How to use a multimeter to measure the quality of components and whether the circuit is working properly
Offline measurement
Measuring components separately without any connection to the working circuit is considered offline measurement. Below is a brief introduction to several types:
1. Inductive type, such as the 220V end of a 1W-500W power transformer, the DC resistance is generally between a few K Ω to several tens of Ω, and the higher the wattage, the lower the resistance value. The coil resistance of the electromagnetic relay is roughly within this range; In addition, the transformers used in switch mode power supplies have relatively low DC resistance, usually between a few tenths of an ohm and a few tens of ohms. The higher the power and frequency, the lower the DC resistance. The DC resistance of small finished inductors is also within this range.
Inductance measurement has a common feature, which is that the test results are the same regardless of whether the measurement is positive, negative, or at any level using a pointer or digital meter.
2. Measurement of semiconductor devices: When measuring diodes, the forward resistance is generally between a few Ω to a few hundred Ω, and the test results may vary depending on the meter and gear. And the reverse resistance will be large, usually between a few megabytes and ∞, but the germanium tube will be smaller, usually above a few hundred K Ω. If using a pointer meter Rx10K to measure a diode with a voltage stabilization value below 9V, it is normal for both forward and reverse resistances to be very low. When measuring NPN or PNP type transistors, b, c, and e can be understood as two diodes connected together. The above method can be used for measurement. For meters with a dedicated transistor gear, direct measurement can be performed using that gear.
3. Measurement of capacitance
For a digital meter, it can be measured directly using the capacitance range. If it is a pointer meter, Rx1 or Rx10 can be used to measure 100 μ For capacitors above F, measure 100 using Rx1K or Rx10K range μ Capacitors below F. It is better to return to the origin after the pointer swings (the larger the capacity, the greater the swing), otherwise there may be leakage or the probe may be exchanged for retesting, because the leakage of the electrolytic capacitor will increase when reverse voltage is applied. For capacitors that have just been removed from the circuit, they must be discharged and measured to prevent damage to the multimeter.
4. Rough measurement of integrated circuits
There should be a good integrated circuit as a reference for this. Compare and contrast the forward and reverse resistances of any two pins separately. If the resistance value between a certain pin is significantly different from that of a good integrated circuit, it can be preliminarily judged that the integrated circuit is damaged.
on-line measurement
Online measurement belongs to live measurement. To do this work, the first step is to understand the normal voltage or current at the measured point. It is best to use a digital meter with high internal resistance when measuring voltage to reduce the impact on the working state of the tested circuit, otherwise the credibility of the test data will be greatly compromised. Online voltage measurement is an important means of judging the quality of a circuit or component.
When measuring the current, the measured point should be disconnected first with the power off, connected in series with a multimeter, and the gear should be set to a position slightly higher than the current of the measured point. Then, the power on test is sufficient.
