Discussion on Portable Digital Multimeter and Related Issues
The basic principle of the portable digital multimeter: the meter head is made of a sensitive magnetoelectric microammeter. For example, when measuring a small current, the meter normally displays the measured value; for example, when measuring a large current, the portable digital multimeter realizes the measurement by shunting or reducing voltage through the parallel connection and series resistance of the meter head. When measuring DC current, connect the shunt resistor in parallel to the meter head to expand the current range, and change the current measurement range by changing the shunt resistance (Figure 1); when measuring DC voltage, connect the drop resistor in series with the meter head to extend the voltage range The resistance value of the multiplier resistor is used to change the measurement range of the voltage (Figure 2); when measuring the AC voltage, a parallel-series half-wave rectifier circuit needs to be added to the meter to convert the AC into a DC (because the meter is a DC meter) , and then the principle is similar to that of measuring DC voltage (Figure 3); when measuring resistance, connect a shunt resistor in parallel with a zero-adjusting resistor in series on the meter head, and connect a battery in series, and measure according to the current through the measured resistance in the diagram The resistance value can be obtained by changing the resistance value of the shunt resistor
Change the range of resistance.
The portable digital multimeter has a wide measurement voltage range and a single power supply. It uses a 9V battery to make the instrument miniaturized and has a high input impedance. Its internal analog switch realizes automatic zero adjustment and polarity conversion.
Precautions for Portable Digital Multimeter in Use
(1) There is no display on the meter screen. First check whether the battery voltage, instrument fuse, current limiting resistor, voltage regulator block, etc. in the multimeter are normal, and then check the circuit board to see if there is a short circuit, open circuit or corrosion. If there is, it should be welded in time, and the circuit board should be cleaned and kept dry. ; If not, measure and display the power input terminal voltage of the integrated block. If it is normal, it means that the integrated block is damaged and needs to be replaced in time. If it is not normal, check whether there is a short circuit around the integrated block.
(2) The displayed value is unstable and there are jumping characters during the measurement. Under normal circumstances, it may be caused by damp or electric leakage on the circuit board. The circuit board should be cleaned first and dried; Make sure that the contact is good or re-solder; if the fault persists, check whether there is a power reduction that causes the resistance to deteriorate or a component is very hot after the test. If so, replace the component in time.
(3) The DC voltage file does not return to zero. The common cause of this phenomenon is that the vicinity of the voltage divider resistor is dirty. You should clean up the area around the resistor first, then adjust the DC potentiometer in the meter, and use a DC standard voltage source to input 1V voltage for calibration.
(4) The indication value is inaccurate when measuring high voltage or the indication value is inaccurate or even unstable during long-term measurement. Most of the faults are caused by the thermal effect caused by the insufficient power of one or several components, which causes the components to become hot within a few seconds after the measurement is stopped, and the faulty components should be replaced in time.
(5) The current file cannot be measured. This problem is mostly caused by improper operation. First check whether the fuse, current-limiting resistor and voltage dividing resistor are blown out. If they are blown out, they need to be replaced; Replace amplifier.
(6) The resistance file cannot be measured. First check whether there is a burning phenomenon of the connecting resistor in the resistance gear circuit, if there is, replace it in time; if not, check each connecting component and replace the damaged component in time; if there is no abnormality in the appearance of the circuit board, check the integrated block , if damaged, replace it in time.
(7) The indication value is not allowed. Under normal circumstances, it is mainly caused by the failure of the resistance or capacitance in the measurement circuit, and the failure resistance or capacitance should be checked and replaced first. If the fault is not eliminated, check the resistance value of the resistor in the circuit (including the resistance value in the thermal reaction), if the resistance value changes or the value of the thermal reaction changes with time, replace the resistor in time; if the fault persists, check the A/D If the resistors and capacitors in the reference voltage circuit of the converter are damaged, replace them in time.
