Tips for using a multimeter

1、 Selection of pointer and number tables:

1. The reading accuracy of pointer meters is poor, but the process of pointer oscillation is relatively intuitive, and the amplitude of its oscillation speed can sometimes objectively reflect the size of the measured object (such as the slight jitter of the TV data bus (SDL) when transmitting data); The reading on the digital meter is intuitive, but the process of numerical changes appears chaotic and difficult to observe.

2. There are usually two batteries inside a pointer meter, one with a low voltage of 1.5V and the other with a high voltage of 9V or 15V. The black probe is the positive terminal relative to the red probe. A 6V or 9V battery is commonly used for digital watches. In the resistance range, the output current of the pointer meter is much larger than that of the digital meter. Using the R × 1 Ω range can make the speaker make a loud "click" sound, and using the R × 10k Ω 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. In some high-voltage microcurrent situations, it is even impossible to measure accurately because its internal resistance can affect the tested circuit (for example, when measuring the acceleration voltage of a TV cathode ray tube, the measured value may be much lower than the actual value). The internal resistance of the voltage range of a digital meter is very high, at least in the megaohm range, and has little impact on the tested circuit. However, the output impedance of * makes it susceptible to the influence of induced voltage, and the data measured in some situations with strong electromagnetic interference may be false.

4. In short, pointer meters are suitable for measuring analog circuits with relatively high current and high voltage, such as televisions and audio amplifiers. Digital meters are suitable for measuring low voltage and low current digital circuits, such as BP machines, mobile phones, etc. No, you can choose a pointer table or a number table according to the situation.

2、 Measurement techniques (if not specified, referring to the use of a pointer gauge):

1. Test speakers, headphones, and dynamic microphones: Use R × 1 Ω mode, connect one probe to one end, and touch the other probe to the other end. Under normal circumstances, a crisp "click" sound will be emitted. 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. Measure capacitance: Use resistance mode to select the appropriate range according to the capacitance, and pay attention to connecting the black probe of the electrolytic capacitor to the positive electrode of the capacitor during measurement. ① Estimating the capacity of microwave capacitors: It can be determined based on experience or by referring to standard capacitors of the same capacity, according to the maximum amplitude of pointer oscillation. The capacitance referred to does not need to have the same withstand voltage value, as long as the capacitance is the same. For example, estimating a capacitance of 100 μ F/250V can be referred to by a capacitance of 100 μ F/25V. As long as their pointer swings by the same magnitude, it can be concluded that the capacitance is the same. ② Estimating the capacitance size of a Pifa level capacitor: It is necessary to use the R × 10k Ω range, but only capacitors above 1000pF can be measured. For capacitors of 1000pF or slightly larger, as long as the pointer swings slightly, it can be considered that the capacity is sufficient. ③ Measure whether the capacitor is leaking: For capacitors above 1000 microfarads, they can be quickly charged using the R × 10 Ω range, and the capacitance can be initially estimated. Then, switch to the R × 1k Ω range and continue measuring for a while. At this point, the pointer should not return, but should stop at or very close to ∞, otherwise there is a leakage phenomenon. For some timing or oscillating capacitors below tens of microfarads (such as oscillating capacitors in color TV switch power supplies), the leakage characteristics are very high. As long as there is a slight leakage, they cannot be used. At this time, they can be charged in the R × 1k Ω range and then switched to the R × 10k Ω range to continue measuring. Similarly, the pointer should stop at ∞ and should not return.

3. In road testing of diodes, transistors, and voltage regulators: Because in actual circuits, the bias resistance of transistors or the peripheral resistance of diodes and voltage regulators are generally large, mostly in the hundreds or thousands of ohms range. Therefore, we can use the R × 10 Ω or R × 1 Ω range of a multimeter to measure the quality of the PN junction on the road. When measuring on the road, the PN junction should have obvious forward and reverse characteristics when measured in the R × 10 Ω range (if the difference in forward and reverse resistance is not significant, the R × 1 Ω range can be used for measurement). Generally, the forward resistance should indicate around 200 Ω when measured in the R × 10 Ω range, and around 30 Ω when measured in the R × 1 Ω range (there may be slight differences depending on different phenotypes). If the measurement result shows that the forward resistance is too high or the reverse resistance is too low, it indicates that there is a problem with the PN junction, and the tube is also problematic. This method is particularly effective for maintenance, as it can quickly identify faulty pipes and even detect pipes that have not yet been damaged but have deteriorated characteristics. For example, when you measure the forward resistance of a PN junction with a low resistance range and it is too high, if you solder it down and measure it again with the commonly used R × 1k Ω range, it may still be normal. In fact, the characteristics of this tube have deteriorated and it cannot work properly or is unstable.

4. Resistance measurement: It is important to choose the appropriate range. When the pointer indicates 1/3 to 2/3 of the full range, the measurement accuracy is highest and the reading is most accurate. It should be noted that when measuring megohm level high resistance resistors with an R × 10k resistor range, do not pinch your fingers on both ends of the resistor, as this will cause the measurement result to be underestimated due to human resistance.

