How does a multimeter quickly detect circuit board failures?

How does a multimeter quickly detect circuit board failures?

Common faults of PCB circuit boards are mainly concentrated on components such as capacitors, resistors, inductors, diodes, triodes, field-effect transistors, etc. The damage of integrated chips and crystal oscillators can be found visually, while some faults need to be detected by using maintenance tools. Today I will explain to you how to troubleshoot circuit board failures with a multimeter:

Intuitive troubleshooting:

1. Check the status of the components

First, observe whether the faulty circuit board has apparent component damage, such as electrolytic capacitor burnout and bulge, resistor burnout, and power device burnout.

2. Check the soldering condition of the circuit board

Check whether the printed circuit board is deformed or warped, whether there are solder joints falling off or soldering, and whether the circuit board's copper cladding is warped, burned, and turned black.

3. Plug-ins for observation components

Ensure that components such as diodes, integrated circuits, circuit board power transformers, etc., are not inserted in the wrong direction.

Multimeter to troubleshoot circuit board failures:

1. Detection of ordinary diodes

Connect the red and black test leads of the multimeter to the two ends of the diode respectively, read the readings, and then exchange the test leads to measure. Judging by comparing the two measurement results, the forward resistance value of low-power germanium diodes is usually 300-500Ω, and the silicon diodes are about 1kΩ or more. If the measured forward and reverse resistances are very small and close to zero, the diode is short-circuited inside; if the forward and reverse resistances are large or tend to be infinite, it means that the inside of the tube is broken.
On the road detection: test the forward and reverse resistance of the diode PN junction; it is easier to judge whether the diode is a breakdown short circuit or an open circuit.

2. Triode detection

Turn the digital multimeter to the diode gear, and measure the PN junction with a test lead. If the forward direction is turned on, the displayed number is the forward voltage drop of the PN junction.
First, determine the collector and the emitter; measure the forward voltage drop of the two PN junctions with a test lead. The emitter e has the larger voltage drop, and the collector c is the smaller one. When testing two junctions, if the red test lead is connected to the common pole, the tested triode is NPN type, and the red test lead is connected to the base b; if the black test lead is connected to the common pole, the tested triode is PNP type, and this pole is the base b. After the triode is damaged, the PN junction has two situations: a breakdown short circuit and an open circuit.
On the road test: The on-road test of the triode judges whether the triode is damaged by testing the forward and reverse resistance of the PN junction. The branch resistance is greater than the forward resistance of the PN junction, and the forward and reverse resistances measured in normal conditions should be significantly different. Otherwise, the PN junction will be damaged. When the branch circuit resistance is less than the forward resistance of the PN junction, the branch circuit should be disconnected. Otherwise, the quality of the triode cannot be judged.

3. Detection of the quality of the MOS tube

1) Connect the black test lead to the D pole, and the red test lead to the S pole, generally with a resistance value of 500-600;
2) On the premise that the black test pen does not move, tap the G pole with the red test pen, and then use the red pen to measure the S pole, and there will be continuity;
3) The red test lead is connected to the D pole, the black test lead is clicked on the G pole and then connected to the S pole. The measured resistance value is the same as that measured by 1, indicating that the MOS tube is usually working.

4. Detection of electrolytic capacitors

Connecting the multimeter's red test lead to the capacitor's negative electrode, and the black one lead to the positive one. At the moment of first contact, the pointer of the multimeter deflects to the right by a large amount and then gradually turns to the left until it stops at a certain position. At this time, the measured resistance value is the electrolytic capacitor's forward leakage resistance. The larger the value, the smaller the leakage current and the better the capacitor performance. Then swap the red and black test pens, and the multimeter pointer will repeat the swing mentioned above. At this time, the measured resistance value is the reverse-phase leakage resistance of the electrolytic capacitor, which is slightly smaller than the forward leakage resistance, which means that the reverse-phase leakage current is larger than the forward leakage current.
In the test, if there is no charging phenomenon in the forward and reverse phases, that is, the needle does not move, it means that the capacitor capacity disappears or there is an internal short circuit; If the measured resistance value is small or zero, it means that the capacitor has large leakage or has been damaged by the breakdown and cannot be used anymore.
On-the-road test: Electrolytic capacitors should only be used to check for severe leakage or breakdown faults. The accuracy of testing minor leakage or small-capacity electrolytic capacitors is poor. In the road test, the influence of other components on the test should also be considered. Otherwise, the read value will be inaccurate, which will affect the normal judgment. Electrolytic capacitors can also use a capacitance meter to detect the capacitance value between the two ends to judge the quality of electrolytic capacitors.