed circuit board, a blade is generally used to make a hole in the copper foil to manufacture a measuring port. In the transistor circuit, if the measured ratio is about zero, the tube is turned off; if the measured current is large, the tube may be saturated.
(2) Note:
1 Measurement of the incision should be selected in a suitable position for easy testing after testing and testing.
2 Pay attention to the polarity of the ammeter. After the connection is reliable, connect the power supply.
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