5. Measurement of voltage regulator diode: The voltage regulator value of the voltage regulator diode we usually use is generally greater than 1.5V, and the resistance range below R × 1k of the pointer meter is powered by the 1.5V battery in the meter. Therefore, measuring the voltage regulator diode with a resistance range below R × 1k is like measuring a diode, with * unidirectional conductivity. But the R × 10k range of the pointer meter is powered by a 9V or 15V battery. When using an R × 10k to measure a voltage regulator with a voltage value less than 9V or 15V, the reverse resistance value will not be ∞, but will have a certain resistance value, but this resistance value will still be much higher than the forward resistance value of the regulator. In this way, we can preliminarily estimate the quality of the voltage regulator. However, a good voltage regulator also needs an accurate voltage regulation value. How to estimate this voltage regulation value under amateur conditions? It's not difficult, just find another pointer table. The method is to first place a meter in the R × 10k range, with its black and red probes connected to the cathode and anode of the voltage regulator, respectively, to simulate the actual working state of the voltage regulator. Then take another meter and place it in the voltage range V × 10V or V × 50V (depending on the voltage regulation value), and connect the red and black probes to the black and red probes of the meter just now. At this point, the measured voltage value is basically the voltage regulation value of this voltage regulator. The reason for saying 'basically' is that the bias current of the voltage regulator on the * block meter is slightly smaller than that 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 that is lower 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 by applying an external power source (it seems that when choosing a pointer meter, using a high-voltage battery voltage of 15V is more suitable than 9V).

6. Measurement of transistor: Usually we need to use the R × 1k Ω range. Whether it is an NPN transistor or a PNP transistor, whether it is a low-power, medium power, or high-power transistor, 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 tube characteristics, it is necessary to change the resistance level for multiple measurements. The method is to set the R × 10 Ω level and measure the PN junction positive conducting resistance, which is around 200 Ω; Set the R × 1 Ω range to measure the forward conducting resistance of the PN junction, which is around 30 Ω. (The above data is obtained from a 47 type meter, and other models have different readings. You can try testing several good tubes and summarize them to have a clear idea.) If the reading is too large, it can be concluded that the characteristics of the tube are not good. You can also place the meter at R × 10k Ω for testing. For tubes with lower withstand voltage (basically, the withstand voltage of a transistor is above 30V), the reverse resistance of its CB junction should also be ∞, but the reverse resistance of its BE junction may be slightly off center, and the pointer may deviate slightly (generally not exceeding 1/3 of the full range, depending on the withstand voltage of the tube). Similarly, when measuring the resistance between ec (for NPN tubes) or ce (for PNP tubes) using the R × 10k Ω range, the gauge needle may slightly deflect, but this does not mean that the tube is damaged. But when measuring the resistance between CE or EC in the range of R × 1k Ω or below, the gauge should indicate infinity, otherwise there is a problem with the tube. It should be noted that the above measurements are for silicon tubes and are not applicable to germanium tubes. However, germanium tubes are also rare now. In addition, the term "reverse" refers to the PN junction, and the direction of NPN and PNP transistors is actually different.

Most commonly used transistors nowadays are plastic sealed. How to accurately determine which of the three pins of the transistor is b, c, or e? The b-pole of a transistor is easy to measure, but how to determine which one is c and which one is e? Here are three recommended methods: * Method: For a pointer meter with a tested transistor hFE socket, first measure the b-pole, then insert the transistor into the socket at will (of course, the b-pole can be inserted accurately), measure the hFE value, and then invert the transistor to measure again. The larger hFE value is measured once, and the correct position of each pin is inserted. The second method: For meters without hFE measuring sockets or for tubes that are too large to be inserted into sockets, this method can be used: for NPN tubes, first measure the b-pole (whether the tube is NPN or PNP and its b-pin are easy to measure, right? ）Place the watch in the R × 1k Ω range, connect the red probe to the assumed e pole (be careful not to touch the tip or pin of the red probe with your hand), and connect the black probe to the assumed c pole. At the same time, pinch the tip and pin of the watch with your fingers, pick up the tube, and use your tongue to lick the b pole. Check that the pointer on the watch head should have a certain deviation. If you connect the probes correctly, the deviation of the pointer will be larger. If you connect them incorrectly, the deviation of the pointer will be smaller. The difference is very obvious. From this, the c and e poles of the tube can be determined. For PNP tubes, connect the black probe to the assumed e pole (do not touch the pen tip or pin), and the red probe to the assumed c pole. At the same time, pinch the pen tip and pin with your fingers, and then lick the b pole with your tongue. If each probe is connected correctly, the pointer of the meter head will deflect significantly. Of course, when measuring, the probes need to be exchanged twice to compare the readings before making a final judgment. This method is applicable to all shapes of transistors and is convenient and practical. Based on the deflection amplitude of the pointer, the amplification 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 transistor and its b-pole, then place the meter in the R × 10k Ω range. For NPN transistors, when the black probe is connected to the e-pole and the red probe is connected to the c-pole, there may be a certain deviation in the needle. For PNP transistors, when the black probe is connected to the c-pole and the red probe is connected to the e-pole, there may be a certain deviation in the needle, and vice versa, there will be no deviation. This can also determine the c and e poles of the transistor. However, for high-pressure pipes, this method is not applicable.

For common imported high-power plastic sealed tubes, the C-pole is mostly in the middle (I haven't seen one with B in the middle yet). Some small and medium power transistors may have b in the middle. For example, the commonly used 9014 transistor and other models of transistors in the series, as well as 2SC1815, 2N5401, 2N5551 and other transistors, have their b-pole in the middle. Of course, they also have a C-pole in the middle. So when repairing or replacing transistors, especially these low-power transistors, they should not be installed directly as they are. They must be tested first.