Multimeter: Different Techniques for Measuring Different Objects
Multimeters, also known as multiplexers, multimeters, triple meters, and multimeters, are indispensable measuring instruments in power electronics and other departments, generally aimed at measuring voltage, current, and resistance. Multimeters are divided into pointer multimeter and digital multimeter according to their display mode. It is a multifunctional and multi range measuring instrument. Generally, a multimeter can measure DC current, DC voltage, AC current, AC voltage, resistance, audio level, etc. Some can also measure AC current, capacitance, inductance, and some parameters of semiconductors (such as β） Etc.
Measurement techniques (if not specified, referring to a pointer table):
1. Measuring speakers, headphones, and dynamic microphones: using R × At 1 Ω level, if any probe is connected to one end and the other probe is touched to the other end, a clear and crisp "click" sound will be emitted normally. If it doesn't make a sound, it means the coil is broken. If the sound is small and sharp, it means there is a problem with wiping the coil and it cannot be used.
2. Capacitance measurement: Using a resistance range, select an appropriate range based on the capacitance, and pay attention to connecting the black probe of the electrolytic capacitor to the positive electrode of the capacitor during measurement Estimating the size of microwave method level capacitor capacity: It can be determined based on experience or referring to standard capacitors of the same capacity, and the maximum amplitude of pointer oscillation. The referenced capacitance does not need to have the same withstand voltage value, as long as the capacity is the same, for example, estimating a 100 μ F/250V capacitor can be used with a 100 μ By referring to the capacitance of F/25V, as long as the maximum amplitude of their pointer oscillation is the same, it can be concluded that the capacity is the same Estimating the capacity of a picosecond capacitor: R should be used × 10k Ω range, but can only measure capacitance above 1000pF. For capacitors of 1000pF or slightly larger, as long as the watch needle swings slightly, the capacity is considered sufficient Test for leakage of capacitance: For capacitors above 1000 microf, R can be used first × Quickly charge it at 10 Ω level and preliminarily estimate the capacitance capacity, then change it to R × Continue measuring at 1k Ω level for a while, and at this point, the pointer should not return, but should stop at or very close to ∞, otherwise there will be leakage. For some timing or oscillating capacitors below tens of microfacies (such as oscillating capacitors in color TV switching power supplies), the leakage characteristics are very high, and they cannot be used as long as there is a slight leakage. In this case, R × After charging at 1k Ω, switch to R × Continue measuring at 10k Ω level, and the pointer should stop at ∞ instead of returning.
3. When testing the quality of diodes, transistors, and voltage regulators on the road: because in actual circuits, the bias resistance of transistors or the peripheral resistance of diodes and voltage regulators are generally relatively large, mostly in the hundreds and thousands of ohms or above. In this way, we can use the R of a multimeter × 10 Ω or R × Measure the quality of the PN junction on the road at 1 Ω level. When measuring on the road, use R × The PN junction measured at 10 Ω should have obvious forward and reverse characteristics (if the difference in forward and reverse resistances is not significant, R can be used instead × 1 Ω gear for measurement), usually the forward resistance is at R × When measuring the 10 Ω gear, the gauge needle should indicate around 200 Ω, at R × When measuring at 1 Ω level, the dial should indicate around 30 Ω (may vary slightly depending on different phenotypes). If the measurement results show that the forward resistance value is too high or the reverse resistance value is too low, it indicates that there is a problem with the PN junction and the pipe. This method is particularly effective for maintenance, as it can quickly identify faulty pipes and even detect pipes that have not yet completely broken but have deteriorated characteristics. For example, if you use a low resistance range to measure the forward resistance of a PN junction, and you solder it down, use the commonly used R × After retesting at 1k Ω, it may still be normal, but in fact, the characteristics of this pipe have deteriorated, making it unable to work properly or unstable.
4. Measuring resistance: It is important to choose a suitable range. When the pointer indicates 1/3 to 2/3 of the full range, the measurement accuracy is the highest and the reading is the most accurate. It should be noted that when using R × When measuring large resistance values in the 10k resistance range, do not pinch your fingers at both ends of the resistance, as this will cause the measurement result to be too small.
5. Measuring voltage regulator diode: The voltage regulator value of the voltage regulator we usually use is generally greater than 1.5V, while the R of the pointer meter × Resistance levels below 1k are powered by a 1.5V battery in the meter, so R × A voltage regulator with a resistance range of less than 1k is like a diode and has complete unidirectional conductivity. But the R of the pointer table × 10k gear is powered by a 9V or 15V battery, while using R × When measuring a voltage regulator with a voltage value less than 9V or 15V at 10k, the reverse resistance value will not be ∞, but there will be a certain resistance value, but this resistance value is still significantly higher than the forward resistance value of the voltage regulator. In this way, we can preliminarily estimate the quality of the voltage regulator. However, a good voltage regulator requires an accurate voltage regulation value. How can we estimate this voltage regulation value under amateur conditions? It's not difficult, just find another pointer table. The method is to first place a table in R × At 10k level, the black and red probes are connected to the cathode and anode of the voltage regulator respectively. At this time, the actual working state of the voltage regulator is simulated, and another meter is placed at voltage level V × 10V or V × At 50V (based on the voltage regulation value), connect the red and black probes to the black and red probes of the previous meter, and the measured voltage value is basically the voltage regulation value of this voltage regulator. Basically, the reason for saying 'basically' is that the bias current of the first meter towards the voltage regulator is slightly smaller than the bias current during normal use, so the measured voltage regulator value may be slightly larger, but the difference is not significant. This method can only estimate the voltage regulator tube whose voltage is less than the voltage of the high-voltage battery on the pointer meter. If the voltage regulation value of the voltage regulator is too high, it can only be measured using an external power source (in this way, when choosing a pointer meter, it seems that using a high-voltage battery voltage of 15V is more suitable than using a 9V one).
6. Test transistor: Usually we use R × In the 1k Ω range, whether it is NPN or PNP tubes, whether it is low power, medium power, or high-power tubes, the be junction and cb junction should exhibit the same unidirectional conductivity as the diode, with infinite reverse resistance and a forward resistance of about 10K. To further estimate the quality of the pipe characteristics, if necessary, multiple measurements should be made by changing the resistance gear. The method is to set R × The positive conduction resistance of the PN junction measured at 10 Ω is around 200 Ω; Set R × The positive and negative conduction resistance of the PN junction measured at 1 Ω level is around 30 Ω. (The above data is obtained from the 47 type meter, while other types of meters may vary slightly. It is recommended to test several good tubes to summarize and have a clear understanding.) If the reading is too large, it can be concluded that the characteristics of the tubes are not good. You can also place the table in R × Measure again at 10k Ω. For tubes with lower voltage resistance (basically, the voltage resistance of the transistor is above 30V), the reverse resistance of the cb junction should also be at ∞, but the reverse resistance of the be junction may be some, and the meter needle may deviate slightly (generally not exceeding 1/3 of the full range, depending on the voltage resistance of the tube). Similarly, when using R × When measuring the resistance between ec (for NPN tubes) or ce (for PNP tubes) at a 10k Ω range, the gauge needle may slightly deflect, but this does not mean that the tube is faulty. But using R × When measuring the resistance between ce or ec at a range below 1k Ω, the indicator on the meter head should be infinite, otherwise there may be a problem with the tube. It should be noted that the above measurements are for silicon tubes and are not applicable to germanium tubes. But now germanium tubes are also very rare. In addition, the term 'reverse' refers to the direction of the PN junction, which is actually different for NPN and PNP pipes.
