Application Techniques of Multimeters

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• 1. Selection of Analog and Digital Multimeters: 1, Mechanical analog multimeters have slight jitter when transmitting data; digital multimeters provide intuitive readings, but the process of digital changes can appear chaotic and hard to read. 2, Analog meters generally have two batteries, one low voltage 1.5V, and one high voltage 9V or 15V, where the black probe is positive and the red probe is negative. Digital meters usually use a 6V or 9V battery. In resistance mode, the output current from the analog meter’s probes is much larger than that of the digital meter, using R×1Ω can make the speaker produce a loud “click” sound, while using R×10kΩ can even light up a light-emitting diode (LED). 3, In voltage mode, the internal resistance of the analog meter is relatively low compared to the digital meter, resulting in lower measurement accuracy. In some high-voltage microcurrent situations, it may not be able to measure accurately because its internal resistance affects the measured circuit (for example, measuring the accelerating voltage of a TV CRT may yield values significantly lower than actual). The internal resistance in the voltage mode of the digital meter is very high, at least in the megaohm range, with minimal impact on the measured circuit. However, the extremely high output impedance makes it susceptible to induced voltages, and in some scenarios with strong electromagnetic interference, the data measured may be false. 4, In summary, analog meters are suitable for measuring large current and high voltage analog circuits, such as TVs and audio amplifiers. Digital meters are suitable for measuring low voltage and small current digital circuits, such as BP machines and mobile phones. It is not absolute and can be selected based on the situation.2. Measurement Techniques (unless specified, analog meters are used): 1, Measuring speakers, headphones, and dynamic microphones: Use R×1Ω range, connect one probe to one end and touch the other end with the other probe; normally it will produce a crisp “click” sound. If there is no sound, the coil is broken; if the sound is small and sharp, there is a rubbing problem, and it cannot be used. 2, Measuring capacitors: Use the resistance range, select an appropriate range based on the capacitor’s capacity, and note that for electrolytic capacitors, the black probe should connect to the positive terminal of the capacitor.

• (1), Estimating the value of microfarad capacitors: You can rely on experience or refer to standard capacitors of the same capacity, judging by the maximum swing of the pointer. The reference capacitor’s voltage rating does not need to be the same, as long as the capacities are the same; for example, to estimate a 100μF/250V capacitor, you can use a 100μF/25V capacitor as a reference, as long as their pointer swings are the same, the capacities can be determined to be the same.(2), Estimating the value of picofarad capacitors: Use R×10kΩ range but can only measure capacitors above 1000pF. For 1000pF or slightly larger capacitors, if the pointer swings slightly, it can be considered sufficient capacity.

