Detailed discussion on the application skills of digital multimeter

1. Selection of pointer meter and digital meter

⒈ The reading accuracy of the pointer meter is poor, but the process of the pointer swing is more intuitive, and the swing speed and amplitude can sometimes objectively reflect the size of the measurement (such as measuring the slight jitter of the TV data bus (SDL) when transmitting data); the reading of the digital meter is intuitive, but the process of digital changes looks very messy and not easy to watch.

⒉ There are usually two batteries in the pointer meter, one low voltage 1.5V, and one high voltage 9V or 15V. The black test lead is the positive terminal relative to the red test lead. A 6V or 9V battery is often used in digital meters. In the resistance range, the output current of the pointer meter lead is much larger than that of the digital meter. Using the R×1Ω range can make the speaker emit a loud "click" sound, and using the R×10kΩ range can even light up the light-emitting diode (LED).

⒊ 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 and micro-current situations, it is even impossible to measure accurately because its internal resistance will affect the circuit being measured (for example, when measuring the acceleration stage voltage of a TV picture tube, the measured value will be much lower than the actual value). The internal resistance of the digital meter voltage range is very large, at least in the megohm level, and has little effect on the circuit being measured. However, the extremely high output impedance makes it susceptible to the influence of induced voltage, and the data measured in some situations with strong electromagnetic interference may be false.

⒋ In short, pointer meters are suitable for measuring analog circuits with relatively large currents and high voltages, such as televisions and audio amplifiers. Digital meters are suitable for measuring digital circuits with low voltages and small currents, such as BP machines and mobile phones. It is not absolute, and pointer meters and digital meters can be used according to the situation.

2. Measurement Techniques

1. Testing speakers, headphones, and dynamic microphones:

Use the R×1Ω setting, connect one probe to one end, and the other probe to the other end. Normally, a clear and loud "click" sound will be emitted. If it does not sound, the coil is broken. If the sound is small and sharp, there is a problem with the coil being rubbed, and it cannot be used.

2. Measure capacitance:

Use the resistance range and select the appropriate range according to the capacitance. When measuring, pay attention to the black test lead of the electrolytic capacitor to be connected to the positive pole of the capacitor.

①. Estimation of the capacity of microwave grade capacitors: You can make a judgment based on experience or by referring to a standard capacitor of the same capacity, according to the maximum swing of the pointer. The reference capacitor does not have to have the same withstand voltage value, as long as the capacity is the same. For example, a 100μF/250V capacitor can be estimated by using a 100μF/25V capacitor as a reference. As long as the maximum swing of their pointers is the same, it can be determined that the capacity is the same.

②. Estimating the capacitance of pico-farad capacitors: Use the R×10kΩ range, but it can only measure capacitance above 1000pF. For capacitance of 1000pF or slightly larger, as long as the needle swings slightly, it can be considered that the capacitance is sufficient.

③. Measure whether the capacitor is leaking: For capacitors above 1,000 microfarads, you can first use the R×10Ω position to quickly charge it and preliminarily estimate the capacitance, then change to the R×1kΩ position and continue measuring for a while. At this time, the pointer should not return, but should stop at or very close to ∞, otherwise there is leakage. For some timing or oscillation capacitors below tens of microfarads (such as the oscillation capacitor of the color TV switching power supply), the leakage characteristics are very high. If there is a slight leakage, it cannot be used. At this time, after charging at the R×1kΩ position, you can switch to the R×10kΩ position to continue measuring. Similarly, the needle should stop at ∞ and should not return.

