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Dongxiang talks about application skills of multimeter simulator
2012-1-9 10:26:13

指针表和数字表的选用: I. Selection of pointer meter and digital meter of multimeter simulator :
In the relatively large current and high voltage analog circuit measurement, pointer tables are suitable, such as televisions and audio amplifiers. In the measurement of low-voltage and small-current digital circuits, digital meters are used, such as BP machines and mobile phones. It is not absolute, you can choose pointer table and digital table according to the situation.


1. The reading accuracy of the pointer table is poor, but the process of pointer swing is relatively intuitive, and its swing speed can sometimes objectively reflect the measured size (such as the slightness of the TV data bus (SDL) when transmitting data). Jitter); digital meter readings are intuitive, but the process of digital changes looks messy and not easy to watch.

2. There are generally two batteries in the pointer meter, a low voltage of 1.5V and a high voltage of 9V or 15V. The black test lead is the positive end compared to the red test lead. 数字表则常用一块6V或9V的电池。 Multimeter emulator digital meters usually use a 6V or 9V battery. In the resistance range, the output current of the test lead of the pointer meter is much larger than that of the digital meter. The R × 1Ω range can make the speaker make a loud “click” sound, and the R × 10kΩ range can even light up the light emitting diode (LED).

3. In the voltage range, the internal resistance of the pointer meter is relatively small compared to the digital meter, and the measurement accuracy is relatively poor. Some high-voltage micro-current occasions cannot even be accurate, because its internal resistance will affect the circuit under test (for example, the measured value will be much lower than the actual value when measuring the acceleration voltage of the TV picture tube). The internal resistance of the voltage meter of the digital meter is very large, at least in the megohm level, and it has little effect on the circuit under test. However, the extremely high output impedance makes it susceptible to the influence of induced voltage. The measured data in some places with strong electromagnetic interference may be false.

的技巧(如不作说明,则指用的是指针表): Second, the skills of measuring multimeter simulator (if not explained, refers to the pointer meter):

1. Measuring speakers, earphones, and dynamic microphones: Use R × 1Ω, one of the test leads is connected to one end, and the other test pen touches the other end, and a “click” sound will be emitted when normal. If it does not sound, the coil is broken. If the sound is small and sharp, there is a problem of rubbing the circle, and it cannot be used.

2. Capacitance measurement: Use a resistance file to select an appropriate range according to the capacitance, and pay attention to the positive electrode of the capacitor for the black test lead of the electrolytic capacitor. ① Estimation of the capacitance of the microwave method capacitor: It can be determined based on the maximum amplitude of the pointer swing based on experience or reference to a standard capacitor of the same capacity. The referenced capacitors do not have to withstand the same voltage, as long as they have the same capacity. For example, when estimating a 100μF / 250V capacitor, you can use a 100μF / 25V capacitor as a reference. As long as their pointers have the same maximum swing amplitude, the same capacity can be determined. ② Estimation of the capacity of picofarad capacitors: R × 10kΩ is used, but only capacitors above 1000pF can be measured. For a capacitor of 1000pF or a little larger, as long as the hands of the meter slightly swing, the capacity can be considered sufficient. ③ Test for capacitance leakage: For capacitors over one thousand microfarads, you can quickly charge them with R × 10Ω, and initially estimate the capacitance, and then change to R × 1kΩ to continue measuring for a while. Should return, but should stop at or very close to ∞, otherwise there is a leakage phenomenon. For some timing or oscillation capacitors below tens of microfarads (such as the oscillation capacitors of color TV switching power supplies), the leakage characteristics are very high. As long as there is a slight leakage, it cannot be used. At this time, it can be charged in the R × 1kΩ file. Then use R × 10kΩ to continue the measurement. Similarly, the hands should stop at ∞ instead of returning.

3. Diodes, triodes, and zener diodes are good or bad during driving test: because in actual circuits, the bias resistance of the triode or the surrounding resistance of the diode and the zener diode are generally large, most of which are in the hundreds or thousands of ohms. , We can use the R × 10Ω or R × 1Ω range of the multimeter to measure the quality of the PN junction on the road. When measuring the road, the PN junction measured with R × 10Ω should have obvious forward and reverse characteristics (if the difference between forward and reverse resistance is not obvious, you can use R × 1Ω to measure). Generally, the forward resistance is at R When measuring in the × 10Ω range, the hands should indicate about 200Ω, and in the R × 1Ω range, the hands should indicate about 30Ω (may differ slightly according to different types). If the measured result is too large or the reverse resistance is too small, it indicates that there is a problem with the PN junction, and the tube is also defective. This method is particularly effective for maintenance, it can quickly find bad pipes, and even measure pipes that have not completely broken but have deteriorated characteristics. For example, when you use a small resistance file to measure the forward resistance of a PN junction, if you solder it down and use a common R × 1kΩ file to measure it, it may still be normal. In fact, the characteristics of this tube have deteriorated. Not working or unstable.

4. Measuring resistance: It is important to choose a good range. When the pointer indicates 1/3 to 2/3 full scale, the measurement accuracy is the highest and the reading is the most accurate. It should be noted that when measuring a large resistance value of megohm level with R × 10k resistance file, do not pinch your fingers at both ends of the resistance, so the resistance of the human body will make the measurement result smaller.

