Multimeters: Different Techniques for Measuring Different Objects
Multimeters, also known as multimeters, multimeters, three-meters, multimeters, etc., are indispensable measuring instruments in power electronics and other departments. Generally, the main purpose is to measure voltage, current and resistance. Multimeters are divided into pointer multimeters and digital multimeters according to the display mode. It is a multi-functional and multi-range measuring instrument. Generally, the multimeter can measure DC current, DC voltage, AC current, AC voltage, resistance and audio level, etc., and some can also measure AC current, capacitance, inductance and semiconductor. Some parameters (such as β) and so on.
Measuring technique (if no explanation is given, it refers to the pointer table):
1. Test speakers, earphones, and dynamic microphones: use the R×1Ω gear, connect any test lead to one end, and the other test lead to touch the other end. It will make a crisp "da" sound under normal conditions. If there is no sound, the coil is broken. If the sound is small and sharp, there is a problem with the ring rubbing, and it cannot be used.
2. Capacitance measurement: use the resistance file, select the appropriate range according to the capacitance capacity, and pay attention to the black test lead of the electrolytic capacitor should be connected to the positive pole of the capacitor when measuring. ①. Estimate the size of the capacitor of the microwave method: it can be judged according to the maximum amplitude of the pointer swing by experience or referring to the standard capacitor of the same capacity. The referenced capacitors do not need to have the same withstand voltage value, as long as the capacity is the same. For example, a 100μF/250V capacitor can be used as a reference to estimate a 100μF/25V capacitor. As long as the maximum swing of their pointers is the same, it can be concluded that the capacity is the same. ②. Estimate the capacitance of picofarad capacitors: R×10kΩ should be used, but only capacitance above 1000pF can be measured. For a capacitance of 1000pF or slightly larger, as long as the hands of the watch swing slightly, the capacity can be considered sufficient. ③. To measure whether the capacitor is leaking: for a capacitor above 1,000 microfarads, you can first use the R×10Ω file to quickly charge it, and initially estimate the capacitor capacity, and then change to the R×1kΩ file to continue measuring for a while. At this time, the pointer does not It should return, but stop at or very close to ∞, otherwise there will be leakage. For some timing or oscillating capacitors below tens of microfarads (such as the oscillating capacitors of color TV switching power supplies), the requirements for their leakage characteristics are very high, as long as there is a slight leakage, they cannot be used. At this time, they can be charged at the R×1kΩ level. Then use the R×10kΩ file to continue the measurement, and the hands should stop at ∞ and should not return.
3. Test the quality of diodes, triodes, and Zener tubes on the road: because in actual circuits, the bias resistance of triodes or the surrounding resistance of diodes and Zener tubes are generally relatively large, mostly in hundreds or thousands of ohms. , we can use the R×10Ω or R×1Ω file of the multimeter to measure the quality of the PN junction on the road. When measuring on the road, use the R×10Ω file to measure the PN junction should have obvious forward and reverse characteristics (if the difference between the forward and reverse resistance is not obvious, you can use the R×1Ω file to measure), generally the forward resistance is at R The hands should indicate about 200Ω when measuring in the ×10Ω range, and about 30Ω when measuring in the R×1Ω range (there may be slight differences depending on the phenotype). If the measurement result shows that the forward resistance is too large or the reverse resistance is too small, it means that there is a problem with the PN junction, and there is also a problem with the tube. This method is particularly effective for maintenance, and can find out bad pipes very quickly, and even detect pipes that have not completely broken but whose characteristics have deteriorated. For example, when you use a small resistance file to measure the forward resistance of a certain PN junction is too large, if you solder it down and use a commonly used R×1kΩ file to measure it, it may still be normal. In fact, the characteristics of this tube have deteriorated. Not working or unstable anymore.
4. Measuring resistance: It is important to select a good range. When the pointer indicates 1/3 to 2/3 of the full scale, the measurement accuracy is the highest and the reading is the most accurate. It should be noted that when using the R×10k resistance file to measure a large resistance of megohm level, do not pinch your fingers at both ends of the resistance, so that the resistance of the human body will make the measurement result smaller.
5. Measure the Zener diode: The voltage regulator value of the Zener diode we usually use is generally greater than 1.5V, and the resistance file below R×1k of the pointer meter is powered by the 1.5V battery in the meter. In this way, use Measuring the Zener tube with a resistance file below R×1k is like measuring a diode, which has complete unidirectional conductivity. However, the R×10k gear of the pointer meter is powered by a 9V or 15V battery. When the R×10k is used to measure a voltage regulator tube with a voltage regulation value less than 9V or 15V, the reverse resistance value will not be ∞, but will have a certain value. Resistance value, but this resistance value is still much higher than the forward resistance value of the Zener tube. In this way, we can initially estimate the quality of the Zener tube. However, a good Zener tube also needs to have an accurate voltage regulation value. How to estimate this voltage regulation value under amateur conditions? It's not difficult, just find another pointer watch. The method is: first place a meter in the R×10k range, and its black and red test leads are respectively connected to the cathode and anode of the voltage regulator tube. At this time, the actual working state of the voltage regulator tube is simulated, and then another meter is placed in the On the voltage file V×10V or V×50V (according to the regulated voltage value), connect the red and black test leads to the black and red test leads of the watch just now, and the measured voltage value at this time is basically this Regulated voltage value of the Zener tube. Saying "basically" is because the bias current of the first meter to the regulator tube is slightly smaller than the bias current in normal use, so the measured voltage regulator value will be slightly larger, but basically the same . This method can only estimate the Zener tube whose voltage regulator value is less than the voltage of the high-voltage battery of the pointer meter. If the regulated voltage value of the Zener tube is too high, it can only be measured with an external power supply (in this way, when we choose a pointer meter, it is more suitable to choose a high-voltage battery with a voltage of 15V than 9V).
6. Measuring the triode: usually we need to use R×1kΩ file, no matter it is NPN tube or PNP tube, no matter it is low power, medium power or high power tube, the be junction and cb junction should show exactly the same unidirectional direction as the diode Electrically, the reverse resistance is infinite, and its forward resistance is about 10K. In order to further estimate the quality of the tube characteristics, if necessary, the resistance gear should be changed for multiple measurements. The method is: set the R×10Ω file to measure the forward conduction resistance of the PN junction is about 200Ω; set the R×1Ω file to measure The forward conduction resistance of the PN junction is about 30Ω, (the above is the data measured by the 47-type meter, other models are probably slightly different, you can test a few more good tubes to summarize, so that you know what you know) If the reading is too large If there are too many, it can be concluded that the characteristics of the pipe are not good. You can also put the meter at R×10kΩ and then measure. For tubes with lower withstand voltage (basically, the withstand voltage of triodes is above 30V), the reverse resistance of the cb junction should also be ∞, but the reverse resistance of the be junction There may be some, and the hands of the watch will deflect slightly (generally not more than 1/3 of the full scale, depending on the pressure resistance of the tube). Similarly, when measuring the resistance between ec (for NPN tube) or ce (for PNP tube) with R×10kΩ file, the needle may be slightly deflected, but this does not mean that the tube is bad. However, when measuring the resistance between ce or ec with a file below R×1kΩ, the indication of the meter head should be infinite, otherwise there is a problem with the tube. It should be noted that the above measurements are for silicon tubes, not for germanium tubes. But germanium tubes are rare now. In addition, the so-called "reverse" is for the PN junction, and the directions of the NPN tube and the PNP tube are actually different.