3. Test the diodes, transistors and voltage regulators on the road:

Because in the actual circuit, the bias resistor of the transistor or the peripheral resistor of the diode and the voltage regulator are generally large, mostly above several hundred or several thousand ohms, so we can use the R×10Ω or R×1Ω range of the multimeter to measure the quality of the PN junction in the circuit. When measuring in the circuit, the PN junction measured with the R×10Ω range should have obvious forward and reverse characteristics (if the difference between the forward and reverse resistances is not obvious, the R×1Ω range can be used instead). Generally, the forward resistance should be measured at about 200Ω when the R×10Ω range is measured, and the forward resistance should be measured at about 30Ω when the R×1Ω range is measured (it may be slightly different depending on the different phenotypes). If the forward resistance value is too large or the reverse resistance value 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 quickly find the bad tube, and can even measure the tube that has not been completely broken but has deteriorated characteristics. For example, when you use a small resistance range to measure the forward resistance of a PN junction and find that it is too large, if you solder it off and measure it again using the commonly used R×1kΩ range, it may still be normal. In fact, the characteristics of this tube have deteriorated and it cannot work normally or is unstable.

4. Measure resistance:

It is important to select a good 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 the R×10k resistance range to measure a large resistance of megohm level, do not pinch your fingers at both ends of the resistor, as the human body resistance will make the measurement result smaller.

5. Measure the voltage zener diode:

The voltage regulator value of the voltage regulator 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. In this way, measuring the voltage regulator with the resistance range below R×1k is like measuring a diode, which has complete unidirectional conductivity. However, the R×10k range of the pointer meter is powered by a 9V or 15V battery. When using R×10k to measure the voltage regulator with a voltage regulator value less than 9V or 15V, the reverse resistance will not be ∞, but 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 preliminarily estimate the quality of the voltage regulator. However, a good voltage regulator must also have an accurate voltage regulator value. How to estimate this voltage regulator value under amateur conditions? It is not difficult. Just find a pointer meter. The method is: first set a meter to the R×10k position, and connect its black and red test leads to the cathode and anode of the voltage regulator tube respectively. At this time, the actual working state of the voltage regulator tube is simulated. Then take another meter and set it to the voltage position V×10V or V×50V (according to the voltage regulation value), and connect the red and black test leads to the black and red test leads of the previous meter respectively. At this time, the measured voltage value is basically the voltage regulation value of this voltage regulator tube. "Basically" is said because the bias current of the first meter for the voltage regulator tube is slightly smaller than the bias current during normal use, so the measured voltage regulation value will be slightly larger, but the difference is not big. This method can only estimate the voltage regulator tube whose voltage regulation value is less than the voltage of the high-voltage battery of the pointer meter. If the voltage regulation value of the voltage regulator tube is too high, it can only be measured by the method of adding an external power supply (so it seems that when we choose a pointer meter, it is more suitable to choose a high-voltage battery voltage of 15V than 9V).

6. Test transistors:

Usually we need to use the R×1kΩ range. No matter it is an NPN tube or a PNP tube, no matter it is a low-power, medium-power, or high-power tube, its be junction and cb junction should show the same unidirectional conductivity as a diode, with infinite reverse resistance and a forward resistance of about 10K. To further estimate the quality of the tube characteristics, if necessary, the resistance range should be changed for multiple measurements. The method is: set the R×10Ω range to measure the PN junction forward conduction resistance, which is about 200Ω; set the R×1Ω range to measure the PN junction forward conduction resistance, which is about 30Ω. (The above data is measured by the 47-type meter. Other models may be slightly different. You can test several good tubes to summarize and have a good idea.) If the reading is too large, it can be concluded that the characteristics of the tube are not good. You can also set the meter to R×10kΩ and measure again. 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 at ∞, but the reverse resistance of the be junction may be a little, and the needle of the meter will deflect slightly (generally not more than 1/3 of the full scale, depending on the withstand voltage of the tube). Similarly, when using the R×10kΩ range to measure the resistance between ec (for NPN tubes) or between ce (for PNP tubes), the needle of the meter may deflect slightly, but this does not mean that the tube is bad. However, when using the range below R×1kΩ to measure the resistance between ce or ec, the meter head should indicate infinity, otherwise the tube is problematic. It should be noted that the above measurement is for silicon tubes and is not applicable to germanium tubes. However, germanium tubes are rare now. In addition, the so-called "reverse" is for PN junctions, and the directions for NPN and PNP tubes are actually different.