5, test voltage regulator diode: the voltage regulator we usually use the voltage value is generally greater than 1.5V, and the pointer table R × 1k below the resistance file is powered by the 1.5V battery in the table, so, use The resistance sizing tube with a resistance below R × 1k is like a diode, and has complete unidirectional conductivity. However, the R × 10k range of the pointer meter is powered by a 9V or 15V battery. When R × 10k is used to measure a voltage regulator with a regulated voltage less than 9V or 15V, the reverse resistance value will not be ∞, but there will be a certain Resistance value, but this resistance value is still much higher than the forward resistance value of the voltage regulator. In this way, we can initially estimate the quality of the voltage regulator. However, a good voltage regulator must have an accurate voltage regulation value. How to estimate this voltage regulation value in amateur conditions? It's not difficult, just find another pointer table. The method is: first put one meter in R × 10k, 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 set to On the voltage range V × 10V or V × 50V (depending on the regulated value), connect the red and black test leads to the black and red test leads of the watch just now. The voltage value measured at this time is basically this. Voltage regulation value of the zener tube. "Basic" is because the bias current of the first meter to the zener tube is slightly smaller than the bias current during normal use, so the measured voltage value will be slightly larger, but the basic difference is not large. . This method can only estimate voltage regulators whose voltage regulation value is lower than the high voltage battery voltage of the pointer meter. If the voltage stabilizing value of the zener tube is too high, it can only be measured by an external power supply method (in this way, when we select the pointer meter, it is more suitable to use a high-voltage battery with a voltage of 15V than 9V).

6. Testing the triode: Usually we need to use R × 1kΩ file, whether it is NPN or PNP tube, whether it is small power, medium power, high power tube, the be junction cb junction should be exactly the same as the diode. Electrically, the reverse resistance is infinite, and its forward resistance is about 10K. In order to further evaluate the quality of the pipe, if necessary, the resistance range should be changed for multiple measurements. The method is: set the R × 10Ω range to measure the forward resistance of the PN junction at about 200Ω; set the R × 1Ω range to measure The forward resistance of the PN junction is about 30 ohms. (The above is the data measured by the 47 type meter. The other models are roughly different. You can test a few more tubes and summarize them to make sure you have the heart.) If the reading is too large Too much, it can be concluded that the characteristics of the pipe are not good. The meter can also be tested at R × 10kΩ, and the tube with a lower withstand voltage (basically the withstand voltage of the triode is above 30V), its cb junction reverse resistance should also be ∞, but its be junction reverse resistance There may be some, the hand will be slightly deflected (generally does not exceed 1/3 of 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, the hand may be slightly deflected, but this does not mean that the tube is bad. However, when measuring the resistance between ce or ec with R × 1kΩ or lower, the indicator on the meter should be infinite, otherwise, the tube is defective. It should be noted that the above measurements are for silicon tubes and not applicable for germanium tubes. But germanium tubes are rare these days. In addition, the "reverse direction" refers to the PN junction, and the direction of the NPN tube and the PNP tube is actually different.

Most of the common triodes are now plastic-sealed. How to accurately determine which of the three pins of the triode are b, c, e? The b pole of a triode is easy to measure, but how do you determine which is c and which is e? Three methods are recommended here: The first method: For the pointer meter with the hFE jack of the triode, first measure the b pole, and then insert the triode into the jack at will (of course, the b pole can be inserted accurately). Take a look at the hFE value, then turn the tube upside down and measure it again. The hFE value is measured once, and the positions of the pins are correct. The second method: For a meter without an hFE measurement jack, or if the tube is too large to be inserted into the jack, you can use this method: For an NPN tube, first measure the b pole (whether the tube is NPN or PNP and its b pin) It ’s easy to measure, right?), Put the meter in the R × 1kΩ range, connect the red test lead to the hypothetical e pole (note that the hand holding the red test lead does not touch the tip or pin of the test lead), and the black test lead is connected to the hypothetical C pole, hold the tip of the test pen and this pin with your fingers at the same time, pick up the tube, lick the b pole with the tip of your tongue, see that the pointer of the meter head should have a certain deflection, if you connect the test pens correctly, the pointer deflection will Larger, if not connected correctly, the pointer deflection will be smaller, the difference is obvious. From this, the c and e poles of the tube can be determined. For the PNP tube, connect the black test lead to the hypothetical e-pole (do not touch the tip or pin of the pen), the red test lead to the hypothetical c-pole, hold the pen tip and the pin with your finger, and then lick b with the tip of the tongue If the test leads are connected correctly, the pointer of the meter head will deflect relatively. Of course, the test leads need to be exchanged and measured twice during the measurement, and the final determination can be made after comparing the readings. This method is applicable to triodes of all shapes, which is convenient and practical. According to the deflection amplitude of the hand, the magnification ability of the tube can also be estimated, of course, this is based on experience. The third method: first determine the NPN or PNP type of the tube and its b pole, then set the meter to the R × 10kΩ range. For NPN tubes, the black test lead is connected to the e pole and the red test lead is connected to the c pole. Deflection. For the PNP tube, when the black test lead is connected to the c-pole and the red test lead is connected to the e-pole, the meter hand may be deflected to a certain degree, and vice versa. This can also determine the c and e poles of the triode. But for pipes with high pressure resistance, this method is not applicable.

For common imported models of high-power plastic sealed tubes, the c pole is basically in the middle (I haven't seen b in the middle). The b of the middle and small power tubes is most likely in the middle. For example, the commonly used 9014 triode and other series of triodes, 2SC1815, 2N5401, 2N5551 and other triodes have some b poles in the middle. Of course, they also have c-pole in the middle. Therefore, when repairing and replacing the triode, especially these low-power triodes, do not use it and install it directly as it is. Be sure to test it first.