The structural principle of the digital multimeter is evolved from the analog principle, namely
1) After dividing the measured voltage by a voltage dividing resistor, it is converted into a test current for the instrument through an operational analog amplifier, and then input into the A/D converter to convert it into a digital signal to form the actual value.
2) After the measured current is shunted through the shunt resistor, it is converted into a current with a certain proportion for the instrument through the operational analog amplifier, and then input to the A/D converter, converted into a digital signal, and displayed as an actual value through the display.
3) Electronic measurement is to divide the measured voltage by resistance, and then convert it into a current with a certain proportion for the instrument through the operational analog amplifier, and then input it to the A/D converter, convert it into a digital signal, and display it as an actual value through the display. .
Common faults and maintenance of digital multimeter
1) No display on the meter First, check whether the battery voltage is normal (usually 9V batteries are used, and new ones should also be measured). Secondly, check whether the fuse is blown, whether the voltage regulator block is normal, and whether the current limiting resistor is open. Then, check whether the circuit board is corroded or short-circuited or short-circuited (especially the main power circuit). If so, the circuit board should be cleaned, and the drying and welding work should be done in time; Is the test voltage normal for the two pins of the power input? If the test voltage is normal, the integrated block is damaged, and the integrated block must be replaced; if the test voltage is abnormal, check whether there are other short-circuit points? If there is, it must be dealt with in time; if not or it is not normal after being dealt with, then the integrated block has been short-circuited and must be replaced.
2) The resistance gear cannot be measured. First, check the circuit board from the appearance. Is there any connected resistor in the resistance gear loop that is burned out? If there is, it must be replaced immediately; if not, each connecting element must be measured, and the damaged ones should be replaced in time. If the periphery is normal, the measurement integrated block is damaged and must be replaced.
3) The displayed value of the voltage gear is inaccurate when measuring high voltage, or the displayed value is inaccurate or even unstable after measuring for a long time. Most of these faults are caused by the insufficient working power of one or several components. If, within a few seconds of stopping the measurement, some components are found to be hot during inspection, which is caused by thermal effects due to insufficient power, the component (or integrated circuit) must be replaced.
4) The current file cannot be measured. Most of these faults are caused by improper operation. Check whether the current limiting resistor and the shunt resistor are burned out? If it burns out, it must be replaced; then check if the connection wire to the amplifier is damaged? If it is damaged, it should be reconnected; if it is not normal, replace the amplifier.
5) The displayed value is unstable and there is word jumping phenomenon. Check whether the whole circuit board is damp or has leakage phenomenon? If there is, the circuit board must be cleaned and dried; whether there is poor contact or virtual soldering in the input circuit (including the test pen), if so, it must be re-soldered; check whether there is resistance deterioration or whether there are components just after testing Supernormal hot hand phenomenon occurs, this phenomenon is caused by its power reduction, if this phenomenon occurs, the component should be replaced.
6) The displayed value is inaccurate This phenomenon is mainly caused by the failure of the resistance or capacitor in the measurement circuit, and the capacitor or resistance must be replaced. Check the resistance value of the resistor in the circuit (including the resistance value in the thermal response), if the resistance value changes or the thermal response changes value, the resistor should be replaced; check the resistance of the resistor in the reference voltage loop of the A/D converter Is the value, capacitor damaged? If damaged, replace it.
The principle of the repair method of digital multimeter is "Finding faults should be done first from the outside before the inside, from the easy to the difficult, breaking it into parts, and making key breakthroughs." The methods can be roughly divided into the following categories:
⒈ Observation method: By means of human vision, hearing, smell, and touch to find and find the fault location. Through visual inspection, it can be found such as disconnection, desoldering, short-circuiting, broken fuse tube, burnt component, mechanical damage, deformation of printed circuit board and lifting and breaking of copper foil on it; you can touch the battery, For the temperature rise of resistors, transistors and integrated blocks, you can refer to the circuit diagram to find out the reason for the abnormal temperature rise according to the circuit principle. In addition, you can also check by hand whether the electronic components are loose, whether the pins of the integrated circuit are firmly inserted, and whether the transfer switch is stuck; you can also hear and smell whether there is any abnormal sound, odor, etc.
⒉ Voltage method Measure whether the working voltage of each key point is normal, and the fault point can be found quickly. Such as measuring the working voltage of the A/D converter, the reference voltage, etc.
⒊Short-circuit method According to the method of checking the A/D converter mentioned above, the short-circuit method is generally used, and this method is used more in the repair of weak and micro-electrical instruments.
⒋Open circuit method Disconnect the suspicious part from the whole machine or unit circuit. If the fault disappears, it means that the fault is in the disconnected circuit. This method is mainly suitable for short circuits in the circuit.
⒌Replacement method When the fault has been reduced to a certain place or a few components, it can be measured online or offline. If necessary, replace with good components. If the fault disappears, the components are broken.
⒍Signal injection method The signal inspection method can improve the accuracy of judging faults. For example, using the induced voltage of the human body as an interference signal to observe the change of the display screen of the watch to be repaired, it is often used to check whether the input circuit and the display part are in good condition.
