How to Use a Multimeter to Measure Leakage Current
1、 Selection of pointer and digital meters:
1. The reading accuracy of the pointer meter is poor, but the process of pointer oscillation is relatively intuitive, and the amplitude of its oscillation speed can sometimes objectively reflect the measured size (such as the slight jitter of the TV data bus (SDL) during data transmission); The reading on the digital meter is intuitive, but the process of changing the numbers looks messy and not easy to watch.
2. There are usually two batteries in a pointer meter, one with a low voltage of 1.5V and the other with a high voltage of 9V or 15V. The black pen is relatively positive compared to the red pen. A digital meter usually uses a 6V or 9V battery. In the resistance range, the output current of the pointer meter is much larger than that of a digital meter, using R × 1 Ω gear can make the speaker emit a loud "click" sound, using R × 10k Ω gear can even light up light-emitting diodes (LEDs).
3. In the voltage range, the internal resistance of a pointer meter is relatively small compared to a digital meter, and the measurement accuracy is relatively poor. In some situations where high voltage and micro current are present, it is even impossible to accurately measure them because their internal resistance can affect the circuit being tested (for example, when measuring the acceleration stage voltage of a television picture tube, the measured value may be much lower than the actual value). The internal resistance of the voltage range of the digital meter is very high, at least at the megaohm level, and has little impact on the circuit being tested. But the extremely high output impedance makes it susceptible to the influence of induced voltage, and the data measured in some places with strong electromagnetic interference may be false.
4. In short, pointer meters are suitable for measuring analog circuits with relatively high currents and voltages, such as television sets and audio amplifiers. Digital meters are suitable for low voltage and low current digital circuit measurements, such as BP machines, mobile phones, etc. Not absolute, you can choose a pointer table and a digital table according to the situation.
2、 Measurement techniques (if not specified, referring to a pointer table):
1. Measuring speakers, headphones, and dynamic microphones: using R × At 1 Ω level, if any probe is connected to one end and the other probe is touched to the other end, a clear and crisp "click" sound will be emitted normally. 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. Capacitance measurement: Using a resistance range, select an appropriate range based on the capacitance, and pay attention to connecting the black probe of the electrolytic capacitor to the positive electrode of the capacitor during measurement Estimating the capacity of microwave level capacitors: It can be determined based on experience or by referring to standard capacitors of the same capacity, and the maximum amplitude of pointer oscillation. The referenced capacitance does not need to have the same withstand voltage value, as long as the capacity is the same, for example, estimating a 100 μ F/250V capacitor can be used with a 100 μ By referring to the capacitance of F/25V, as long as the maximum amplitude of their pointer oscillation is the same, it can be concluded that the capacity is the same Estimating the capacity of a picosecond capacitor: R should be used × 10k Ω range, but can only measure capacitance above 1000pF. For capacitors of 1000pF or slightly larger, as long as the watch needle swings slightly, the capacity is considered sufficient Test for leakage of capacitance: For capacitors above 1000 microf, R can be used first × Quickly charge it at 10 Ω level and preliminarily estimate the capacitance capacity, then change it to R × Continue measuring at 1k Ω level for a while, and at this point, the pointer should not return, but should stop at or very close to ∞, otherwise there will be leakage. For some timing or oscillating capacitors below tens of microfacies (such as oscillating capacitors in color TV switching power supplies), the leakage characteristics are very high, and they cannot be used as long as there is a slight leakage. In this case, R × After charging at 1k Ω, switch to R × Continue measuring at 10k Ω level, and the pointer should stop at ∞ instead of returning.
3. When testing the quality of diodes, transistors, and voltage regulators on the road: because in actual circuits, the bias resistance of transistors or the peripheral resistance of diodes and voltage regulators are generally relatively large, mostly in the hundreds and thousands of ohms or above. In this way, we can use the R of a multimeter × 10 Ω or R × Measure the quality of the PN junction on the road at 1 Ω level. When measuring on the road, use R × The PN junction measured at 10 Ω should have obvious forward and reverse characteristics (if the difference in forward and reverse resistances is not significant, R can be used instead × 1 Ω gear for measurement), usually the forward resistance is at R × When measuring the 10 Ω gear, the gauge needle should indicate around 200 Ω, at R × When measuring at 1 Ω level, the dial should indicate around 30 Ω (may vary slightly depending on different phenotypes). If the measurement results show that the forward resistance value is too high or the reverse resistance value is too low, it indicates that there is a problem with the PN junction and the pipe. This method is particularly effective for maintenance, as it can quickly identify faulty pipes and even detect pipes that have not yet completely broken but have deteriorated characteristics. For example, if you use a low resistance range to measure the forward resistance of a PN junction, and you solder it down, use the commonly used R × After retesting at 1k Ω, it may still be normal, but in fact, the characteristics of this pipe have deteriorated, making it unable to work properly or unstable.