• (3), Measuring capacitor leakage: For capacitors above 1000μF, you can first use R×10Ω range to quickly charge it and preliminarily estimate its capacity, then switch to R×1kΩ range to continue measuring; the pointer should not return but should stop at or very close to ∞; otherwise, there is a leakage phenomenon. For timing or oscillating capacitors below several tens of microfarads (like oscillating capacitors in color TV switch mode power supplies), the leakage characteristics are very high; any leakage makes them unusable. In this case, after charging with R×1kΩ, switch to R×10kΩ range to continue measuring; similarly, the pointer should stop at ∞ and not return. 3, In-circuit testing of diodes, transistors, and voltage regulators: Because in actual circuits, the bias resistors of transistors or surrounding resistors of diodes and voltage regulators are generally quite large, mostly in the hundreds or thousands of ohms, we can use the multimeter in R×10Ω or R×1Ω range to measure the PN junction in-circuit. When measuring in-circuit, using R×10Ω range, the PN junction should exhibit obvious forward and reverse characteristics (if the forward and reverse resistances are not significantly different, you can switch to R×1Ω range to measure). Generally, the forward resistance should indicate around 200Ω when measuring with R×10Ω range, and around 30Ω when measuring with R×1Ω range (this may vary slightly depending on the meter type). If the forward resistance is too high or the reverse resistance too low, it indicates a problem with the PN junction, and the component is faulty. This method is particularly effective for repairs, allowing for a quick identification of faulty components, even those that have not completely failed but have degraded characteristics. For example, if you measure a PN junction’s forward resistance with a low resistance range and find it too high, if you desolder it and measure it with a common R×1kΩ range, it may still appear normal; however, its characteristics have already degraded, making it unstable or unable to work properly. 4, Measuring resistance: It is important to select the right range; when the pointer indicates between 1/3 and 2/3 of full scale, the measurement accuracy is highest and the reading most accurate. Note that when measuring megaohm-level high resistance with R×10k resistance range, do not touch the ends of the resistor with your fingers, as the body’s resistance will cause the measurement result to be lower. 5 , Measuring zener diodes: The zener diodes we commonly use usually have a zener voltage greater than 1.5V, while the resistance ranges below R×1k of the analog meter are powered by the 1.5V battery inside; thus, measuring zener diodes with resistance ranges below R×1k will behave like measuring diodes, exhibiting complete unidirectional conductivity. However, the R×10k range is powered by a 9V or 15V battery; when measuring zener diodes with a zener voltage less than 9V or 15V, the reverse resistance will not be ∞ but will have a certain resistance, which is still significantly higher than the forward resistance of the zener diode. Thus, we can preliminarily estimate the condition of the zener diode. However, a good zener diode must also have an accurate zener voltage; how can we estimate this zener voltage under amateur conditions? It’s not difficult; just find another analog meter. The method is: first set one meter to R×10k range, connecting the black and red probes to the cathode and anode of the zener diode, simulating the actual working state of the zener diode, then use another meter set to voltage mode V×10V or V×50V (depending on the zener voltage) to connect the red and black probes to the black and red probes of the first meter; the voltage measured is essentially the zener voltage of this zener diode. The term “essentially” is used because the first meter’s bias current is slightly lower than the normal operating bias current, so the measured zener voltage will be slightly higher, but the difference is minimal. This method can only estimate zener voltages less than the high voltage battery voltage of the analog meter. If the zener voltage is too high, we can only measure it using an external power supply (thus, when selecting an analog meter, it’s more suitable to use one with a high voltage battery of 15V rather than 9V). 6 , Measuring transistors: Typically, we should use R×1kΩ range, regardless of whether it’s an NPN or PNP transistor, whether it’s low, medium, or high power; measuring the be and cb junctions should exhibit the same unidirectional conductivity as diodes, with reverse resistance being infinite, and the forward resistance around 10K. To further estimate the characteristics of the transistor, it is necessary to switch resistance ranges and measure multiple times; the method is: set to R×10Ω range, the forward conduction resistance of the PN junction should be around 200Ω; set to R×1Ω range, the forward conduction resistance of the PN junction should be around 30Ω; (the above data is measured with a 47 model meter, other models may vary slightly; you can measure several good transistors and summarize to have a better understanding). If the readings are too high, it can be concluded that the transistor’s characteristics are poor. You can also set the meter to R×10kΩ and measure; even for low voltage rated transistors (generally, transistors have a voltage rating above 30V), the cb junction should also be ∞, but the reverse resistance of the be junction may show some resistance, causing the pointer to slightly deflect (generally not exceeding 1/3 of full scale, depending on the transistor’s voltage rating). Similarly, when measuring the resistance between ec (for NPN) or ce (for PNP) using R×10kΩ range, the pointer may slightly deflect, but this does not indicate the transistor is faulty.However, when measuring the resistance between ce or ec with ranges below R×1kΩ, the meter should indicate infinity; otherwise, the transistor is faulty. It should be noted that the above measurements are applicable to silicon transistors and do not apply to germanium transistors, which are now rare. Additionally, the term “reverse” refers to the PN junction, and the direction for NPN and PNP transistors is actually different.

• Most commonly seen transistors are now plastic-encapsulated; how to accurately identify which of the three leads is b, c, e? The b lead is easy to measure, but how to determine which is c and which is e? Here are three recommended methods: The first method: For analog meters with an hFE socket, first measure the b lead, then insert the transistor into the socket (of course, the b lead must be inserted correctly) and measure the hFE value; then flip the transistor and measure again; the configuration with the larger hFE value indicates the correct connections. The second method: For meters without an hFE measurement socket, or if the transistor is too large to fit into the socket: For NPN transistors, first measure the b lead, set to R×1kΩ range, connect the red probe to the assumed e lead (be careful not to touch the probe tip or lead), and the black probe to the assumed c lead; while holding the probe tip and this lead, lick the b lead with your tongue; if the probes are connected correctly, the pointer will deflect more; if not, the deflection will be smaller, and the difference is quite obvious. Thus, you can determine the leads of the transistor as c and e. For PNP transistors, connect the black probe to the assumed e lead (again, be careful not to touch the probe tip or lead), the red probe to the assumed c lead; while holding the probe tip and this lead, lick the b lead; if the probes are connected correctly, the pointer will deflect more. Of course, you should switch the probes and measure twice, comparing the readings to make a final determination. This method is applicable to all shapes of transistors, making it convenient and practical. Based on the pointer’s deflection, you can also estimate the transistor’s amplification capability, of course, based on experience. The third method: First determine whether the transistor is NPN or PNP type and its b lead, then set the meter to R×10kΩ range; for NPN transistors, connect the black probe to the e lead and the red probe to the c lead; the pointer may deflect slightly; for PNP transistors, connect the black probe to the c lead and the red probe to the e lead; the pointer may deflect slightly, but reversing the probes will not cause deflection. This way, you can also determine the c and e leads of the transistor. However, this method is not applicable for high voltage rated transistors. For commonly seen imported models of high-power plastic-encapsulated transistors, the c lead is typically in the middle (I have never seen a b lead in the middle). For small and medium power transistors, sometimes the b lead may be in the middle. For example, common 9014 transistors and others in the same series, such as 2SC1815, 2N5401, 2N5551, etc., often have the b lead in the middle. Therefore, when repairing and replacing transistors, especially these small power transistors, do not just install them directly as they were; always measure first.

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