How To Use A Multimeter To Test Whether A Circuit Is Faulty

Common PCB circuit board faults are mainly concentrated in the components above, such as capacitors, resistors, inductors, diodes, audiodes, field effect tubes, etc., the obvious damage of integrated chips and crystal oscillators, these faults can be found visually with the eyes, and some faults need to borrow tools, today small series to explain how to multimeter troubleshooting circuit board faults:

Visual Troubleshooting

1, look at the state of the component

Get a faulty circuit board, first observe the circuit board has no obvious component damage, such as electrolytic capacitor burn and bulge, resistance burn and power device burn.

2. Look at the welding of the circuit board

If the printed circuit board is deformed and warped; There is no solder joint falling off, obvious virtual welding; The copper cladding of the circuit board has not warped, burned and blackened.

3, observe the plug-in components

Such as integrated circuit, diode, circuit board power transformer and other directions are not inserted wrong.

Multimeter Check Circuit Board Faults

First, the detection of ordinary diodes

Measure with MF47 multimeter, connect the red and black pen to both ends of the diode, read the reading, and then switch the pen to measure. According to the results of the two measurements, the forward resistance value of the low-power germanium diode is usually 300-500Ω, 

and the silicon diode is about 1kΩ or larger. The reverse phase resistance of the germanium tube is tens of kiloohms, and the reverse resistance of the silicon tube is above 500kΩ (the value of the high-power diode is much smaller). Good diode forward resistance is lower,

 the reverse resistance is larger, the greater the positive and negative resistance difference, the better. If the positive and reverse resistances measured are very small and close to zero, it indicates that the diode has a short circuit inside;

 If the positive and reverse resistance is large or tends to infinity, it means that the inside of the pipe has been broken. In both cases the diode needs to be scrapped.

In the road test: testing the positive and negative resistance of the diode PN junction, it is easier to determine whether the diode is a breakdown short circuit or a break.

Second, triode detection

Turn the digital multimeter to the diode and use the pen to measure the PN junction. If the positive pilot is on, the number displayed is the positive voltage drop of the PN junction.

First determine the collector and emitter; The forward pressure drop of the two PN junctions is measured with a pen. The one with large pressure drop is emitter e and the one with small pressure drop is collector c. In the test of two junctions,

 the red pen is connected to the common pole, then the tested triode is NPN type, and the red pen is connected to the base b; If the black watch pen is connected to a common pole, the triode under test is PNP type, and this pole is base b. 

After the triode is damaged, the PN junction has two conditions: breakdown short circuit and open circuit.

In the road test: In the road test of the transistor, in fact, by testing the positive and reverse resistance of the PN junction, to determine whether the transistor is damaged. The branch resistance is greater than the forward resistance of the PN junction,

 and the positive and reverse resistance measured in normal times should be significantly different, otherwise the PN junction is damaged. When the branch resistance is less than the PN junction forward resistance, the branch should be disconnected, otherwise it is impossible to judge the quality of the triode.

Third, the experience of MOS tube is good or bad

1: The black watch pen is connected to the D pole, the red watch pen is connected to the S pole, and there is generally a 500-600 resistance value

2: Under the premise that the black watch pen does not move, use the red watch pen to point the G pole, and then use the red pen to measure the S pole, there will be conduction

3: The red pen is connected to the D pole, and the resistance measured by the G pole below the black pen point and then the S pole is the same as that measured by 1, indicating that the MOS tube works normally

4. Detection of electrolytic capacitors

When measuring with MF47 multimeter, the appropriate range of the multimeter should be selected for electrolytic capacitors with different capacities. According to experience, under normal circumstances, electrolytic capacitors below 47μF can be measured in R×1K, 

and electrolytic capacitors greater than 47μF can be measured in R×100.

Connect the red pen of the multimeter to the negative electrode of the capacitor, and the black pen to the positive electrode. At the moment of contact, the multimeter pointer deflects to the right by a large margin, 

and then gradually turns to the left until it stops at a certain position (returning to infinity). The resistance value at this time is the forward leakage resistance of the electrolytic capacitor. The larger the value, the smaller the leakage current and the better the performance of the capacitor. 

Then, switch the red and black watch pen, and the multimeter pointer will repeat the above-mentioned swinging phenomenon. However, the resistance measured at this time is the reverse phase leakage resistance of the electrolytic capacitor, which is slightly less than the forward leakage resistance. 

That is, the reverse leakage current is larger than the forward leakage current. Practical experience shows that the leakage resistance of electrolytic capacitors should generally be more than a few hundred thousand ohms, otherwise it will not work normally.

In the test, if there is no charging phenomenon in the forward and reverse phase, that is, the watch hand does not move, it indicates that the capacitor capacity disappears or the internal short circuit; If the resistance value measured is small or zero,

 it indicates that the capacitor has large leakage or has been broken down and damaged, and can no longer be used.

On-road test: On-road test of electrolytic capacitors should only check the fault of serious leakage or breakdown, and the accuracy of slight leakage or small-capacity electrolytic capacitors test is very poor.

 The influence of other components on the test should also be considered in the road test, otherwise the value read out will be inaccurate and will affect the normal judgment. 

Electrolytic capacitors can also use capacitance meters to detect the capacitance value between the two ends to judge the quality of electrolytic capacitors.