Discussion on special usage skills of digital multimeter
First, the structure of the digital multimeter
The digital multimeter is composed of a digital voltmeter and a corresponding functional conversion circuit. It can directly measure various parameters such as AC and DC voltage, AC, DC current, resistance, capacitance and frequency. A digital voltmeter usually uses an integrated circuit chip, which integrates the A/D converter and the display logic controller that can directly drive the display, and is surrounded by related resistors, capacitors and displays to form a digital multimeter head. . It only measures DC voltage, and other parameters must be converted into a DC voltage proportional to its own size before it can be measured. The overall performance of the digital multimeter is mainly determined by the performance of this digital meter. The digital voltmeter is the core of the digital multimeter, and the A/D converter is the core of the digital voltmeter. Different A/D converters constitute a digital multimeter with different principles. Function conversion circuit is an essential circuit for digital multimeter to realize multi-parameter measurement. Voltage and current measurement circuits are generally composed of passive voltage divider and shunt resistor networks; AC and DC conversion circuits and conversion circuits for measuring electrical parameters such as resistance and capacitance are generally realized by a network composed of active devices. The function selection can be realized by the switching of the mechanical switch, the range selection can be switched by the changeover switch, and can also be realized by the automatic range switching circuit.
2. Use the diode file and 200MΩ file to identify the triode
1. Put the switch of the multimeter in the diode gear, because the diode gear of the digital multimeter has a voltage output of about 2.7V, use the unidirectional conductivity of the PN junction to judge the b-pole and judge the NPN and PNP transistors.
(1) Assuming that one pole of the triode is the b pole, connect the red test lead to the hypothetical b pole, and connect the black test lead to the other two poles to measure its resistance. If the resistance measured twice is low and roughly equal, At this time, change the meter pens to test whether the resistances are high and equal, then the red pen is connected to the b pole you are looking for, and it is judged to be an NPN type tube.
(2) If the red test lead is connected to the hypothetical b pole, and the measurement results are all high and equal in value according to the above method, and the resistances measured by changing the test lead are all low and equal, then the black test lead is connected to the b pole, and it is PNP tube.
(3) If one of the results measured by the above method is a low resistance value and the other is a high resistance value, then the b pole of the original assumption is wrong, and the other pin must be assumed to be the b pole until the requirements are met. When the three measured results do not have the same resistance value, the triode is a bad tube.
2. Set the multimeter switch to the resistance 200MΩ range. For NPN tubes, first assume that one pole is c pole, connect the red test lead to the assumed c pole, and connect the black test lead to e pole, or pinch b pole and c pole by hand. poles, but cannot touch. This is to connect a bias resistor between bc to add a forward current to the base of the triode to turn on the triode. Write down the resistance value at this time, then exchange the red and black test leads for re-testing, and also record the resistance value, compare the resistance value twice, and which resistance value is smaller indicates which assumption is correct, then the The second red test lead is connected to the C pole. On the contrary, for the PNP type tube, the black test lead is connected to the C pole.
3. Use a digital multimeter as a thermometer
Many multimeters do not have a temperature measurement file, and the temperature needs to be recorded in each experiment class, and the temperature needs to be controlled when the PCB is made, and the mercury thermometer is easily damaged. The author can measure the temperature by connecting a simple temperature measuring circuit. The measurement circuit includes a temperature-voltage conversion circuit and a high-precision reference voltage source. The temperature-voltage conversion circuit is composed of an integrated temperature sensor, an integrated operational amplifier, and a resistor. The high-precision voltage reference consists of an integrated voltage regulator, capacitors and resistors, etc. The function is to compensate the difference between 273.2K and 0°C and realize dimension conversion. Through debugging, the voltage sensitivity of the measuring circuit is 1mv/℃. Then, set the digital multimeter switch to the voltage 200mV range, and use the digital multimeter to directly display the measured temperature value.
